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Equity Incentive Schemes and Stock Compensation Charges - Summary of RSU and PSU Activity (Details)	12 Months Ended
	Dec. 31, 2023 $ / shares shares
Performance Share Unit (PSUs)
Outstanding number of shares [Roll Forward]
Outstanding at beginning of period (in shares) \| shares	152,420
Granted (in shares) \| shares	60,374
Shares vested (in shares) \| shares	(47,026)
Forfeited (in shares) \| shares	(60,512)
Outstanding at ending of period (in shares) \| shares	105,256
Weighted Average Fair Value
Outstanding at beginning of period (USD per share) \| $ / shares	$ 192.29
Granted (USD per share) \| $ / shares	232.51
Shares vested (USD per share) \| $ / shares	159.57
Forfeited (USD per share) \| $ / shares	198.70
Outstanding at end of period (USD per share) \| $ / shares	$ 226.29
Restricted Stock Units (RSUs)
Outstanding number of shares [Roll Forward]
Outstanding at beginning of period (in shares) \| shares	582,612
Granted (in shares) \| shares	308,963
Shares vested (in shares) \| shares	(188,800)
Forfeited (in shares) \| shares	(81,764)
Outstanding at ending of period (in shares) \| shares	621,011
Weighted Average Fair Value
Outstanding at beginning of period (USD per share) \| $ / shares	$ 207.73
Granted (USD per share) \| $ / shares	218.86
Shares vested (USD per share) \| $ / shares	187.68
Forfeited (USD per share) \| $ / shares	216.03
Outstanding at end of period (USD per share) \| $ / shares	$ 218.27

Equity Incentive Schemes and Stock Compensation Charges - Outstanding and Exercisable Share Options (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range exercise price (USD per share)	$ 20.83
Maximum range exercise price (USD per share)	233.88
Range 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range exercise price (USD per share)	20.83
Maximum range exercise price (USD per share)	96.15
Range 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range exercise price (USD per share)	103.81
Maximum range exercise price (USD per share)	121.68
Range 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range exercise price (USD per share)	125.74
Maximum range exercise price (USD per share)	147.26
Range 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum range exercise price (USD per share)	159.33
Maximum range exercise price (USD per share)	$ 233.88
Stock Option and Award Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares (in shares)	902,806
Weighted Average Remaining Contractual Life	4 years 5 months 1 day
Weighted average exercise price (USD per share)	$ 142.96	$ 119.86	$ 110.38	$ 108.53
Number of shares (in shares)	654,386
Vested and exercisable at end of period (USD per share)	$ 119.67
Stock Option and Award Plans \| Range 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares (in shares)	201,306
Weighted Average Remaining Contractual Life	2 years 3 months 14 days
Number of shares (in shares)	201,306
Stock Option and Award Plans \| Range 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares (in shares)	132,964
Weighted Average Remaining Contractual Life	4 years 6 months 14 days
Number of shares (in shares)	132,964
Stock Option and Award Plans \| Range 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares (in shares)	249,194
Weighted Average Remaining Contractual Life	4 years 5 months 12 days
Number of shares (in shares)	233,371
Stock Option and Award Plans \| Range 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares (in shares)	319,342
Weighted Average Remaining Contractual Life	5 years 8 months 8 days
Number of shares (in shares)	86,745

Contract Balances - Contracts with Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue from Contract with Customer [Abstract]
Billed services (accounts receivable)	$ 1,821,855	$ 1,751,950
Allowance for credit losses (note 5)	(31,533)	(20,562)	$ (7,081)
Accounts receivable (net)	1,790,322	1,731,388
Unbilled services (unbilled revenue)	951,936	957,655
Accounts receivable and unbilled revenue, net	2,742,258	2,689,043
Unearned revenue (payments on account)	(1,654,507)	(1,507,449)
Net balance	(702,571)	$ (549,794)
Change in unbilled receivables	(5,719)
Change in unearned revenue (payments on account)	(147,058)
Change in advance payments netted against unbilled contracts receivable	$ (152,777)
Change in unbilled receivables (in percent)	(0.60%)
Change in unearned revenue (payments on account) (in percent)	9.80%
Change in advance payments netted against unbilled contracts receivable (in percent)	27.80%

Income Taxes

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Income Taxes

The Company's United States and Irish based subsidiaries file income tax returns in the United States and Ireland respectively. Other foreign subsidiaries are taxed separately under the laws of their respective countries.

The components of income before income tax expense are as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Ireland	$	624,642	$	432,963	$	231,893
United States	(352,532)		(270,440)		(278,413)
Other	352,357		405,328		243,200
Income before income tax expense	$	624,467	$	567,851	$	196,680

The components of income tax expense are as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Income tax expense:
Current tax (benefit)/expense:
Ireland	$	80,427	$	53,248	$	18,469
United States	(42,428)		60,753		35,478
Other	59,153		70,395		48,003
Total current tax expense	97,152		184,396		101,950

Deferred tax (benefit)/expense:
Ireland	45,253		(6,166)		553
United States	(143,323)		(118,475)		(52,717)
Other	12,667		(344)		(8,452)
Total deferred tax benefit	(85,403)		(124,985)		(60,616)

Income tax expense allocated to continuing operations	11,749		59,411		41,334

Income tax expense was allocated to the following components of other comprehensive income:

Currency impact on long term funding	(3,903)		7,211		1,776

Cash flow hedge	301		—		—
Total	$	8,147	$	66,622	$	43,110

Ireland's statutory trading income tax rate, the rate of our country of domicile, is 12.5%. The Company's consolidated reported income tax expense differed from the amount that would result from applying the Irish statutory rate as set forth below:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Taxes at Irish statutory rate of 12.5% (2022:12.5%; 2021:12.5%)	$	78,058	$	70,980	$	24,586
Rate differential from amortization of intangible assets	(71,223)		(59,330)		(31,228)
Foreign and other income taxed at higher rates	35,778		52,464		51,273
Research & development tax incentives	(3,868)		(2,608)		(3,120)
Movement in valuation allowance	(1,068)		(777)		3,101
Effects of change in tax rates	3,154		(300)		(128)
(Decrease)/ increase in unrecognized tax benefits	(54,347)		8,392		5,246
Investor tax expense on foreign subsidiary earnings	39,165		—		—
Impact of stock compensation	(11,487)		(8,756)		(9,083)

Other	(2,413)		(654)		687
Income tax expense	$	11,749	$	59,411	$	41,334

The tax effects of temporary differences and carryforwards that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below:


	December 31, 2023		December 31, 2022
	(in thousands)
Deferred tax liabilities:
Property, plant and equipment	$	7,547	$	10,927
Operating right-of-use-assets	16,108		23,260
Goodwill	39,014		37,150
Intangible assets	950,055		1,078,302
Investments in foreign subsidiaries	52,408		1,587
Other	7,982		7,467
Total deferred tax liabilities recognized	1,073,114		1,158,693

Deferred tax assets:
Operating loss and tax credits carryforwards	124,150		88,697
Property, plant and equipment	9,082		6,010
Operating lease liabilities	20,190		27,593
Intangible assets	2,166		3,602
Stock compensation	17,605		21,862
Other liabilities	84,928		66,933
Unearned revenue	23,748		66,565
Other	8,774		9,155
Total deferred tax assets	290,643		290,417
Valuation allowance for deferred tax assets	(42,967)		(43,379)
Deferred tax assets recognized	247,676		247,038
Overall net deferred tax liability	$	(825,438)	$	(911,655)

At December 31, 2023 Ireland subsidiaries had tax credit carryforwards for income tax purposes that may be carried forward indefinitely, available for offset against future tax liabilities, if any, of $16.9 million.

At December 31, 2023 U.S. subsidiaries had U.S. federal and state net operating loss ("NOL") carryforwards of approximately $1.7 million and $331.4 million, respectively. These NOLs are available for offset against future taxable income and the expiry dates are shown in the table below. Of the $1.7 million U.S. federal NOLs, approximately $1.2 million is currently available for offset against future U.S. federal taxable income. The subsidiaries' ability to use the remaining U.S. federal and state NOL carryforwards is limited on an annual basis due to change of ownership in 2014, 2017, and 2019, as defined by Section 382 of the Internal Revenue Code of 1986, as amended. All $1.7 million of the U.S. federal NOLs are limited by Section 382 and expire between 2024 - 2037. As of December 31, 2023, U.S subsidiaries also had disallowed interest carryforwards of $204 million that can be carried forward indefinitely. These carryforwards are available for offset against future taxable income in the event that the U.S subsidiaries have excess capacity for interest deductions in future years.

The expected expiry dates of the US NOLs are as follows:


	Federal NOL's		State NOL's
	(in thousands)
2024-2037	$	1,744	$	271,434
2038-2042	—		50,685
Indefinite	—		9,251
	$	1,744	$	331,370

In addition, we also have general business tax credit carryforwards of approximately $0.8 million that are available to reduce future U.S. federal and state income taxes. The general business tax credits are non-refundable and are due to expire between the years 2026-2038.

At December 31, 2023 other than those in the U.S. and Ireland, we had operating loss carryforwards for income tax purposes that may be carried forward indefinitely, available to offset against future taxable income, if any, of approximately $42.9 million. At December 31, 2023 those subsidiaries also had additional operating loss carryforwards of $12.9 million which are due to expire between 2024 and 2030. In addition, at December 31, 2023 those subsidiaries had tax credit carryforwards for income tax purposes that may be carried forward indefinitely, available to offset against future tax liabilities, if any, of $4.3 million.

The valuation allowance at December 31, 2023 was approximately $43.0 million. The valuation allowance for deferred tax assets as of December 31, 2022 and December 31, 2021 was $43.4 million and $45.5 million respectively. The net change in the total valuation allowance was a decrease of $0.4 million during 2023 and a decrease of $2.1 million during 2022. Of the total decrease of $0.4 million in 2023, $1.1 million was recognized within income tax expense and a increase of $0.7 million was recognized in Other Comprehensive Income. Of the total decrease of $2.1 million in 2022, $0.8 million was recognized within income tax expenses and $1.3 million was recognized in Other Comprehensive Income.

The valuation allowances at December 31, 2023 and December 31, 2022 were primarily related to operating losses and tax credits carried forward that, in the judgment of management, are not more likely than not to be realized. In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers the scheduled reversal of deferred tax liabilities, loss utilization, projected future taxable income and tax planning strategies in making this assessment. In respect of deferred tax assets not subject to a valuation allowance, management considers that it is more likely than not that these deferred tax assets will be realized on the basis that there will be sufficient reversals of deferred tax liabilities and taxable income in future periods.

The Company has recognized a deferred tax liability of $52.4 million (2022: $1.6 million) for investments in foreign subsidiaries where the Company does not consider the earnings to be indefinitely reinvested. Given changes in various tax laws during 2023, the group has taken the decision to repatriate certain excess earnings which will not be permanently reinvested and consequently have provided for the relevant tax in the current year. For the deferred tax liability not recognized in respect of temporary differences related to investments in foreign subsidiaries which are considered to be indefinitely reinvested, it is not practicable to calculate the exact unrecognized deferred tax liability, however it is not expected to be material as Ireland allows a tax credit in respect of distributions from foreign subsidiaries at the statutory tax rate in the jurisdiction of the subsidiary so that no material tax liability would be expected to arise in Ireland in the event these earnings were ever remitted. In addition, withholding taxes applicable to remittances from foreign subsidiaries would not be expected to be material given Ireland’s tax treaty network and the EU parent subsidiary directive.

A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows:


	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Unrecognized tax benefits at start of year	$	217,584	$	202,065	$	19,078
Increase related to acquired tax positions	—		—		170,047
Increase related to prior year tax positions	1,161		16,098		204
Decrease related to prior year tax positions	(918)		(5,442)		(1,695)
Increase related to current year tax positions	4,552		5,701		18,613
Settlements	—		—		(844)
Lapse of statute of limitations	(62,363)		(838)		(3,338)
Unrecognized tax benefits at end of year	$	160,016	$	217,584	$	202,065

The relevant statute of limitations for unrecognized tax benefits totaling $65.0 million could potentially expire during 2024.

Included in the balance of total unrecognized tax benefits at December 31, 2023 were potential benefits of $160.0 million, which if recognized, would affect the effective rate on income from continuing operations. The balance of total unrecognized tax benefits at December 31, 2022 and December 31, 2021 included potential benefits which, if recognized, would affect the effective rate of income tax from continuing operations of $217.6 million and $202.1 million respectively.

Interest and penalties recognized during the year ended December 31, 2023 amounted to a net charge of $4.2 million (2022: $7.1 million, 2021: $1.9 million) and are included within the income tax expense. Total accrued interest and penalties as of December 31, 2023 and December 31, 2022 were $27.1 million and $22.6 million respectively and are included in closing income taxes payable at those dates.

Our major tax jurisdictions are Ireland and the United States. We may potentially be subjected to tax audits in both our major jurisdictions. In Ireland, tax periods open to audit include the years ended December 31, 2019, December 31, 2020, December 31, 2021, December 31, 2022 and December 31, 2023. In the United States, tax periods open to audit include the years ended December 31, 2019, December 31, 2020, December 31, 2021, December 31, 2022 and December 31, 2023. During such audits, local tax authorities may challenge the positions taken by us in our tax returns.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

Disaggregation of Revenue

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue

Revenue disaggregated by customer profile is as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Top client	$	721,309	$	683,546	$	441,173
Clients 2-5	1,453,508		1,506,087		1,291,946
Clients 6-10	1,188,943		1,112,636		752,325
Clients 11-25	1,743,539		1,585,739		1,077,073
Other	3,012,877		2,853,378		1,918,309
Total	$8,120,176		$7,741,386		$5,480,826

Our customers have similar profiles and economic characteristics, and therefore have similar degrees of risk and growth opportunities.Contract Balances

Accounts receivable and unbilled revenue are as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Billed services (accounts receivable)	$	1,821,855	$	1,751,950
Allowance for credit losses (note 5)	(31,533)		(20,562)
Accounts receivable (net)	1,790,322		1,731,388
Unbilled services (unbilled revenue)	951,936		957,655
Accounts receivable and unbilled revenue, net	$	2,742,258	$	2,689,043

Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows:


(in thousands, except percentages)	December 31, 2023		December 31, 2022		$ Change		% Change

Unbilled services (unbilled revenue)	$	951,936	$	957,655	$	(5,719)	(0.6)	%
Unearned revenue (payments on account)	(1,654,507)		(1,507,449)		(147,058)		9.8	%
Net balance	$	(702,571)	$	(549,794)	$	(152,777)	27.8	%

Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected. These assets are recorded as unbilled revenue and therefore contract assets rather than accounts receivable when receipt of the consideration is conditional on something other than the passage of time. Liabilities are recorded for amounts that are collected in advance of the satisfaction of performance obligations or billed in advance of the revenue being earned.

Unbilled services/revenue balances arise where invoicing or billing is based on the timing of agreed milestones related to service contracts for clinical research. Contractual billing arrangements in respect of certain reimbursable expenses (principally investigators) require billing by the investigator to the Company prior to billing by the Company to the customer. As there is no contractual right of set-off between unbilled services (contract assets) and unearned revenue (contract liabilities), each are separately presented gross on the Consolidated Balance Sheet.

The Company is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support a clinical trial. The progress towards completion for clinical service contracts is measured based on total project costs (including reimbursable costs). Amounts owed to investigators and others in respect of reimbursable expenses at December 31, 2023 and December 31, 2022 were $333.0 million and $406.3 million (see note 12 - Other liabilities).

Unbilled services as at December 31, 2023 decreased by $5.7 million as compared to December 31, 2022. Unearned revenue increased by $147.1 million over the same period resulting in an increase of $152.8 million in the net balance of unbilled services and unearned revenue or payments on account between December 31, 2022 and December 31, 2023. These fluctuations are primarily due to the timing of payments and invoicing related to the Company's clinical trial management contracts. Billings and payments are established by contractual provisions including predetermined payment schedules which may or may not correspond to the timing of the transfer of control of the Company's services under the contract. Unbilled services arise from long-term contracts when a cost-based input method of revenue recognition is applied and revenue recognized exceeds the amount billed to the customer.

The credit loss expense recognized on the Company's receivables and unbilled services was $24.6 million and $17.8 million for the year ended December 31, 2023 and 2022, respectively.

As of December 31, 2023 approximately $14.8 billion of revenue is expected to be recognized in the future in respect of unsatisfied performance obligations compared to $13.7 billion as of December 31, 2022. As of December 31, 2023 the Company expects to recognize revenue on approximately 52% of the unsatisfied performance obligation over the next twelve months compared to 52% as of December 31, 2022, with the remainder recognized thereafter over the duration of the customer contracts.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Employee Benefits - Plan Asset Fair Value Measurements (Details) - Level 1 - ICON Development Solutions Limited pension plan - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 27,320	$ 26,050
Government Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21,312	22,887
Diversified Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,008	$ 3,163

- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References + Details

Description of business (Details)	12 Months Ended
Description of business (Details)	Dec. 31, 2023 Employee Location Country	Jul. 01, 2021 Employee
Product Information [Line Items]
Number of employees \| Employee	41,100	38,000
Number of locations in which company operates \| Location	106
Number of countries in which company operates \| Country	53
Geographic Concentration Risk \| United States \| Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	40.40%
Geographic Concentration Risk \| Europe \| Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	48.70%
Geographic Concentration Risk \| Rest of World \| Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	10.90%

Cover	12 Months Ended
Cover	Dec. 31, 2023 shares
Document Information [Line Items]
Document Type	20-F
Document Registration Statement	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Document Shell Company Report	false
Entity File Number	333-08704
Entity Registrant Name	ICON PLC
Entity Incorporation, State or Country Code	L2
Entity Address, Address Line One	South County Business Park
Entity Address, Address Line Two	Leopardstown
Entity Address, City or Town	Dublin 18
Entity Address, Postal Zip Code	D18 X5R3
Entity Address, Country	IE
Title of 12(b) Security	ORDINARY SHARES, PAR VALUE €0.06 EACH
Trading Symbol	ICLR
Security Exchange Name	NASDAQ
Entity common stock, shares outstanding (in shares)	82,495,086
Entity Well Known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filler Category	Large Accelerated Filer
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction [Flag]	false
Document Accounting Standard	U.S. GAAP
Entity Shell Company	false
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Central Index Key	0001060955
Business Contact
Document Information [Line Items]
Entity Address, Address Line One	South County Business Park
Entity Address, Address Line Two	Leopardstown
Entity Address, City or Town	Dublin 18
Entity Address, Postal Zip Code	D18 X5R3
Entity Address, Country	IE
Contact Personnel Name	Brendan Brennan
Contact Personnel Email Address	Brendan.Brennan@iconplc.com
Country Region	353
City Area Code	1
Local Phone Number	291-2000

Bank Credit Lines and Loan Facilities - Schedule of Principal payments (Details) - Senior Secured Term Loan - Senior Secured Term Loan - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022
Debt Instrument [Line Items]
Periodic principal payment	$ 250,000	$ 300,000	$ 150,000	$ 250,000	$ 200,000	$ 200,000	$ 100,000	$ 300,000
Total	$ 950,000				$ 800,000

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive income (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Currency translation adjustments	$ (149,148)	$ (175,369)
Actuarial gain on defined benefit pension plan	7,803	7,559
Loss on cash flow hedge	(2,161)	(3,728)
Total	$ (143,506)	$ (171,538)

- Definition

Amount, after tax, of accumulated increase (decrease) in equity from transaction and other event and circumstance from nonowner source.

+ References + Details

Business Combinations (Tables)

12 Months Ended

Dec. 31, 2023

Business Combinations [Abstract]

Schedule of Business Acquisitions, by Acquisition

The Merger Date fair value of the consideration transferred consisted of the following:


	(in thousands)
Fair value of cash consideration	$	5,308,646
Fair value of ordinary shares issued to acquiree stockholders	5,658,126
Fair value of replacement share-based awards issued to acquiree employees	209,399
Repayment of term loan obligations and accrued interest *	865,800
	$	12,041,971

* This represents the portion of PRA debt paid by ICON. PRA also paid $401.6 million from available cash to settle debt obligations that existed at the Merger Date.

Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed

The following table summarizes the allocation of the consideration transferred based on the Merger Date fair values of assets acquired and liabilities assumed, with the excess of the purchase price over the estimated fair values of the identifiable net assets acquired recorded as goodwill:


	July 1, 2021
	(in thousands)
Cash and cash equivalents	$	259,971
Accounts receivable and unbilled revenue	934,308
Other current assets	125,156
Fixed assets	156,851
Operating lease right-of-use assets	180,601
Goodwill *	8,084,314
Intangible assets	4,919,000
Deferred tax assets	25,190
Other assets	33,928
Accounts payable	(50,259)
Accrued expenses and other current liabilities	(380,048)
Current portion of operating lease liabilities	(36,506)
Unearned revenue	(739,278)
Non-current portion of operating lease liabilities	(147,204)
Deferred tax liabilities	(1,119,762)
Other non-current liabilities	(204,291)
Net assets acquired	$	12,041,971

* The goodwill in connection with the Merger is primarily attributable to the assembled workforce of PRA and the expected synergies of the Merger. None of the goodwill recognized is deductible for income tax purposes.

Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination

The following table summarizes the fair value of identified intangible assets and their respective useful lives as of the Merger Date:


	Estimated Fair Value		Estimated Useful Life
	(in thousands)
Customer relationships	$	3,938,000	23 years
Order backlog	500,000		3 years
Trade names	202,000		3 years
Patient database	168,000		7 years
Technology assets	111,000		5 years
	$	4,919,000

Schedule of measurement period adjustments

In the period since the Merger Date, the Company recognized certain measurement period adjustments as shown in the table below:


	Measurement period adjustments

	(in thousands)
Cash and cash equivalents	$	—
Accounts receivable and unbilled revenue	—
Other current assets	14,465
Fixed assets	(6,137)
Operating lease right-of-use assets	(11,744)
Goodwill	70,436
Intangible assets *	44,000
Deferred tax assets	(147,039)
Other assets	(1,166)
Accounts payable	—
Accrued expenses and other current liabilities	(37,496)
Current portion of operating lease liabilities	1,865
Unearned revenue **	19,623
Non-current portion of operating lease liabilities	10,454
Non-current deferred tax liabilities	193,837
Other non-current liabilities	(151,098)

* In the year ended December 31, 2022, the Company incurred a $2.2 million amortization expense which related to the year ended December 31, 2021 due to the timing of the measurement period adjustment.

** The unearned revenue measurement period adjustment also includes $16.0 million as a result of the early adoption of ASU 2021-08 'Business Combinations (Topic 805) - Accounting for Contract Assets and Contract Liabilities from Contracts with Customers' in Quarter 4 2021.

Supplemental Disclosure of Cash Flow Information (Tables)

12 Months Ended

Dec. 31, 2023

Supplemental Cash Flow Elements [Abstract]

Schedule of Supplemental Disclosure of Cash Flow Information


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Cash paid for interest	$	317,975	$	210,918	$	106,205
Cash paid for income taxes (net of refunds)	$	163,778	$	116,322	$	55,105

Equity Incentive Schemes and Stock Compensation Charges - Summary of Stock Option Activity (Details) - Stock Option and Award Plans - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Options Granted Under Plans
Beginning balance (in shares)	1,378,119	1,695,460	553,746
Assumed through business combination (in shares)			2,177,130
Granted (in shares)	82,472	108,643	100,299
Exercised (in shares)	(535,705)	(348,286)	(1,065,529)
Canceled/expired (in shares)	(22,080)	(77,698)	(70,186)
Ending balance (in shares)	902,806	1,378,119	1,695,460
Vested and exercisable at end of period (in shares)	654,386
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 119.86	$ 110.38	$ 108.53
Assumed through business combination (USD per share)			108.78
Granted (USD per share)	232.48	229.94	177.76
Exercised (USD per share)	95.12	102.87	111.29
Canceled/expired (USD per share)	196.20	143.08	128.46
Ending balance (USD per share)	142.96	$ 119.86	$ 110.38
Vested and exercisable at end of period (USD per share)	$ 119.67

Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Other Liabilities Disclosure [Abstract]
General trade and overhead liabilities	$ 463,882	$ 530,204
Personnel related liabilities	385,499	395,862
Operating lease liabilities	36,414	43,657
Facility related liabilities	11,078	16,896
Other liabilities	13,532	12,852
Restructuring liabilities	4,951	5,512
Short term government grants	43	42
Other liabilities	915,399	1,005,025
Amounts due to third parties for reimbursable expenses	$ 333,000	$ 406,300

Contract Balances - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Amounts due to third parties for reimbursable expenses	$ 333,000	$ 406,300
Change in unbilled receivables	(5,719)
Change in unearned revenue (payments on account)	(147,058)
Increase in advance payments netted against unbilled contracts receivable	(152,777)
Bad debt expense	24,600	17,800
Revenue, remaining performance obligation, amount	$ 14,800,000	$ 13,700,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation (in percent)		52.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation (in percent)	52.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period		12 months

Allowances for Credit Losses (Tables)

12 Months Ended

Dec. 31, 2023

Accounts Receivable, after Allowance for Credit Loss [Abstract]

Schedule of Provision for Credit Losses

The Company does business with most major international pharmaceutical companies. Provision for credit losses at December 31, 2023 and December 31, 2022 comprises:


	December 31, 2023		December 31, 2022
	(in thousands)
Opening provision	$	20,562	$	7,081
Amounts used during the year	(13,358)		(3,913)
Amounts provided during the year	24,550		17,800
Foreign exchange	(221)		(406)
Closing provision	$	31,533	$	20,562

Employee Benefits - Assumptions Used in Determining Benefit Obligation (Details) - United Kingdom - ICON Development Solutions Limited pension plan	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.80%	4.90%
Rate of compensation increase	3.00%	3.60%

Income Taxes - Consolidated Reported Provision for Income Taxes Differed from Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Taxes at Irish statutory rate of 12.5% (2022:12.5%; 2021:12.5%)	$ 78,058	$ 70,980	$ 24,586
Rate differential from amortization of intangible assets	(71,223)	(59,330)	(31,228)
Foreign and other income taxed at higher rates	35,778	52,464	51,273
Research & development tax incentives	(3,868)	(2,608)	(3,120)
Movement in valuation allowance	(1,068)	(777)	3,101
Effects of change in tax rates	3,154	(300)	(128)
(Decrease)/ increase in unrecognized tax benefits	(54,347)	8,392	5,246
Investor tax expense on foreign subsidiary earnings	39,165
Impact of stock compensation	(11,487)	(8,756)	(9,083)
Other	(2,413)	(654)	687
Income tax expense allocated to continuing operations	$ 11,749	$ 59,411	$ 41,334

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Weighted Average Fair Values and Assumptions Used (Details) - Employee Stock Option - $ / shares		12 Months Ended
	Jul. 01, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value (USD per share)	$ 107.21	$ 85.12	$ 68.42	$ 49.15
Assumptions:
Expected volatility (in percent)	30.00%	33.00%	31.00%	30.00%
Dividend yield (in percent)	0.00%	0.00%	0.00%	0.00%
Risk free interest rate (in percent)	0.56%	4.18%	1.86%	0.78%
Expected life	3 years 6 months	5 years	5 years	5 years

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 35
-Paragraph 1D
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-URI https://asc.fasb.org//1943274/2147480483/718-10-35-1D

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Business Combinations - Narrative, Biotel Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 02, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Fair value of cash consideration		$ 71,766	$ 0	$ 5,914,475
Goodwill		$ 9,022,075	$ 8,971,670	$ 9,037,931
Biotel Research LLC
Business Acquisition [Line Items]
Fair value of cash consideration	$ 68,100
Cash acquired from acquisition	1,400
Intangible assets	36,400
Goodwill	$ 23,400

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References + Details

Employee Benefits - Components of Net Periodic Benefit Cost (Details) - United Kingdom - ICON Development Solutions Limited pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 117	$ 134
Interest cost	975	672	665
Expected return on plan assets	(1,015)	(1,258)	(1,171)
Amortization of net (gain)/loss	(274)	228	625
Net periodic benefit (credit)/cost	$ (272)	$ (241)	$ 253

Allowances for Credit Losses

12 Months Ended

Dec. 31, 2023

Accounts Receivable, after Allowance for Credit Loss [Abstract]

Allowance for Credit Losses

The Company does business with most major international pharmaceutical companies. Provision for credit losses at December 31, 2023 and December 31, 2022 comprises:


	December 31, 2023		December 31, 2022
	(in thousands)
Opening provision	$	20,562	$	7,081
Amounts used during the year	(13,358)		(3,913)
Amounts provided during the year	24,550		17,800
Foreign exchange	(221)		(406)
Closing provision	$	31,533	$	20,562

Business Combinations - Schedule of Pro-Forma Information, PRA Acquisition (Details) - PRA Health Sciences, Inc - USD ($) $ in Thousands	12 Months Ended	18 Months Ended
	Dec. 31, 2022	Dec. 31, 2022
Business Acquisition [Line Items]
Cash and cash equivalents		$ 0
Accounts receivable and unbilled revenue		0
Other current assets		14,465
Fixed assets		(6,137)
Operating lease right-of-use assets		(11,744)
Goodwill		70,436
Intangible assets		44,000
Deferred tax assets		(147,039)
Other assets		(1,166)
Accounts payable		0
Accrued expenses and other current liabilities		(37,496)
Current portion of operating lease liabilities		1,865
Unearned revenue		19,623
Non-current portion of operating lease liabilities		10,454
Non-current deferred tax liabilities		193,837
Other non-current liabilities		$ (151,098)
Amortization adjustment	$ 2,200
Accounting Standards Update 2021-08
Business Acquisition [Line Items]
Unearned revenue	$ 16,000

Investments	12 Months Ended
Investments	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	Investments (a) Available for sale investments (short-term) As at December 31, 2023, the Company held $2.0 million of available for sale investments (December 31, 2022: $1.7 million). Short-term investments comprise highly liquid investments with maturities of greater than three months and minimum "A-" rated fixed term deposits. The Company classifies its short- term investments as available for sale. Short term investments at December 31, 2023 have an average maturity of 3.6 years compared to 2.8 years at December 31, 2022. The contractual maturity of certain investments in the portfolio is greater than 12 months; however, classification as short-term investments reflects the Company's practice and intention in respect of these investments. The Company recognizes the unrealized losses in equity as these unrealized losses on short-term investments have been considered as temporary. (b) Available for sale investments (long-term) The Company entered into subscription agreements with a number of funds. During the year ended December 31, 2023, capital totaling $14.0 million (December 31, 2022: $3.7 million) had been advanced under the terms of the subscription agreements. The Company determined that the interest in funds meets the definition of equity securities without readily determinable fair values. The Company therefore concluded that the interests held at December 31, 2023 and December 31, 2022 qualify for the NAV practical expedient in ASC 820. There was an increase in fair value of $0.2 million (December 31, 2022: $6.3 million) recognized in net income during the year bringing the carrying value of the subscriptions to $46.8 million at December 31, 2023 (December 31, 2022: $32.6 million). At December 31, 2023, the Company had committed to future investments of $66.4 million in respect of these funds. (c) Equity method investments On July 24, 2020, the Company obtained a 49% interest in the voting share capital of Oncacare in exchange for consideration of $4.9 million. At that time, the Company’s investment in Oncacare was accounted for under the equity method due to the Company's ability to exercise significant influence over Oncacare. The Company recorded its pro rata share of the earnings/losses of this investment in 'Share of losses in equity investments' in the Consolidated Statement of Operations. The Company has recorded losses of $0.4 million, $3.1 million and $2.2 million representing its pro rata share of the losses in Oncacare during the year ended December 31, 2023, December 31, 2022 and December 31, 2021, respectively. On April 20, 2023, the Company completed the purchase of the majority investor's 51% majority voting share capital of Oncacare, such that Oncacare and its subsidiaries became wholly-owned subsidiaries of the ICON Group.

Summary of Significant accounting policies

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Summary of Significant accounting policies

Summary of Significant Accounting Policies

The accounting policies noted below were applied in the preparation of the accompanying financial statements of the Company and are in conformity with accounting principles generally accepted in the United States.

Basis of consolidation

The consolidated financial statements include the financial statements of the Company and all of its subsidiaries. All significant intercompany profits, transactions and account balances have been eliminated. The results of subsidiary undertakings acquired in the period are included in the Consolidated Statement of Operations from the date of acquisition.

Use of estimates

The preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reported period. Actual results could differ from those estimates. The principal management estimates and judgments used in preparing the financial statements relate to revenue recognition, intangible assets acquired in a business combination and income taxation.

Disclosure of fair value of financial instruments

Cash, cash equivalents, other receivables, available for sale investments, accounts receivable, accounts payable, investigator payments and income taxes payable have carrying amounts that approximate fair value due to the short term maturities of these instruments. Other liabilities' carrying amounts approximate fair value based on the net present value of estimated future cash flows. Debt is measured at amortized cost.

Financial instruments are measured in the Consolidated Balance Sheet at amortized cost or fair value using a fair value hierarchy of valuation inputs. The fair value hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. These levels are:

•Level 1: Inputs are based upon unadjusted quoted prices for identical instruments traded in active markets.

•Level 2: Inputs are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

•Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability.

The Company classifies its investments in short-term debt or equity as available for sale, as it does not actively trade such securities nor does it intend to hold them to maturity. The fair value of short term investments are represented by Level 1 fair value measurements – quoted prices in active markets for identical assets. The changes in fair value are recognized in equity until disposal or sale, at which time, those unrealized movements from prior periods are recognized in the Consolidated Statement of Operations. Losses, other than temporary, which reduce the carrying amount below cost are recognized in the Consolidated Statement of Operations.

Business combinations

The cost of a business combination is measured as the aggregate of the fair value of assets received, liabilities assumed and equity instruments issued in exchange for control. The Company records and allocates to its reporting units the excess of the cost over the fair value of the net assets acquired, known as goodwill. Where a business combination agreement provides for an adjustment to the cost of the acquisition which is contingent upon future events, the amount of the estimated adjustment is recognized at the acquisition date at the fair value of the contingent consideration. Accounting for contingent consideration, after the acquisition, depends on the classification of the contingent consideration. Equity-classified contingent consideration is not remeasured after the acquisition date and subsequent settlement is accounted for within equity. Contingent consideration, which is classified as a liability or an asset, is remeasured to fair value at each reporting date until the contingency is resolved. The changes in fair value are recognized in earnings, unless the arrangement is a derivative which has been designated as a hedging instrument in a cash flow hedging relationship, in which case, the changes are initially recognized in other comprehensive income/loss.

The assets, liabilities and contingent liabilities of businesses acquired are measured at their fair values at the date of acquisition. In the case of a business combination which is completed in stages, the fair values of the identifiable assets, liabilities and contingent liabilities are determined at the date of each exchange transaction. When the initial accounting for a business combination is determined provisionally, any subsequent adjustments to the provisional values allocated to the identifiable assets, liabilities and any contingent liabilities are made within twelve months of the acquisition date and presented as adjustments to goodwill in the reporting period in which the adjustments are determined.

Foreign currencies and translation of subsidiaries

The Company's financial statements are prepared in United States dollars. The financial statements of subsidiaries with other functional currencies are translated at period end rates for the Consolidated Balance Sheets and average rates for the Consolidated Statements of Operations. Translation gains and losses arising are reported as a movement on accumulated other comprehensive income/loss.

Transactions in currencies other than the functional currency of the subsidiaries of the Company are recorded at the rate ruling at the date of the transaction. Monetary assets and liabilities denominated in currencies other than the functional currency of the subsidiaries of the Company are translated into the functional currency of that entity at exchange rates prevailing at the balance sheet date. Adjustments resulting from these translations are charged or credited to income. Foreign currency gains and losses on intercompany transactions classified as long-term investments are reported in other comprehensive income/loss as currency translation adjustments. Amounts charged or credited to the Consolidated Statements of Operations for the years ended December 31, 2023, December 31, 2022 and December 31, 2021 were as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Amounts charged/(credited)	$	12,916	$	(25,997)	$	(14,316)

Revenue recognition

The Company earns revenues by providing a number of different services to its customers. These services, which are integral elements of the clinical development process, include clinical trials management, consulting, contract staffing, data services and laboratory services. These services, which are described below, can be purchased collectively or individually as part of a clinical trial contract. There is not significant variability in how economic factors affect these services. Contracts range in duration from a number of months to several years.

ASC 606 requires application of five steps: (1) identify the contract(s) with a customer; (2) identify the performance obligation in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when (or as) the entity satisfies the performance obligation(s), which have been applied to revenue recognized from each service described below.

Clinical trial service revenue

A clinical trial service is a single performance obligation satisfied over time, i.e. the full-service obligation in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Promises offered to the customer are not distinct within the context of the contract. ICON is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support the clinical research projects. The transaction price is determined by reference to the contract or change order value (total service revenue and pass-through/ reimbursable expenses) adjusted to reflect a realizable contract value. Revenue is recognized over time as the single performance obligation is satisfied. The progress towards completion for clinical service contracts is measured based on an input measure being total project costs incurred (inclusive of pass-through/ reimbursable expenses) at each reporting period as a percentage of forecasted total project costs.

Laboratory services revenue

Revenue is recognized when, or as, obligations under the terms of a contract are satisfied, which occurs when control of the products or services are transferred to the customer. Revenue for laboratory services is measured as the amount of consideration we expect to receive in exchange for transferring products or services. Where contracts with customers contain multiple performance obligations, the transaction price is allocated to each performance obligation based on the estimated relative selling price of the promised good or service. Service revenue is recognized over time as the services are delivered to the customer based on the extent of progress towards completion of the performance obligation. The determination of the methodology to measure progress requires judgment and is based on the nature of services provided. This requires an assessment of the transfer of value to the customer. The right to invoice measure of progress is generally related to rate per unit contracts, as the extent of progress towards completion is measured based on discrete service or time-based increments, such as samples tested or labor hours incurred. Revenue is recorded in the amount invoiced since that amount corresponds to the value of the Company's performance and the transfer of value to the customer.

Contracting services revenue

The Company has availed of the practical expedient which results in recognition of revenue on a right to invoice basis. Application of the practical expedient reflects the right to consideration from the customer in an amount that corresponds directly with the value to the customer of the performance completion to date. This reflects hours performed by contract staff.

Consulting services revenue

Our consulting services contracts represent a single performance obligation satisfied over time. The transaction price is determined by reference to contract or change order value. Revenue is recognized over time as the performance obligation is satisfied. The progress towards completion for consulting contracts is measured based on total project inputs (time) at each reporting period as a percentage of forecasted total project inputs.

Data services revenue

The Company provides data reports and analytics to customers based on agreed-upon specifications, including the timing of delivery, which is typically either weekly, monthly, or quarterly. If a customer requests more than one type of data report or series of data reports within a contract, each distinct type of data report is a separate performance obligation. The contracts provide for the Company to be compensated for the value of each deliverable. The transaction price is determined using list prices, discount agreements, if any, and negotiations with the customers, and generally includes any out-of-pocket expenses. Typically, the Company bills in advance of services being provided with the amount being recorded as unearned revenue.

When multiple performance obligations exist, the transaction price is allocated to performance obligations on a relative standalone selling price basis. In cases where the Company contracts to provide a series of data reports, or in some cases data, the Company recognizes revenue over time using the “units delivered” output method as the data or reports are delivered. Expense reimbursements are recorded to revenue as the expenses are incurred as they relate directly to the services performed.

Certain arrangements include upfront customization or consultative services for customers. These arrangements often include payments based on the achievement of certain contractual milestones. Under these arrangements, the Company contracts with a customer to carry out a specific study, ultimately resulting in delivery of a custom report or data product. These arrangements are a single performance obligation given the integrated nature of the service being provided. The Company typically recognizes revenue under these contracts over time, using an output-based measure, generally time elapsed, to measure progress and transfer of control of the performance obligation to the customer. Expense reimbursements are recorded to revenue as the expenses are incurred as they relate directly to the service performed.

The Company enters into contracts with some of its larger data suppliers that involve non-monetary terms. The Company issues purchase credits to be used toward the data supplier's purchase of the Company's services based on the fair value of the data obtained. In exchange, the Company receives monetary discounts on the data received from the data suppliers. The fair value of the revenue earned from the customer purchases is recognized as services are delivered as described above.

At the end of the contract year, any unused customer purchase credits may be forfeited or carried over to the next contract year based on the terms of the data supplier contract.

Commissions

Incremental costs of obtaining a contract are recognized as an asset on the Consolidated Balance Sheet in respect of those contracts that exceed one year. Where commission costs relate to contracts that are less than one year, the practical expedient is applied as the amortization period of the asset which would arise on deferral would be one year or less.

Reimbursable expenses

Reimbursable expenses comprise investigator payments and certain other costs which are reimbursed by clients under terms specific to each contract to the investigators. The Company includes reimbursed expenses in revenue and direct costs as the Company is primarily responsible for fulfilling the promise to provide the specified service, including integration of the related services into a combined output to the customer.

Direct costs

Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs.

Reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred. Payments are made based on predetermined contractual arrangements. Timing of payments may differ from the timing of the expense.

Cash and cash equivalents

Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value.

Investments in debt, equity and other

Available for sale investments

The Company classifies short-term investments as available for sale. The investments are reported at fair value, with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of the investments are represented by accrued interest and unrealized gains/losses. Realized gains and losses are determined using specific identification.

Long term investments

The Company classifies its interests in funds having considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2023 and December 31, 2022 meet the definition of equity securities without readily determinable fair values. The Company concluded that the interests held at December 31, 2023 and December 31, 2022 qualify for the Net Asset Value ("NAV") practical expedient in ASC Topic 820, Fair value Measurements ("ASC 820"). Any increases or decreases in fair value are recognized in earnings in the period.

Equity method investments

The Company’s investments that are not consolidated are accounted for under the equity method if the Company exercises significant influence that is considered to be greater than minor. These investments are classified as equity method investments on the Consolidated Balance Sheet. The Company records its pro rata share of the earnings/losses of these investments in Share of losses in equity investments in the Consolidated Statement of Operations. The Company reviews equity method investments for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable.

Accounts receivable, net and unbilled revenue

Accounts receivable and unbilled revenue are recorded at fair value less an estimate of the credit losses expected to be incurred on the Company's accounts receivable portfolio. The Company's estimate of expected credit losses considers historical credit loss information that is adjusted, where necessary, for current conditions and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. The Company's receivables and unbilled services are predominantly due from large and mid-tier pharmaceutical and biotechnology companies that share similar risk characteristics. The Company monitors their portfolio of receivables and unbilled services for any deterioration in current or expected credit quality (for example, expected delinquency level), and adjusts the allowance for credit losses as required.

Changes in the allowance for credit losses are recorded as a provision for (or reversal of) credit loss expense in the Consolidated Statement of Operations. Losses are charged against the allowance when management believes the uncollectibility of a previously provisioned amount is confirmed.

Accounts receivable early payment discounting

Where the Company enters into an agreement to sell certain portfolios of its accounts receivable balances, the sale is accounted for in accordance with ASC Topic 860, Transfers and Servicing ("ASC 860"). Agreements which result in true sales of the transferred receivables, as defined in ASC 860, are excluded from amounts reported in the Consolidated Balance Sheet. Cash proceeds received from such sales are included in operating cash flows.

Property, plant and equipment

Property, plant and equipment is stated at cost less accumulated depreciation. Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below:


			Estimated Useful Life
Buildings			40 years
Computer equipment and software			2 - 8 years
Office furniture and fixtures			8 years
Laboratory equipment			5 years
Motor vehicles			5 years

Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter.

Leases

The Company determines if an arrangement is a lease at inception and reassess if there are changes in terms and conditions of the contract. Finance leases, if any, are depreciated on the same basis as property, plant and equipment. At December 31, 2023 and December 31, 2022, the Company did not account for any leases as finance leases.

Operating leases are included in operating right-of-use ("ROU") assets, other liabilities and non-current operating lease liabilities on our Consolidated Balance Sheet with the lease charge recognized on a straight-line basis over the lease term. ROU assets and lease liabilities are recognized based on the present value of future minimum lease payments over the lease term at the lease commencement date. Our lease terms may also include options to extend or terminate. The Company actively reviews options to extend or terminate leases and adjusts the ROU asset and lease liability when it is reasonably certain the option will be exercised. The ROU asset is adjusted for any prepayments made at the date of commencement and any initial direct costs incurred. As most of the Company's leases do not provide an implicit rate, the discount rate used is based on the rate of traded corporate bonds available at the commencement date adjusted for country risk, liquidity and lease term.

The Company accounts for lease and non-lease components separately with lease components flowing through the Consolidated Balance Sheet and non-lease components expensed directly to the Consolidated Statements of Operations.

Leasehold improvements are amortized over the shorter of the depreciable lives of the corresponding fixed assets or the lease term including any applicable renewals. Certain property leases include variable lease payments resulting from periodic rent increases based on an index which cannot be reasonably estimated at the lease commencement date. These costs are expensed as incurred on the Consolidated Statements of Operations.

In some cases, the Company enters into sublease agreements and becomes both a lessee and a lessor for the same underlying asset. Subleases are accounted for as operating leases separately from the lease they relate to, but similar in manner as all other leases.

ROU assets for operating leases are occasionally reduced by impairment losses. The Company uses the long-lived assets impairment guidance in ASC Subtopic 360-10, Property, Plant, and Equipment – Overall, to determine whether an ROU asset is impaired, and if so, the amount of the impairment loss to recognize.

Intangible assets

Intangible assets are measured at fair value at the date of acquisition and amortized on a straight-line basis over their respective estimated useful lives. The Company has no indefinite-lived intangible assets. The Company evaluates its intangible assets for impairment when indicators of impairment exist.

Intangible assets are amortized on a straight-line basis over their estimated useful lives, as set forth in the table below:


			Estimated Useful Life
Customer relationships			16 - 23 years
Order backlog			3 years
Trade names			3 years
Patient database			7 years
Technology assets			5 years

The Company periodically assesses the estimated useful lives of intangible assets to evaluate whether what was established at acquisition continues to be appropriate.

Impairment of goodwill and long-lived assets

Goodwill is tested for impairment annually, or more frequently, if an event or circumstance indicates that an impairment loss may have been incurred. The annual impairment test for goodwill includes an option to perform a qualitative assessment of whether it is more likely than not that a reporting unit's fair value is less than its carrying value. Reporting units are businesses with discrete financial information that is available and reviewed by management. If the Company determines that it is more likely than not that the fair value of a reporting unit is less than its carrying value, then the Company performs the quantitative goodwill impairment test. The Company may also chose to bypass the qualitative assessment for any reporting unit in its goodwill assessment and proceed directly to performing the quantitative assessment. The Company recognizes an impairment charge for the amount by which the reporting unit's carrying amount exceeds its fair value.

Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured at the amount by which the carrying amount of the asset exceeds the fair value of the asset. Fair value is determined through various valuation techniques including discounted cash flow models and third-party independent appraisals, as considered necessary. Assets to be disposed of are reported at the lower of the carrying amount of the asset or fair value less selling costs.

Debt issuance costs

Debt issuance costs relating to the Company’s long-term debt are recorded as a direct reduction of long-term debt; these costs are deferred and amortized to interest expense using the effective interest method, over the respective terms of the related debt. Debt issuance costs relating to the Company’s revolving credit facilities are recorded as an asset; these costs are deferred and amortized to interest expense using the straight-line method. Early repayment of debt facilities can result in modification of the debt and the acceleration of the amortization of debt issuance costs.

Derivative financial instruments

The Company uses derivative financial instruments to reduce exposures to interest rates. Derivatives are recorded on the Consolidated Balance Sheet at fair value at each balance sheet date utilizing pricing models for non-exchange-traded contracts.

Our accounting policies for derivative financial instruments are based on whether they meet the criteria for designation as cash flow or fair value hedges. A designated hedge of the exposure to variability in the future cash flows of an asset or a liability, or of a forecast transaction, is referred to as a cash flow hedge. A designated hedge of the exposure to changes in fair value of an asset or a liability is referred to as a fair value hedge. The criteria for designating a derivative as a hedge include the assessment of the instrument's effectiveness in risk reduction, matching of the derivative instrument to its underlying transaction and the probability that the underlying transaction will occur. For derivatives with cash flow hedge accounting designation, we report the gain or loss from the effective portion of the hedge as a component of Other Comprehensive Income and reclassify it

into earnings in the same period or periods in which the hedged transaction affects earnings and within the same Consolidated Statement of Operations line item as the impact of the hedged transaction. For derivatives with fair value hedge accounting designation, we recognize gains or losses from the change in fair value of these derivatives, as well as the offsetting change in the fair value of the underlying hedged item, in earnings. Fair value gains and losses arising on derivative financial instruments not qualifying for hedge accounting are reported in our Consolidated Statement of Operations.

Income taxes

The Company applies the asset and liability method of accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amount of existing assets and liabilities and their respective tax bases and for operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which these temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the provision of income taxes in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance to the amount that is more likely than not to be realized. The Company recognizes the effect of income tax positions only if those positions will more likely than not be sustained. Recognized income tax positions are measured at the largest amount of tax benefit that is greater than 50 percent likely of being realized upon settlement. Interest and penalties related to income taxes are included in income tax expense and classified with the related liability on the Consolidated Balance Sheet. The Company accounts for the impact of Global Intangible Low-Taxed Income ("GILTI") in the period it arises and has therefore not provided for deferred taxes in respect of this item.

Government grants

Government grants received relating to capital expenditures are shown by deducting the grant from the asset's carrying amount and crediting them to income on a basis consistent with the depreciation policy of the relevant assets. Grants relating to categories of operating expenditures are shown as deferred income and credited to income in the period in which the expenditure to which they relate is charged.

Under the grant agreements, amounts received may become repayable in full should certain circumstances specified within the grant agreements occur, including downsizing by the Company, disposing of the related assets, ceasing to carry on its business or the appointment of a receiver over any of its assets. The Company has not recognized any loss contingency having assessed as remote the likelihood of these events arising.

Research and development credits

Research and development credits are available to the Company under the tax laws in certain jurisdictions, based on qualifying research and development spend as defined under those tax laws. Research and development credits may be recognized as a reduction of income tax expense. However, certain tax jurisdictions provide refundable credits that are not wholly dependent on the Company's ongoing income tax status or income tax position. In these circumstances the benefit of these credits is not recorded as a reduction to income tax expense, but rather as a reduction of operating expenditure.

Pension costs

The Company contributes to defined contribution plans covering all eligible employees. The Company contributes to these plans based upon various fixed percentages of employee compensation and such contributions are expensed as incurred.

The Company operates, through certain subsidiaries, a defined benefit plan for certain employees located in the United Kingdom and Switzerland. The Company accounts for these plans in accordance with ASC Subtopic 715-30, Defined Benefit Plans – Pension. The Company also maintains various retirement plans across the Group, many of which are required by local employment laws.

Share-based compensation

The Company accounts for its share options, Restricted Share Units ("RSUs") and Performance Share Units ("PSUs") in accordance with the provisions of ASC Topic 718, Compensation – Stock Compensation ("ASC 718").

Share-based compensation expense for share options awarded to employees and directors is estimated at the grant date based on each option's fair value as calculated using the Black-Scholes option-pricing model. Share-based compensation for RSUs and PSUs awarded to employees and directors is calculated based on the market value of the Company's shares on the date of award of the RSUs and PSUs. The value of awards expected to vest is recognized as an expense over the requisite service periods. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates.

Estimating the grant date fair value of share options as of the grant date using an option-pricing model, such as the Black-Scholes model, is affected by the Company's share price as well as assumptions regarding a number of complex

variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards.

Liability classified awards are measured at the fair value of the award on the grant date and remeasured at each reporting period at fair value until the award is settled.

Replacement awards

In connection with the completion of the Merger, the company issued replacement awards to the holders of PRA equity awards on July 1, 2021. An exchange of share-based compensation awards in a business combination is treated as a modification under ASC 718. The replacement awards and the original acquiree awards are measured at fair value at the acquisition date and calculated using the fair-value-based measurement principles in ASC 718. Amounts attributable to pre-combination vesting are accounted for as part of the consideration transferred for the acquiree. Amounts attributable to post-combination vesting are accounted for separate from the business combination and are recognized as compensation cost in the post-combination period.

Transaction and integration-related expenses

Transaction and integration-related expenses are the incremental costs directly attributable to completion and integration activities associated with the Company’s recent acquisitions. The costs consist of investment banking fees, advisory costs, professional fees, retention agreements with employees, accelerated share-based compensation charges and ongoing integration activities. The Company accounts for these transaction and integration-related costs as expenses in the period in which the costs are incurred and the services are received.

Restructuring

Restructuring charges reflect certain one-time costs arising from reorganization programs announced by Company management. These programs generally result in asset impairments and workforce reductions in order to optimize the Company’s structure and facilitate improved long-term performance. Asset related impairment charges are taken when the fair value is less than the asset’s carrying value. Workforce related charges are taken when an approved reorganization program is communicated to the relevant employee groups.

Net income per ordinary share

Basic net income per ordinary share attributable to the Company has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares. There is no difference in net income used for basic and diluted net income per ordinary share.

Impact of Recently Issued or Adopted Accounting Standards

Accounting pronouncements, effective for the year ended December 31, 2023, did not have any significant impact on the Company’s consolidated financial statements.

The following accounting pronouncements have been issued, but not yet adopted, as of December 31, 2023:

In November 2023, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2023-07, Segment Reporting – Improvements to Reportable Segment Disclosures (“ASU 2023-07”), which amends the guidance in ASC Topic 280, Segment Reporting. The amended guidance requires disclosure of significant segment expenses and other segment items on an annual and interim basis. Furthermore, all disclosures about a reportable segment’s profit or loss, which are currently required annually, will be mandated for interim periods. The additional disclosures required by ASU 2023-07 apply to entities with a single reportable segment. The amended guidance is effective for fiscal periods beginning after December 15, 2023, and for interim periods within fiscal years beginning after December 15, 2024, with retrospective application required. The Company is currently evaluating the impact of this ASU on disclosures within its consolidated financial statements.

In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures, which requires disaggregated information about an entity’s effective tax rate reconciliation and additional discloses on income taxes paid. The new requirements are effective for annual periods beginning after December 15, 2024. The guidance is to be applied prospectively, with an option for retrospective application. The Company is currently evaluating the impact of this new guidance on disclosures within its consolidated financial statements.

Operating Leases

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

Operating Leases

Lease costs recorded under operating leases for the years ended December 31, 2023, 2022 and 2021 were as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)

Operating lease costs	$	46,820	$	53,880	$	51,200
Income from sub-leases	(1,103)		(1,165)		(1,338)
Net operating lease costs	$	45,717	$	52,715	$	49,862

Of the total cost of $45.7 million incurred in the year ended December 31, 2023, $37.9 million is recorded within selling, general and administration costs and $7.8 million is recorded within direct costs. Of the total cost of $52.7 million incurred in the year ended December 31, 2022, $48.3 million is recorded within selling, general and administration costs and $4.4 million is recorded within direct costs. Of the total cost of $49.9 million incurred in the year ended December 31, 2021, $47.5 million is recorded within selling, general and administration costs and $2.4 million is recorded within direct costs.

Right-of-use assets obtained, in exchange for lease obligations, during the years ended December 31, 2023 and December 31, 2022 totaled $37.7 million and $28.7 million, respectively.

During the years ended December 31, 2023, 2022 and 2021, impairments of operating right-of-use assets were recognized within restructuring charges for $8.7 million, $24.8 million, and $15.4 million, respectively, as part of an office consolidation program (see note 19 - Restructuring Charges).

The weighted average remaining lease term and weighted-average discount rate at December 31, 2023 were 6.72 years and 3.29%, respectively. The weighted average remaining lease term and weighted-average discount rate at December 31, 2022 were 6.90 years and 2.45%, respectively.

Future minimum lease payments under non-cancelable leases as of December 31, 2023 were as follows:


	Minimum rental payments
	(in thousands)
2024	$	40,894
2025	34,585
2026	29,172
2027	23,254
2028	15,023
Thereafter	33,998
Total future minimum lease payments	176,926
Lease imputed interest	(14,191)
Total	$	162,735

Operating lease liabilities are presented as current and non-current. Operating lease liabilities of $36.4 million and $43.7 million have been included in other liabilities as at December 31, 2023 and December 31, 2022, respectively.

Bank Credit Lines and Loan Facilities - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	May 02, 2023	Jul. 01, 2021
Debt Instrument [Line Items]
Total financing costs					$ 336,699	$ 229,731	$ 182,423
Drawdown of credit lines and facilities					370,000	75,000	5,905,100
Closing Balance	$ 55,000				55,000	0
Repayment					315,000	75,000
Long-term debt at fair value	3,793,500				3,793,500	$ 4,650,300
Senior Secured Revolving Loan Facility
Debt Instrument [Line Items]
Closing Balance	$ 445,000				$ 445,000
Senior Secured Term Loan \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					0.50%
Senior Secured Term Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					2.25%
Senior secured revolving loan facility \| Variable rate component one \| Base rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					0.60%
Senior secured revolving loan facility \| Variable rate component one \| Base rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					1.00%
Senior secured revolving loan facility \| Variable rate component one \| Base rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					0.25%
Senior secured revolving loan facility \| Variable rate component two \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					1.60%
Senior secured revolving loan facility \| Variable rate component two \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					2.00%
Senior secured revolving loan facility \| Variable rate component two \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)					1.25%
Senior Secured Term Loan \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt instrument, face amount									$ 5,515,000
Debt instrument, interest rate (in percent)	7.86%				7.86%	7.092%			1.00%
Accelerated charge on repayments	$ 7,900				$ 7,900	$ 7,800
Senior Secured Term Loan \| Senior secured credit facility and senior secured notes
Debt Instrument [Line Items]
Debt discount paid									$ 27,600
Line of credit \| Senior secured revolving loan facility \| Senior Secured Revolving Loan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 500,000	300,000
Drawdown of credit lines and facilities	$ 65,000	$ 75,000	$ 50,000	$ 180,000	$ 370,000	$ 0
Senior Secured Notes \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, face amount									$ 500,000
Debt instrument, interest rate (in percent)	2.875%				2.875%	2.875%			2.875%
PRA Health Sciences, Inc
Debt Instrument [Line Items]
Total financing costs					$ 16,400	$ 17,700	$ 86,700

Employee Benefits - Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
United Kingdom \| ICON Development Solutions Limited pension plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 26,050	$ 36,198
Expected return on plan assets	1,015	1,258	$ 1,171
Actuarial loss	(910)	(7,305)
Employer contributions	70	70
Plan participants' contributions	19	19
Benefits paid	(318)	(514)
Foreign currency exchange rate changes	1,394	(3,676)
Fair value of plan assets at end of year	27,320	26,050	36,198
Switzerland \| Aptiv Solutions pension plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	5,681	6,964
Expected return on plan assets	133	29
Actuarial loss	356	(987)
Scheme contributions	215	114
Plan participants' contributions	93	82
Benefits paid and transferred balances	134	182
Settlement	(645)	(218)
Foreign currency exchange rate changes	562	(121)
Fair value of plan assets at end of year	6,261	5,681	6,964
Switzerland \| PRA Switzerland AG pension plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,059	3,017
Expected return on plan assets	101	29
Actuarial loss	(52)	87
Scheme contributions	456	325
Plan participants' contributions	456	325
Benefits paid and transferred balances	795	1,125
Settlement	(753)	(844)
Foreign currency exchange rate changes	467	(5)
Fair value of plan assets at end of year	$ 5,529	$ 4,059	$ 3,017

Employee Benefits - Expected Rate of Return and Actual Plan Asset Allocation (Details) - ICON Development Solutions Limited pension plan	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	100.00%	100.00%
Government Bonds
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	78.00%	88.00%
Diversified Bonds
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	22.00%	12.00%

Employee Benefits - Schedule of Amounts Recognized in Balance Sheet (Details) - Pension Plan - United States - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Other receivables	$ 6,321	$ 6,492
Non-current other liabilities (note 16)	$ (14,738)	$ (13,033)

- Definition

Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

+ References + Details

Equity Incentive Schemes and Stock Compensation Charges (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Schedule of Transactions for Company's Share Option Plans

The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2023, December 31, 2022 and December 31, 2021:


	Options Granted Under Plans	Weighted Average Exercise Price
Outstanding at December 31, 2020	553,746	$	108.53
Assumed through business combinations *	2,177,130	$	108.78
Granted	100,299	$	177.76
Exercised	(1,065,529)	$	111.29
Canceled	(70,186)	$	128.46
Outstanding at December 31, 2021	1,695,460	$	110.38
Granted	108,643	$	229.94
Exercised	(348,286)	$	102.87
Canceled/expired	(77,698)	$	143.08
Outstanding at December 31, 2022	1,378,119	$	119.86
Granted	82,472	$	232.48
Exercised	(535,705)	$	95.12
Canceled/expired	(22,080)	$	196.20
Outstanding at December 31, 2023	902,806	$	142.96
Vested and exercisable at December 31, 2023	654,386	$	119.67

*Represents stock options issued as replacement awards in connection with the Merger.

Summary of Information Concerning Outstanding and Exercisable Share Options

The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2023:


Options Outstanding					Options Exercisable
Range Exercise Price	Number of Shares	Weighted Average Remaining Contractual Life	Weighted Average Exercise Price		Number of Shares	Weighted Average Exercise Price
$20.83 - 96.15	201,306	2.29	—		201,306	—
$103.81 - 121.68	132,964	4.54	—		132,964	—
$125.74 - 147.26	249,194	4.45	—		233,371	—
$159.33 - 233.88	319,342	5.69	—		86,745	—

$20.83 - 233.88	902,806	4.42	$	142.96	654,386	$	119.67

Summary of Weighted Average Fair Values and Assumptions Used

The weighted average fair values and assumptions were as follows:


	Year Ended
	December 31, 2023			December 31, 2022			December 31, 2021
Weighted average fair value	$	85.12		$	68.42		$	49.15
Assumptions:
Expected volatility	33		%	31		%	30		%
Dividend yield	—		%	—		%	—		%
Risk-free interest rate	4.18		%	1.86		%	0.78		%
Expected life	5.0 years			5.0 years			5.0 years

The weighted average fair value of options assumed on the date of the Merger was calculated using the Black-Scholes option pricing model. The weighted average fair values on the date of the Merger and assumptions used were as follows:


	July 1, 2021
Weighted average grant date fair value	$	107.21
Assumptions:
Expected volatility	30		%
Dividend yield	—		%
Risk-free interest rate	0.56		%
Expected life	3.5 years

Summary of RSU and PSU Activity

The following table summarizes RSU and PSU activity for the year ended December 31, 2023:


	PSU Outstanding Number of Shares	PSU Weighted Average Grant Date Fair Value		RSU Outstanding Number of Shares	RSU Weighted Average Grant Date Fair Value
Outstanding at December 31, 2022	152,420	$	192.29	582,612	$	207.73

Granted	60,374	$	232.51	308,963	$	218.86
Shares vested	(47,026)	$	159.57	(188,800)	$	187.68
Forfeited	(60,512)	$	198.70	(81,764)	$	216.03
Outstanding at December 31, 2023	105,256	$	226.29	621,011	$	218.27

Schedule of Non-cash Stock Compensation Expense

Stock compensation expense has been allocated as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Direct costs	$	26,595	$	22,854	$	18,551
Selling, general and administrative	29,072		47,669		41,457
Transaction and integration related *	—		—		73,836
Total compensation costs	$	55,667	$	70,523	$	133,844

* Represents the post combination portion of the accelerated vesting of awards following the completion of the Merger

Disaggregation of Revenue (Tables)

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

Schedule of Disaggregation of Revenue

Revenue disaggregated by customer profile is as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Top client	$	721,309	$	683,546	$	441,173
Clients 2-5	1,453,508		1,506,087		1,291,946
Clients 6-10	1,188,943		1,112,636		752,325
Clients 11-25	1,743,539		1,585,739		1,077,073
Other	3,012,877		2,853,378		1,918,309
Total	$8,120,176		$7,741,386		$5,480,826

Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue	$ 8,120,176	$ 7,741,386	$ 5,480,826
Top client
Disaggregation of Revenue [Line Items]
Revenue	721,309	683,546	441,173
Clients 2-5
Disaggregation of Revenue [Line Items]
Revenue	1,453,508	1,506,087	1,291,946
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue	1,188,943	1,112,636	752,325
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue	1,743,539	1,585,739	1,077,073
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 3,012,877	$ 2,853,378	$ 1,918,309

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479941/924-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-30

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-40

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-41

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-4

+ Details

Business Segment and Geographical Information - Distribution of Net Revenue by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenue	$ 8,120,176	$ 7,741,386	$ 5,480,826
Ireland
Segment Reporting Information [Line Items]
Revenue	2,377,104	1,984,567	1,365,909
Rest of Europe
Segment Reporting Information [Line Items]
Revenue	1,574,783	1,618,350	1,175,515
U.S.
Segment Reporting Information [Line Items]
Revenue	3,283,790	3,574,610	2,581,007
Other
Segment Reporting Information [Line Items]
Revenue	$ 884,499	$ 563,859	$ 358,395

- Definition

+ References

+ Details

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
Revenue	$ 8,120,176	$ 7,741,386	$ 5,480,826
Costs and expenses:
Direct costs (excluding depreciation and amortization)	5,719,949	5,527,045	3,972,612
Selling, general and administrative	768,559	778,753	585,330
Depreciation and amortization	585,950	569,513	314,987
Transaction and integration related (Note 6)	44,176	39,695	198,263
Restructuring (Note 19)	45,390	31,143	31,105
Total costs and expenses	7,164,024	6,946,149	5,102,297
Income from operations	956,152	795,237	378,529
Interest income	5,014	2,345	574
Interest expense (Note 13)	(336,699)	(229,731)	(182,423)
Income before income tax expense	624,467	567,851	196,680
Income tax expense (Note 20)	(11,749)	(59,411)	(41,334)
Income before share of losses from equity method investments	612,718	508,440	155,346
Share of losses from equity method investments	(383)	(3,136)	(2,161)
Net income	$ 612,335	$ 505,304	$ 153,185
Net income per ordinary share (Note 24):
Basic (USD per share)	$ 7.46	$ 6.20	$ 2.28
Diluted (USD per share)	$ 7.40	$ 6.13	$ 2.25
Weighted average number of ordinary shares outstanding (Note 24):
Basic (in shares)	82,101,813	81,532,320	67,110,186
Diluted (in shares)	82,717,640	82,468,363	68,068,311

Operating Leases Operating Leases - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	$ 45,717	$ 52,715	$ 49,862
Right-of-use assets obtained in exchange for lease obligations	37,700	28,700
Impairment of long lived assets	$ 8,700	$ 24,800	15,400
Weighted average remaining lease term	6 years 8 months 19 days	6 years 10 months 24 days
Weighted average discount rate (in percent)	3.29%	2.45%
Operating lease liabilities	$ 36,414	$ 43,657
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other liabilities (Note 12)	Other liabilities (Note 12)
Selling, general and administrative
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	$ 37,900	$ 48,300	47,500
Cost of Sales
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	$ 7,800	$ 4,400	$ 2,400

Employee Benefits - Net Periodic Pension Benefit Cost Assumptions (Details) - ICON Development Solutions Limited pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Expected rate of return on plan assets	3.80%	3.80%
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.90%	1.80%	1.50%
Rate of compensation increase	3.60%	3.70%	3.40%
Expected rate of return on plan assets	3.80%	3.80%	3.40%
Other comprehensive income/(loss)
Actuarial gain - benefit obligation	$ (335)	$ (18,636)	$ (2,097)
Actuarial loss/(gain) - plan assets	910	7,305	(1,176)
Amortization of net gain/(loss) recognized in net periodic benefit (credit)/cost	274	(228)	(625)
Total	$ 849	$ (11,559)	$ (3,898)

Income Taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Current tax (benefit)/expense:
Current tax expense	$ 97,152	$ 184,396	$ 101,950
Deferred tax (benefit)/expense:
Deferred tax (benefit)/expense	(85,403)	(124,985)	(60,616)
Income tax expense allocated to continuing operations	11,749	59,411	41,334
Income tax expense was allocated to the following components of other comprehensive income:
Currency impact on long term funding	(3,903)	7,211	1,776
Fair value of cash flow hedge	301	0	0
Total	8,147	66,622	43,110
Ireland
Current tax (benefit)/expense:
Current tax expense	80,427	53,248	18,469
Deferred tax (benefit)/expense:
Deferred tax (benefit)/expense	45,253	(6,166)	553
United States
Current tax (benefit)/expense:
Current tax expense	(42,428)	60,753	35,478
Deferred tax (benefit)/expense:
Deferred tax (benefit)/expense	(143,323)	(118,475)	(52,717)
Other
Current tax (benefit)/expense:
Current tax expense	59,153	70,395	48,003
Deferred tax (benefit)/expense:
Deferred tax (benefit)/expense	$ 12,667	$ (344)	$ (8,452)

Net Income Per Ordinary Share

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Net Income Per Ordinary Share

Basic net income per ordinary share has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period.

Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares.

There is no difference in net income used for basic and diluted net income per ordinary share.

The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows:


	Year ended
	December 31, 2023	December 31, 2022	December 31, 2021
Weighted average number of ordinary shares outstanding for basic net income per ordinary share	82,101,813	81,532,320	67,110,186
Effect of dilutive share options and other awards outstanding under share based compensation programs	615,827	936,043	958,125
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share	82,717,640	82,468,363	68,068,311








	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
Net income per ordinary share:
Basic	$	7.46	$	6.20	$	2.28
Diluted	$	7.40	$	6.13	$	2.25

Share Capital (Details) $ / shares in Units, $ in Thousands				12 Months Ended
Share Capital (Details) $ / shares in Units, $ in Thousands	Feb. 18, 2022 USD ($) shares	Jul. 01, 2021 shares	Jul. 22, 2016	Dec. 31, 2023 USD ($) vote $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares
Equity, Class of Treasury Stock [Line Items]
Number of votes \| vote				1
Total proceeds from exercise of stock options by employees \| $				$ 50,973	$ 35,844	$ 118,589
Ordinary shares redeemed, value \| $					$ 99,983
Buyback program
Equity, Class of Treasury Stock [Line Items]
Ordinary shares redeemed (in shares)	420,530
Ordinary shares redeemed, value \| $	$ 100,000
Maximum \| Buyback program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized percentage (in percent)			10.00%
Employee Stock Option
Equity, Class of Treasury Stock [Line Items]
Options exercised by employees (in shares)				535,705	348,286	1,065,529
Average exercise price of option per share (USD per share) \| $ / shares				$ 95.12	$ 102.87	$ 111.29
Total proceeds from exercise of stock options by employees \| $				$ 51,000	$ 35,800	$ 118,600
Restricted Stock Units (RSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain RSUs previously awarded by the Company (in shares)				188,800	195,029	446,404
Performance Share Unit (PSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain PSUs previously awarded by the Company (in shares)				47,026	46,087	44,132
Ordinary Shares
Equity, Class of Treasury Stock [Line Items]
Options exercised by employees (in shares)				535,705	348,286	1,065,529
Issue of shares associated with a business combination (in shares)		27,372,427				27,372,427
Ordinary shares redeemed (in shares)					420,530
Ordinary shares redeemed, value \| $					$ 28

Contract Balances (Tables)

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

Schedule of Contracts with Customers, Asset and Liabilities

Accounts receivable and unbilled revenue are as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Billed services (accounts receivable)	$	1,821,855	$	1,751,950
Allowance for credit losses (note 5)	(31,533)		(20,562)
Accounts receivable (net)	1,790,322		1,731,388
Unbilled services (unbilled revenue)	951,936		957,655
Accounts receivable and unbilled revenue, net	$	2,742,258	$	2,689,043

Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows:


(in thousands, except percentages)	December 31, 2023		December 31, 2022		$ Change		% Change

Unbilled services (unbilled revenue)	$	951,936	$	957,655	$	(5,719)	(0.6)	%
Unearned revenue (payments on account)	(1,654,507)		(1,507,449)		(147,058)		9.8	%
Net balance	$	(702,571)	$	(549,794)	$	(152,777)	27.8	%

Derivatives - Narrative (Details) $ in Thousands	12 Months Ended
Derivatives - Narrative (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Nov. 29, 2022 USD ($) instrument
Derivative [Line Items]
Derivative Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other liabilities (Note 12)	Other liabilities (Note 12)
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other receivables
Derivative Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Non-current other liabilities (Note 16)	Non-current other liabilities (Note 16)
Gain/(loss) on cash flow hedge	$ (1,567)	$ 3,728	$ (778)
Other comprehensive income (loss), cash flow hedge, gain (loss), reclassification, before tax	2,400	100
Interest rate cash flow hedge gain (loss) to be reclassified euring next 12 months, net	3,200
Interest Rate Caps \| Total derivatives designated as hedging instruments
Derivative [Line Items]
Number of interest rate derivatives held \| instrument				2
Derivative, notional amount				$ 2,101,000
Derivative, fixed interest rate				0.42%
Interest Rate Caps \| Total derivatives designated as hedging instruments \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Derivative [Line Items]
Derivative, variable interest rate, reference rate in excess of				4.75%
Interest Rate Swap
Derivative [Line Items]
Derivative, notional amount				$ 1,101,000
Derivative, fixed interest rate				3.40%
Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments \| Level 2
Derivative [Line Items]
Derivative liability, current	1,900	3,300
Derivative asset, current		10
Derivative liability, noncurrent	$ 500	$ 400

Net Income Per Ordinary Share - Reconciliation of Net Income Attributable to the Group Per the Statement of Operating Income and Net Income Used For Net Income Per Ordinary Share (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net income per ordinary share (Note 24):
Basic (USD per share)	$ 7.46	$ 6.20	$ 2.28
Diluted (USD per share)	$ 7.40	$ 6.13	$ 2.25

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
-SubTopic 10
-Name Accounting Standards Codification
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-URI https://asc.fasb.org//1943274/2147483443/250-10-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 250
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-Paragraph 3
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-URI https://asc.fasb.org//1943274/2147483443/250-10-50-3

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+ Details

Property, Plant and Equipment, net (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Land	$ 3,724	$ 3,724
Buildings	70,072	70,880
Computer equipment and software	550,119	500,135
Office furniture and fixtures	45,856	50,600
Laboratory equipment	56,217	59,946
Leasehold improvements	55,000	65,167
Motor vehicles	79	41
Property, plant and equipment, gross	781,067	750,493
Less accumulated depreciation and asset write offs	(419,883)	(400,173)
Property, plant and equipment (net)	$ 361,184	$ 350,320

Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
United Kingdom \| ICON Development Solutions Limited pension plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	$ 19,558	$ 41,813
Service cost	42	117	$ 134
Interest cost	975	672	665
Plan participants' contributions	19	19
Benefits paid	(318)	(514)
Actuarial gain	(335)	(18,636)
Foreign currency exchange rate changes	1,058	(3,913)
Benefit obligation at end of year	20,999	19,558	41,813
Switzerland \| Aptiv Solutions pension plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,806	7,643
Service cost	109	146
Interest cost	136	30
Plan participants' contributions	93	82
Settlement	(645)	(218)
Prior service cost	(13)	(23)
Benefits paid and transferred balances	(134)	(182)
Actuarial gain	511	(1,527)
Foreign currency exchange rate changes	578	(145)
Benefit obligation at end of year	6,441	5,806	7,643
Switzerland \| PRA Switzerland AG pension plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,345	4,990
Service cost	417	404
Interest cost	132	20
Plan participants' contributions	456	325
Settlement	(753)	(844)
Benefits paid and transferred balances	795	1,125
Actuarial gain	711	(627)
Foreign currency exchange rate changes	644	(48)
Benefit obligation at end of year	$ 7,747	$ 5,345	$ 4,990

Bank Credit Lines and Loan Facilities (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of Long-term Debt Instruments

The Company had the following debt outstanding as of December 31, 2023 and 2022:


		Interest rate as of				Principal amount as of
	Maturity Date	December 31, 2023		December 31, 2022		December 31, 2023		December 31, 2022
						(in thousands)
Senior Secured Term Loan	July 2028	7.860	%	7.092	%	$	3,251,213	$	4,201,213
Senior Secured Notes	July 2026	2.875	%	2.875	%	500,000		500,000
Senior Secured Revolving Loan Facility	January 2024	6.720	%	—		55,000		—



Total debt						3,806,213		4,701,213

Less current portion of debt						(110,150)		(55,150)
Total long-term debt						3,696,063		4,646,063
Less debt issuance costs and debt discount						(30,624)		(47,026)
Total long-term debt, net						$	3,665,439	$	4,599,037


	Drawdown		Repayment		Closing Balance
	(in thousands)
December 31, 2022	$	—	$	—	$	—
Quarter 1, 2023	180,000		100,000		80,000
Quarter 2, 2023	50,000		80,000		50,000
Quarter 3, 2023	75,000		50,000		75,000
Quarter 4, 2023	65,000		85,000		55,000
	$	370,000	$	315,000

Schedule of Financing Cost


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Interest expense on drawn facilities	$	311,019	$	209,189	$	93,809
Amortization of merger related financing fees	16,402		17,749		12,890
Transaction and one time financing costs	—		—		75,391
Other financing costs*	9,278		2,793		333
Total financing costs	$	336,699	$	229,731	$	182,423

*Includes costs associated with the senior secured revolving loan facility.

Schedule of Maturities of Long-term Debt

As of December 31, 2023, the contractual maturities of the Company's debt obligations were as follows:


Current maturities of debt:	(in thousands)
2024	$	110,150
2025	55,150
2026	555,150
2027	55,150
2028 and thereafter	3,030,613
Total	$	3,806,213

Schedule of Principal Payments on Debt Instrument

Principal repayments, comprising mandatory and voluntary repayments, during the year ended December 31, 2023 and December 31, 2022 were as follows:


Principal repayments	December 31, 2023		December 31, 2022
	(in thousands)
Quarter 1	$	250,000	$	300,000
Quarter 2	150,000		100,000
Quarter 3	300,000		200,000
Quarter 4	250,000		200,000
Total	$	950,000	$	800,000

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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+ Details

Equity Incentive Schemes and Stock Compensation Charges - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended			24 Months Ended	252 Months Ended
	Feb. 14, 2017	May 11, 2015	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017	May 18, 2020	May 16, 2019	May 31, 2018	Feb. 13, 2017
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Weighted average contractual term of options outstanding			4 years 5 months 1 day	4 years 8 months 8 days
Exercisable - weighted average remaining contractual life			3 years 10 months 20 days	4 years 3 months
Total compensation costs			$ 55,667	$ 70,523	$ 133,844
Income tax benefit related to stock compensation			20,000	12,900	22,700
Cash tax benefit related to stock options exercised			$ 10,900	7,700	$ 23,900
The 2020 Legacy PRA Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)								2,500,000
The 2018 Legacy PRA Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)										2,000,000
Consultants Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)	400,000										1,000,000
Limit of shares issued or to be issued pursuant to options granted (in shares)	1,000,000
Employee Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)	6,000,000
Maximum number of shares per employee (in shares)	400,000
Option Plans 2008 \| Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Percentage of option price for fair value of ordinary share (in percent)	100.00%
Employee Stock Option
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period			5 years
Employee Stock Option \| Award date, prior to 2018
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period							5 years
Shares expiration period							8 years
Restricted Stock Units 2013
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)		4,100,000
Additional number of ordinary shares which have been reserved for issuance (in shares)		2,500,000
Consultants Restricted Stock Units 2019
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)									250,000
Consultants Restricted Stock Units 2019 \| Non-executive director
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period						12 months
Restricted Stock Units (RSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 35,400	$ 34,100
Restricted Stock Units (RSUs) \| Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)			$ 159.33	$ 137.47
Restricted Stock Units (RSUs) \| Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)			$ 265.96	$ 265.96
Performance Share Unit (PSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 7,500	$ 6,500
Performance Share Unit (PSUs) \| Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)			$ 159.33	$ 137.47
Performance Share Unit (PSUs) \| Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)			$ 166.51	$ 166.51
PSUs Based on Service and EPS Targets \| Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units to be granted (in shares)			49,340

Income Taxes - Tax Effects of Temporary Differences That Give Rise to Significant Portions of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred tax liabilities:
Property, plant and equipment	$ 7,547	$ 10,927
Operating right-of-use-assets	16,108	23,260
Goodwill	39,014	37,150
Intangible assets	950,055	1,078,302
Investments in foreign subsidiaries	52,408	1,587
Other	7,982	7,467
Total deferred tax liabilities recognized	1,073,114	1,158,693
Deferred tax assets:
Operating loss and tax credits carryforwards	124,150	88,697
Property, plant and equipment	9,082	6,010
Operating lease liabilities	20,190	27,593
Intangible assets	2,166	3,602
Stock compensation	17,605	21,862
Other liabilities	84,928	66,933
Unearned revenue	23,748	66,565
Other	8,774	9,155
Total deferred tax assets	290,643	290,417
Valuation allowance for deferred tax assets	(42,967)	(43,379)	$ (45,500)
Deferred tax assets recognized	247,676	247,038
Overall net deferred tax liability	$ (825,438)	$ (911,655)

Business Combinations - Fair Value of Identified Intangible Assets Acquired, PRA Acquisition (Details) - PRA Health Sciences, Inc $ in Thousands	Jul. 01, 2021 USD ($)
Business Acquisition [Line Items]
Estimated Fair Value	$ 4,919,000
Customer relationships
Business Acquisition [Line Items]
Estimated Fair Value	$ 3,938,000
Estimated Useful Life	23 years
Order backlog
Business Acquisition [Line Items]
Estimated Fair Value	$ 500,000
Estimated Useful Life	3 years
Trade names
Business Acquisition [Line Items]
Estimated Fair Value	$ 202,000
Estimated Useful Life	3 years
Patient database
Business Acquisition [Line Items]
Estimated Fair Value	$ 168,000
Estimated Useful Life	7 years
Technology assets
Business Acquisition [Line Items]
Estimated Fair Value	$ 111,000
Estimated Useful Life	5 years

Business Segment and Geographical Information - Distribution of Long-lived Assets, Net, by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Long-lived assets	$ 501,517	$ 504,152
Ireland
Segment Reporting Information [Line Items]
Long-lived assets	199,051	143,025
Rest of Europe
Segment Reporting Information [Line Items]
Long-lived assets	94,046	99,721
U.S.
Segment Reporting Information [Line Items]
Long-lived assets	159,245	213,311
Other
Segment Reporting Information [Line Items]
Long-lived assets	$ 49,175	$ 48,095

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 612,335	$ 505,304	$ 153,185
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	585,950	569,513	314,987
Impairment of long lived assets	8,686	28,767	20,037
Reduction in carrying value of operating right-of-use assets	41,546	45,215	45,339
Loss on equity method investments	383	3,136	2,161
Acquisition-related gain	(6,160)	0	0
Charge on cash flow hedge	2,407	0	0
Amortization of financing costs and debt discount	16,402	17,749	12,890
Stock compensation expense	55,667	70,523	133,844
Loss on extinguishment of debt	0	0	14,434
Loss on issuance of debt	0	0	59,460
Deferred tax benefit	(85,403)	(124,985)	(60,616)
Unrealized foreign exchange movements	19,706	(13,009)	(6,054)
Other non-cash items	24,332	11,324	4,480
Changes in operating assets and liabilities:
Accounts receivable	(83,296)	(420,695)	113,513
Unbilled revenue	4,716	(332,592)	(17,656)
Unearned revenue	134,566	192,944	(69,121)
Other net assets	(170,810)	10,121	108,259
Net cash provided by operating activities	1,161,027	563,315	829,142
Cash flows from investing activities:
Purchase of property, plant and equipment	(140,692)	(142,160)	(93,750)
Purchase of subsidiary undertakings (net of cash acquired)	(71,766)	0	(5,914,475)
Investment in equity method investments	0	0	(2,450)
Loan to equity method investment	0	0	(10,000)
Sale of available for sale investments	2,616	481	497
Purchase of available for sale investments	(2,857)	(482)	(480)
Proceeds from investments in equity - long term	0	1,906	500
Purchase of investments in equity - long term	(13,954)	(5,612)	(4,077)
Net cash used in investing activities	(226,653)	(145,867)	(6,024,235)
Cash flows from financing activities:
Financing costs	0	0	(30,328)
Drawdown of credit lines and facilities	370,000	75,000	5,905,100
Repayment of credit lines and facilities	(1,265,000)	(875,000)	(877,780)
Proceeds from the exercise of equity compensation	50,973	35,844	118,589
Share issue costs	(16)	(17)	(853)
Repurchase of ordinary shares	0	(99,983)	0
Share repurchase costs	0	(17)	0
Net cash (used in)/provided by financing activities	(844,043)	(864,173)	5,114,728
Effect of exchange rate movements on cash	(997)	(16,720)	(7,727)
Net increase/(decrease) in cash and cash equivalents	89,334	(463,445)	(88,092)
Cash and cash equivalents at beginning of year	288,768	752,213	840,305
Cash and cash equivalents at end of year	$ 378,102	$ 288,768	$ 752,213

Employee Benefits (Tables)

12 Months Ended

Dec. 31, 2023

Retirement Benefits [Abstract]

Schedule of Amounts Recognized in Balance Sheet

The balances recorded to the balance sheet are as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Other receivables	$	6,321	$	6,492
Non-current other liabilities (note 16)	(14,738)		(13,033)

Schedule of Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan


Funded surplus	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(20,999)	$	(19,558)
Fair value of plan assets	27,320		26,050
Funded surplus	$	6,321	$	6,492

The funded status as at December 31, 2023 and December 31, 2022 is included in other long-term receivables on the Consolidated Balance Sheet.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of year	$	19,558	$	41,813
Service cost	42		117
Interest cost	975		672
Plan participants' contributions	19		19

Benefits paid	(318)		(514)
Actuarial gain	(335)		(18,636)
Foreign currency exchange rate changes	1,058		(3,913)
Benefit obligation at end of year	$	20,999	$	19,558


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of year	$	26,050	$	36,198
Expected return on plan assets	1,015		1,258
Actuarial loss	(910)		(7,305)
Employer contributions	70		70
Plan participants' contributions	19		19
Benefits paid	(318)		(514)
Foreign currency exchange rate changes	1,394		(3,676)
Fair value of plan assets at end of year	$	27,320	$	26,050


Funded deficit	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(6,441)	$	(5,806)
Fair value of plan assets	6,261		5,681
Funded deficit	$	(180)	$	(125)

The funded deficit at December 31, 2023 and December 31, 2022 are included in non-current other liabilities on the Consolidated Balance Sheet.

The change in benefit obligation is presented in the following table. The discount rates used in calculating the benefit obligation in years ended December 31, 2023 and December 31, 2022 were 1.5% and 2.3%, respectively.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of year	$	5,806	$	7,643
Service cost	109		146
Interest cost	136		30
Plan participants' contributions	93		82
Settlement	(645)		(218)
Prior service cost	(13)		(23)
Benefits paid and transferred balances	(134)		(182)
Actuarial loss/(gain)	511		(1,527)
Foreign currency exchange rate changes	578		(145)
Benefit obligation at end of year	$	6,441	$	5,806


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of year	$	5,681	$	6,964
Expected return on plan assets	133		29
Actuarial gain/(loss)	356		(987)
Scheme contributions	215		114
Plan participants' contributions	93		82
Benefits paid and transferred balances	(134)		(182)
Settlement	(645)		(218)
Foreign currency exchange rate changes	562		(121)
Fair value of plan assets at end of year	$	6,261	$	5,681

Pension plan disclosures are presented in the following tables in accordance with the requirements of ASC 715-20, Defined Benefit Plans - General.


Funded deficit	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(7,747)	$	(5,345)
Fair value of plan assets	5,529		4,059
Funded deficit	$	(2,218)	$	(1,286)

The funded deficit at December 31, 2023 and December 31, 2022 are included in non-current other liabilities on the Consolidated Balance Sheet.

The change in benefit obligation is presented in the following table. The discount rate used in calculating the benefit obligation in years ended December 31, 2023 and December 31, 2022 was 1.5% and 2.3%, respectively.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of period	$	5,345	$	4,990
Service cost	417		404
Interest cost	132		20
Plan participants’ contributions	456		325


Settlement	(753)		(844)
Benefits paid and transferred balances	795		1,125
Actuarial loss/(gain)	711		(627)
Foreign currency exchange rate changes	644		(48)
Benefit obligation at end of year	$	7,747	$	5,345


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of period	$	4,059	$	3,017
Expected return on plan assets	101		29

Actuarial (loss)/gain	(52)		87
Scheme contributions	456		325
Plan participants’ contributions	456		325
Benefits paid and transferred balances	795		1,125

Settlement	(753)		(844)
Foreign currency exchange rate changes	467		(5)
Fair value of plan assets at end of year	$	5,529	$	4,059

Schedule of Components of Net Periodic Benefit Cost

The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit (credit)/cost:


	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Service cost	$	42	$	117	$	134
Interest cost	975		672		665
Expected return on plan assets	(1,015)		(1,258)		(1,171)
Amortization of net (gain)/loss	(274)		228		625

Net periodic benefit (credit)/cost	$	(272)	$	(241)	$	253

Summary of Assumptions Used in Calculating Pension Benefit Obligations

The following assumptions were used at the commencement of the year in determining the net periodic pension benefit (credit)/cost for the years ended December 31, 2023, December 31, 2022 and December 31, 2021:


	December 31, 2023		December 31, 2022		December 31, 2021
Discount rate	4.9	%	1.8	%	1.5	%
Rate of compensation increase	3.6	%	3.7	%	3.4	%
Expected rate of return on plan assets	3.8	%	3.8	%	3.4	%

The following assumptions were used in determining the benefit obligation at December 31, 2023 and December 31, 2022:


	December 31, 2023		December 31, 2022
Discount rate	4.8	%	4.9	%
Rate of compensation increase	3.0	%	3.6	%

Schedule of Amounts Recognized in Other Comprehensive Income (Loss)


Other comprehensive income/(loss)	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Actuarial gain - benefit obligation	$	(335)	$	(18,636)	$	(2,097)
Actuarial loss/(gain) - plan assets	910		7,305		(1,176)
Amortization of net gain/(loss) recognized in net periodic benefit (credit)/cost	274		(228)		(625)
Total	$	849	$	(11,559)	$	(3,898)

Schedule of Underlying Asset Split of Fund

The Company's pension plan asset allocation is as follows:


Asset Category	December 31, 2023		December 31, 2022
Government Bonds	78	%	88	%
Diversified Bonds	22	%	12	%


	100	%	100	%

Schedule of Plan Asset Fair Value Measurements


	December 31, 2023		December 31, 2022
	(in thousands)
Government Bonds	$	21,312	$	22,887
Diversified Bonds	$	6,008	$	3,163

	$	27,320	$	26,050

Schedule of Annual Benefit Payments which Reflect Expected Future Service

The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid.


	(in thousands)
2024	$	379
2025	420
2026	679
2027	662
2028	612
Years 2029 - 2033	4,028

Business Segment and Geographical Information - Distribution of Income from Operations, Excluding Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 956,152	$ 795,237	$ 378,529
Ireland
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	432,026	218,088	131,961
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	153,450	253,799	177,863
U.S.
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	310,343	254,849	39,132
Other
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 60,333	$ 68,501	$ 29,573

Goodwill

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill

The change in the carrying amount of goodwill for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Opening goodwill	$	8,971,670	$	9,037,931
Current year acquisitions (note 6)	36,750		—
Prior period acquisition (note 6)	—		(35,692)
Foreign exchange movement	13,655		(30,569)
Closing goodwill	$	9,022,075	$	8,971,670

There were no goodwill impairment losses for the years ended December 31, 2023 and 2022.

CONSOLIDATED BALANCE SHEETS (Parenthetical) - € / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Ordinary shares, par value (in EUR per share)	€ 6	€ 6
Ordinary shares, shares authorized (in shares)	100,000,000	100,000,000
Ordinary shares, shares issued (in shares)	82,495,086	81,723,555
Ordinary shares, shares outstanding (in shares)	82,495,086	81,723,555

Related Parties	12 Months Ended
Related Parties	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Parties	Related Parties Subsidiaries of the Company earned revenue of $243,000 (December 31, 2022: $428,000) from Corvus Pharmaceuticals during the year. Dr. Linda Grais serves as a Director and shareholder of Corvus Pharmaceuticals. $101,000 (December 31, 2022: $231,000) was noted as due from Corvus Pharmaceuticals at December 31, 2023. Subsidiaries of the Company earned revenue of $45,000 (December 31, 2022: $235,000) from Afimmune Limited during the year. Dr. John Climax is Chief Executive Officer and a Director and shareholder of Afimmune Limited. $47,000 was noted as due from Afimmune Limited at December 31, 2023 (December 31, 2022: $263,000). On July 24, 2020, a subsidiary of the Company, ICON Clinical Research Limited, entered into an agreement to jointly establish a new company, Oncacare Limited ("Oncacare"), a specialized oncology site network in the US and EMEA regions, with a third party. The Company invested $4.9 million to obtain a 49% interest in the voting share capital of Oncacare. On April 20, 2023, the Company completed the purchase of the majority investor’s 51% majority voting share capital of Oncacare. The consideration paid by ICON to purchase the 51% majority voting share capital was $5.1 million. As a result of this transaction (the "Oncacare acquisition"), Oncacare and its subsidiaries became wholly owned subsidiaries of the ICON Group. Prior to the Oncacare acquisition, the Company recorded losses of $0.4 million and $3.1 million representing its pro rata share of the losses in Oncacare during the year ended December 31, 2023 and December 31, 2022, respectively. The Oncacare acquisition also resulted in goodwill of $13.4 million and gave rise to an acquisition-related gain of $6.2 million.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-6

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(g)(3))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(e))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//850/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-6

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483326/850-10-50-1

+ Details

Business Combinations

12 Months Ended

Dec. 31, 2023

Business Combinations [Abstract]

Business Combinations

BioTel Research LLC Acquisition

On October 2, 2023, the Company acquired the entire outstanding equity interests of BioTel Research LLC (“BioTel”), a leading provider of medical imaging and cardiac safety monitoring services, from BioTelemetry Inc. in exchange for initial cash consideration of $68.1 million. Cash acquired amounted to $1.4 million. The purchase price allocation, as of the date of acquisition, was based on a preliminary valuation and may be subject to revision. Preliminarily, the BioTel acquisition resulted in the initial recognition of intangible assets of $36.4 million and goodwill of $23.4 million. Preliminary goodwill arising in connection with the acquisition is primarily attributable to the assembled workforce of BioTel and the expected synergies of the acquisition. The goodwill recognized is deductible for income tax purposes.

Oncacare Limited Acquisition

On April 20, 2023, the Company completed the purchase of the majority investor's 51% majority voting share capital of Oncacare Limited ("Oncacare") for $5.1 million, such that Oncacare and its subsidiaries became wholly-owned subsidiaries of the ICON Group. The Oncacare acquisition resulted in goodwill of $13.4 million and also gave rise to an acquisition-related gain of $6.2 million.

PRA Health Sciences, Inc. Acquisition

On July 1, 2021 (the "Merger Date"), the Company completed the acquisition of PRA by means of a merger whereby Indigo Merger Sub, Inc., a Delaware corporation and subsidiary of ICON, merged with and into PRA Health Sciences, Inc., the parent of PRA Health Sciences ("the Acquisition" and "the Merger"). The combined company has retained the name ICON and brought together approximately 38,000 (as at the Merger date) employees across the globe, creating the world's largest clinical research organization with a singular focus on clinical research and commercialization. The Merger was accounted for as a business combination using the acquisition method of accounting in accordance with ASC Topic 805, Business Combinations.

The combined company leverages its enhanced operations to transform clinical trials and accelerate biopharma customers’ commercial success through the development of much needed medicines and medical devices. The combined company has a renewed focus on leveraging data, applying technology and accessing diverse patient populations to speed up drug development.

Upon completion of the Merger, pursuant to the terms of the Merger Agreement, PRA became a wholly owned subsidiary of the ICON Group. Under the terms of the Merger, PRA shareholders received, per share, $80 in cash and 0.4125 shares of ICON stock.

In the years ended December 31, 2023, December 31, 2022 and December 31, 2021, the Company incurred Merger-related expenses of $44.2 million, $39.7 million and $198.3 million, respectively, which were accounted for separately from the business combination and expensed as incurred within the “Transaction and integration related” line item of the Consolidated Statement of Operations. The costs consist of investment banking fees, advisory costs, professional fees, retention agreements with employees, accelerated share-based compensation charges and ongoing integration activities.

In the years ended December 31, 2023, December 31, 2022 and December 31, 2021 the Company incurred approximately $16.4 million, $17.7 million and $86.7 million of Merger-related financing fees which are included in the “Interest expense” line item in the Consolidated Statement of Operations.

The Merger Date fair value of the consideration transferred consisted of the following:


	(in thousands)
Fair value of cash consideration	$	5,308,646
Fair value of ordinary shares issued to acquiree stockholders	5,658,126
Fair value of replacement share-based awards issued to acquiree employees	209,399
Repayment of term loan obligations and accrued interest *	865,800
	$	12,041,971

* This represents the portion of PRA debt paid by ICON. PRA also paid $401.6 million from available cash to settle debt obligations that existed at the Merger Date.


	July 1, 2021
	(in thousands)
Cash and cash equivalents	$	259,971
Accounts receivable and unbilled revenue	934,308
Other current assets	125,156
Fixed assets	156,851
Operating lease right-of-use assets	180,601
Goodwill *	8,084,314
Intangible assets	4,919,000
Deferred tax assets	25,190
Other assets	33,928
Accounts payable	(50,259)
Accrued expenses and other current liabilities	(380,048)
Current portion of operating lease liabilities	(36,506)
Unearned revenue	(739,278)
Non-current portion of operating lease liabilities	(147,204)
Deferred tax liabilities	(1,119,762)
Other non-current liabilities	(204,291)
Net assets acquired	$	12,041,971

The following table summarizes the fair value of identified intangible assets and their respective useful lives as of the Merger Date:


	Estimated Fair Value		Estimated Useful Life
	(in thousands)
Customer relationships	$	3,938,000	23 years
Order backlog	500,000		3 years
Trade names	202,000		3 years
Patient database	168,000		7 years
Technology assets	111,000		5 years
	$	4,919,000

At June 30, 2022, the Company completed its review of the July 1, 2021 acquisition balance sheet of PRA and completed the final valuation associated with certain assets acquired and liabilities assumed. In the period since the Merger Date, the Company recognized certain measurement period adjustments as shown in the table below:


	Measurement period adjustments

	(in thousands)
Cash and cash equivalents	$	—
Accounts receivable and unbilled revenue	—
Other current assets	14,465
Fixed assets	(6,137)
Operating lease right-of-use assets	(11,744)
Goodwill	70,436
Intangible assets *	44,000
Deferred tax assets	(147,039)
Other assets	(1,166)
Accounts payable	—
Accrued expenses and other current liabilities	(37,496)
Current portion of operating lease liabilities	1,865
Unearned revenue **	19,623
Non-current portion of operating lease liabilities	10,454
Non-current deferred tax liabilities	193,837
Other non-current liabilities	(151,098)

Property, Plant and Equipment, net

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment, net

The carrying amount of Property, Plant and Equipment for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Cost
Land	$	3,724	$	3,724
Buildings	70,072		70,880
Computer equipment and software	550,119		500,135
Office furniture and fixtures	45,856		50,600
Laboratory equipment	56,217		59,946
Leasehold improvements	55,000		65,167
Motor vehicles	79		41

	781,067		750,493
Less accumulated depreciation and asset write offs	(419,883)		(400,173)

Property, plant and equipment (net)	$	361,184	$	350,320

The depreciation expense recognized by the Company was $126.1 million, $106.4 million, and $75.5 million for the twelve months ended December 31, 2023, 2022, and 2021, respectively.

The Company regularly updates its register of property, plant and equipment and during the year ended December 31, 2023 and the year ended December 31, 2022, certain fully depreciated assets were written off as they were no longer used by the Company.

Equity Incentive Schemes and Stock Compensation Charges

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Equity Incentive Schemes and Stock Compensation Charges

Share Options

On July 21, 2008 the Company adopted the Employee Share Option Plan 2008 (the "2008 Employee Plan") pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any employee, or any Director holding a salaried office or employment with the Company or a Subsidiary for the purchase of ordinary shares. On the same date, the Company also adopted the Consultants Share Option Plan 2008 (the "2008 Consultants Plan"), pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any consultant, adviser or non-executive Director retained by the Company or any Subsidiary for the purchase of ordinary shares.

On February 14, 2017 both the 2008 Employee Plan and the 2008 Consultants Plan (together the "2008 Option Plans") were amended and restated in order to increase the number of options that can be issued under the 2008 Consultants Plan from 0.4 million to 1.0 million and to extend the date for options to be granted under the 2008 Option Plans. An aggregate of 6.0 million ordinary shares have been reserved under the 2008 Employee Plan, as reduced by any shares issued or to be issued pursuant to options granted under the 2008 Consultants Plan, under which a limit of 1.0 million shares applies. Further, the maximum number of ordinary shares with respect to which options may be granted under the 2008 Employee Option Plan, during any calendar year to any employee shall be 0.4 million ordinary shares. There is no individual limit under the 2008 Consultants Plan. No options may be granted under the 2008 Option Plans after February 14, 2027.

Each option granted under the 2008 Option Plans is an employee stock option, or Nonqualified Stock Option ("NSO"), as described in Section 422 or 423 of the Internal Revenue Code. Each grant of an option under the 2008 Options Plans is evidenced by a Stock Option Agreement between the optionee and the Company. The exercise price is specified in each Stock Option Agreement, however option prices are not less than 100% of the fair market value of an ordinary share on the date the option is granted. Share option awards are granted with an exercise price equal to the market price of the Company's shares at date of grant. Share options typically vest over a period of five years from date of grant and expire eight years from date of grant.

PRA Equity Incentive Plans

The following represent the PRA equity incentive plans, that have been terminated as of July 1, 2021, as to grants of future awards.

Pursuant to the Merger Agreement, effective on July 1, 2021, each stock option and restricted stock unit under the PRA Plans was assumed by the Company and converted into a stock option or Restricted Share Unit exercisable for or payable in Ordinary Shares based on the ratio of the average trading price per Ordinary Share for the ten days prior to July 1, 2021, and the corresponding value of the Merger consideration for each PRA Share. Accordingly, the plans as detailed below were assumed by the Company.

PRA Health Sciences, Inc. 2020 Stock Incentive Plan (the "2020 Plan”), 2018 Stock Incentive Plan (the "2018 Plan"), 2014 Omnibus Incentive Plan (the "2014 Plan"), and 2013 Stock Incentive Plan (the "2013 Plan") were amended and restated and assumed by the Registrant effective as of July 1, 2021.

The 2020 Stock Incentive Plan, was approved by the PRA stockholders at their annual meeting on May 18, 2020. The 2020 Plan allowed for the issuance of stock options, stock appreciation rights, restricted shares and restricted stock units, other stock-based awards, and performance compensation awards as permitted by applicable laws. The 2020 Plan authorized the issuance of 2.5 million shares of common stock plus all shares that remained available under the prior plan on May 18, 2020.

The 2018 Stock Incentive Plan (the “2018 Plan”), was approved by the PRA stockholders at their annual meeting on May 31, 2018. The 2018 Plan allowed for the issuance of stock options, stock appreciation rights, restricted shares and restricted stock units, other stock-based awards, and performance compensation awards as permitted by applicable laws. The 2018 Plan authorized the issuance of 2.0 million shares of common stock plus all shares that remained available under the 2014 Plan on May 31, 2018 (which included shares carried over from the 2013 Plan).

On November 23, 2014, the PRA Health Sciences, Inc. Board of Directors approved the formation of the 2014 Plan for Key PRA Employees. The 2014 Plan allowed for the issuance of stock options, stock appreciation rights, restricted shares and restricted stock units, other stock-based awards, and performance compensation awards as permitted by applicable laws.

The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2023, December 31, 2022 and December 31, 2021:


	Options Granted Under Plans	Weighted Average Exercise Price
Outstanding at December 31, 2020	553,746	$	108.53
Assumed through business combinations *	2,177,130	$	108.78
Granted	100,299	$	177.76
Exercised	(1,065,529)	$	111.29
Canceled	(70,186)	$	128.46
Outstanding at December 31, 2021	1,695,460	$	110.38
Granted	108,643	$	229.94
Exercised	(348,286)	$	102.87
Canceled/expired	(77,698)	$	143.08
Outstanding at December 31, 2022	1,378,119	$	119.86
Granted	82,472	$	232.48
Exercised	(535,705)	$	95.12
Canceled/expired	(22,080)	$	196.20
Outstanding at December 31, 2023	902,806	$	142.96
Vested and exercisable at December 31, 2023	654,386	$	119.67

*Represents stock options issued as replacement awards in connection with the Merger.

The weighted average remaining contractual life of options outstanding and options exercisable at December 31, 2023, was 4.42 years and 3.89 years respectively (2022: 4.69 years and 4.25 years respectively).

Outstanding and exercisable share options:

The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2023:


Options Outstanding					Options Exercisable
Range Exercise Price	Number of Shares	Weighted Average Remaining Contractual Life	Weighted Average Exercise Price		Number of Shares	Weighted Average Exercise Price
$20.83 - 96.15	201,306	2.29	—		201,306	—
$103.81 - 121.68	132,964	4.54	—		132,964	—
$125.74 - 147.26	249,194	4.45	—		233,371	—
$159.33 - 233.88	319,342	5.69	—		86,745	—

$20.83 - 233.88	902,806	4.42	$	142.96	654,386	$	119.67

Options outstanding include both vested and unvested options as at December 31, 2023. Options exercisable represent options which have vested at December 31, 2023. From the date of grant, substantially all options vest over a five year period.

Fair value of Stock Options Assumptions

The weighted average fair value of options granted during the years ended December 31, 2023, December 31, 2022 and December 31, 2021 was calculated using the Black-Scholes option pricing model. The weighted average fair values and assumptions were as follows:


	Year Ended
	December 31, 2023			December 31, 2022			December 31, 2021
Weighted average fair value	$	85.12		$	68.42		$	49.15
Assumptions:
Expected volatility	33		%	31		%	30		%
Dividend yield	—		%	—		%	—		%
Risk-free interest rate	4.18		%	1.86		%	0.78		%
Expected life	5.0 years			5.0 years			5.0 years


	July 1, 2021
Weighted average grant date fair value	$	107.21
Assumptions:
Expected volatility	30		%
Dividend yield	—		%
Risk-free interest rate	0.56		%
Expected life	3.5 years

Expected volatility is based on the historical volatility of our common stock over a period equal to the expected term of the options; the expected life represents the weighted average period of time that options granted are expected to be outstanding given consideration to vesting schedules and our historical experience of past vesting and termination patterns. The risk-free rate is based on the U.S. government zero-coupon bonds yield curve in effect at time of the grant for periods corresponding with the expected life of the option.

Restricted Share Units and Performance Share Units

On April 23, 2013 the Company adopted the 2013 Employees Restricted Share Unit and Performance Share Unit Plan (the "2013 RSU Plan") pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may select any employee, or any Director holding a salaried office or employment with the Company, or a Subsidiary to receive an award under the plan. On May 11, 2015 the 2013 RSU Plan was amended and restated in order to increase the number of shares that can be issued under the RSU Plan by 2.5 million shares. Accordingly, an aggregate of 4.1 million ordinary shares have been reserved for issuance under the 2013 RSU Plan. The shares are awarded at par value and vest over a service period. Awards under the 2013 RSU Plan may be settled in cash or shares at the option of the Company. No awards may be granted under the 2013 RSU Plan after May 11, 2025.

On April 30 2019, the Company approved the 2019 Consultants and Directors Restricted Share Unit Plan (the “2019 Consultants RSU Plan”), which was effective as of May 16, 2019, pursuant to which the Compensation and Organization Committee of the Company’s Board of Directors may select any consultant, adviser or non-executive Director retained by the Company, or a Subsidiary to receive an award under the plan. 250,000 ordinary shares have been reserved for issuance under the 2019 Consultants RSU Plan. The awards are at par value and vest over a service period. Awards granted to non-executive directors vest over twelve months.

The Company has awarded RSUs and PSUs to certain key individuals of the Group. The following table summarizes RSU and PSU activity for the year ended December 31, 2023:


	PSU Outstanding Number of Shares	PSU Weighted Average Grant Date Fair Value		RSU Outstanding Number of Shares	RSU Weighted Average Grant Date Fair Value
Outstanding at December 31, 2022	152,420	$	192.29	582,612	$	207.73

Granted	60,374	$	232.51	308,963	$	218.86
Shares vested	(47,026)	$	159.57	(188,800)	$	187.68
Forfeited	(60,512)	$	198.70	(81,764)	$	216.03
Outstanding at December 31, 2023	105,256	$	226.29	621,011	$	218.27

The fair value of RSUs vested for the year ended December 31, 2023 totaled $35.4 million (2022: $34.1 million). The share price range for the year was $159.33 - $265.96 (2022: $137.47 - $265.96).

The fair value of PSUs vested for the year ended December 31, 2023 totaled $7.5 million (2022: $6.5 million). The share price range for the year was $159.33 - $166.51 (2022: $137.47 - $166.51).

The PSUs vest based on service and specified EPS targets over the period 2021 – 2023, 2022 – 2024, and 2023 - 2025. Depending on the actual amount of EPS from 2021 to 2025, up to an additional 49,340 PSUs may also be granted.

Stock compensation expense

Income from operations for the year ended December 31, 2023 is stated after charging $55.7 million in respect of stock compensation expense (inclusive of employer related taxes). Stock compensation expense has been allocated as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Direct costs	$	26,595	$	22,854	$	18,551
Selling, general and administrative	29,072		47,669		41,457
Transaction and integration related *	—		—		73,836
Total compensation costs	$	55,667	$	70,523	$	133,844

* Represents the post combination portion of the accelerated vesting of awards following the completion of the Merger

The income tax expense for the year ended December 31, 2023 reflects a net income tax benefit of $20.0 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2023 was $10.9 million. The income tax expense for the year ended December 31, 2022 reflects a net income tax benefit of $12.9 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2022 was $7.7 million. The income tax expense for the year ended December 31, 2021 reflects a net income tax benefit of $22.7 million in connection with stock compensation (including excess tax benefits) and the total tax benefit realized in connection with stock options exercised during 2021 was $23.9 million.

Goodwill - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Opening goodwill	$ 8,971,670	$ 9,037,931
Current year acquisitions (note 6)	36,750	0
Prior period acquisition (note 6)	0	(35,692)
Foreign exchange movement	13,655	(30,569)
Closing goodwill	$ 9,022,075	$ 8,971,670

- Definition

+ References + Details

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Litigation We do not expect any litigation to have a materially adverse effect on our financial condition or results of operations. However, from time to time, we may become involved in various lawsuits and legal proceedings which arise in the ordinary course of business. Litigation is subject to inherent uncertainties, and an adverse result in these or other matters may arise from time to time that may harm our business. Operating Leases The Company has several non-cancelable operating leases, primarily for facilities, that expire over the next sixteen years. These leases generally contain renewal options and require the Company to pay all executory costs such as maintenance and insurance. See note 15 - Operating leases for rental expense pursuant to ASC 842 for the years ended December 31, 2023, 2022 and 2021 and future minimum rental commitments as of December 31, 2023.

Related Parties (Details) - USD ($) $ in Thousands		12 Months Ended
Related Parties (Details) - USD ($) $ in Thousands	Apr. 20, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jul. 24, 2020
Related Party Transaction [Line Items]
Equity method investments					$ 4,900
Share of losses from equity method investments		$ 383	$ 3,136	$ 2,161
Goodwill		9,022,075	8,971,670	9,037,931
Acquisition related costs		$ 6,160	0	0
Oncacare
Related Party Transaction [Line Items]
Percentage of share capital acquired	51.00%
Business combination, consideration transferred	$ 5,100
Goodwill	13,400
Acquisition related costs	$ 6,200
Oncacare
Related Party Transaction [Line Items]
Equity method investments					$ 4,900
Equity method investment, ownership percentage		49.00%			49.00%
Share of losses from equity method investments		$ 400	3,100	$ 2,200
Corvus Pharmaceuticals \| Subsidiaries
Related Party Transaction [Line Items]
Revenues		243	428
Other receivables		101	231
Afimmune Limited \| Subsidiaries
Related Party Transaction [Line Items]
Revenues		45	235
Other receivables		$ 47	$ 263

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Non-cash Stock Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 55,667	$ 70,523	$ 133,844
Direct costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	26,595	22,854	18,551
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	29,072	47,669	41,457
Transaction and integration related
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 0	$ 0	$ 73,836

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

Bank Credit Lines and Loan Facilities - Schedule of Finance Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Disclosure [Abstract]
Interest expense on drawn facilities	$ 311,019	$ 209,189	$ 93,809
Amortization of financing costs and debt discount	16,402	17,749	12,890
Transaction and one time financing costs	0	0	75,391
Other financing costs	9,278	2,793	333
Total financing costs	$ 336,699	$ 229,731	$ 182,423

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References + Details

Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 317,975	$ 210,918	$ 106,205
Cash paid for income taxes (net of refunds)	$ 163,778	$ 116,322	$ 55,105

Restructuring charges (Tables)

12 Months Ended

Dec. 31, 2023

Restructuring and Related Activities [Abstract]

Schedule of Restructuring Charges


	Year Ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Restructuring charges	$	45,390	$	31,143	$	31,105
Net charge	$	45,390	$	31,143	$	31,105

Schedule of Restructuring Reserve by Type of Cost


	Year Ended
	December 31, 2023		December 31, 2022
	(in thousands)
Opening provision	$	6,022	$	10,311
Additional provision in the year	36,704		4,364
Utilization	(35,727)		(8,653)
Ending provision	$	6,999	$	6,022

Fair Value (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value, Liabilities Measured on Recurring Basis

As of December 31, 2023, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,954	$	—	$	—	$	—	$	1,954
Available for sale investments (long-term) (b)	—		—		—		46,804		46,804
Derivative instruments (c)	—		—		—		—		—
Total assets	$	1,954	$	—	$	—	$	46,804	$	48,758

Liabilities:
Derivative instruments (c)	—		2,411		—		—		2,411
Total Liabilities	$	—	$	2,411	$	—	$	—	$	2,411

As of December 31, 2022, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,713	$	—	$	—	$	—	$	1,713
Available for sale investments (long-term) (b)	—		—		—		32,631		32,631
Derivative instruments (c)			12						12
Total assets	$	1,713	$	12	$	—	$	32,631	$	34,356

Liabilities:
Derivative instruments (c)	—		3,670		—		—		3,670
Total liabilities	$	—	$	3,670	$	—	$	—	$	3,670

(a) Represents the fair value of highly liquid investments with maturities greater than three months, a minimum "A-" rated fixed term deposit and are based on quoted market prices.

(b) To determine the classification of interests in long-term investments, the Company considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2023 and December 31, 2022 meet the definition of equity securities without readily determinable fair values. The Company concluded that the interests held at December 31, 2023 and December 31, 2022 qualify for the NAV practical expedient in ASC 820. Any increases or decreases in fair value are recognized in net income in the period.

Fair Value, Assets Measured on Recurring Basis

As of December 31, 2023, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,954	$	—	$	—	$	—	$	1,954
Available for sale investments (long-term) (b)	—		—		—		46,804		46,804
Derivative instruments (c)	—		—		—		—		—
Total assets	$	1,954	$	—	$	—	$	46,804	$	48,758

Liabilities:
Derivative instruments (c)	—		2,411		—		—		2,411
Total Liabilities	$	—	$	2,411	$	—	$	—	$	2,411

As of December 31, 2022, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,713	$	—	$	—	$	—	$	1,713
Available for sale investments (long-term) (b)	—		—		—		32,631		32,631
Derivative instruments (c)			12						12
Total assets	$	1,713	$	12	$	—	$	32,631	$	34,356

Liabilities:
Derivative instruments (c)	—		3,670		—		—		3,670
Total liabilities	$	—	$	3,670	$	—	$	—	$	3,670

(a) Represents the fair value of highly liquid investments with maturities greater than three months, a minimum "A-" rated fixed term deposit and are based on quoted market prices.

Summary of Significant accounting policies (Policies)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Basis of Consolidation

Basis of consolidation

Use of Estimates

Use of estimates

Disclosure of Fair Value of Financial Instruments

Disclosure of fair value of financial instruments

•Level 1: Inputs are based upon unadjusted quoted prices for identical instruments traded in active markets.

•Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability.

Business Combinations

Business combinations

Transaction and integration-related expenses

Foreign Currencies and Translation of Subsidiaries

Foreign currencies and translation of subsidiaries


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Amounts charged/(credited)	$	12,916	$	(25,997)	$	(14,316)

Revenue recognition

Clinical trial service revenue

Laboratory services revenue

Contracting services revenue

Consulting services revenue

Data services revenue

At the end of the contract year, any unused customer purchase credits may be forfeited or carried over to the next contract year based on the terms of the data supplier contract.

Commissions

Reimbursable expenses and Direct costs

Reimbursable expenses

Direct costs

Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs.

Cash and Cash Equivalents

Cash and cash equivalents

Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value.

Investment in Debt, Equity and Other

Investments in debt, equity and other

Available for sale investments

Long term investments

Equity method investments

Accounts Receivable, Net and Unbilled Revenue

Accounts receivable, net and unbilled revenue

Accounts receivable early payment discounting

Property, Plant and Equipment

Property, plant and equipment


			Estimated Useful Life
Buildings			40 years
Computer equipment and software			2 - 8 years
Office furniture and fixtures			8 years
Laboratory equipment			5 years
Motor vehicles			5 years

Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter.

Leases

Intangible Assets

Intangible assets

Intangible assets are amortized on a straight-line basis over their estimated useful lives, as set forth in the table below:


			Estimated Useful Life
Customer relationships			16 - 23 years
Order backlog			3 years
Trade names			3 years
Patient database			7 years
Technology assets			5 years

The Company periodically assesses the estimated useful lives of intangible assets to evaluate whether what was established at acquisition continues to be appropriate.

Impairment of goodwill and long-lived assets

Debt Issuance Costs

Debt issuance costs

Derivative Financial Instruments

Derivative financial instruments

Income Taxes

Income taxes

Government grants

Research and development credits

Pension Costs

Pension costs

Share-Based Compensation

Share-based compensation

variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards.

Liability classified awards are measured at the fair value of the award on the grant date and remeasured at each reporting period at fair value until the award is settled.

Replacement awards

Transaction and Integration-related Expenses

Business combinations

Transaction and integration-related expenses

Restructuring

Net Income per Ordinary Share

Net income per ordinary share

Impact of Recently Issued or Adopted Accounting Standards

Accounting pronouncements, effective for the year ended December 31, 2023, did not have any significant impact on the Company’s consolidated financial statements.

The following accounting pronouncements have been issued, but not yet adopted, as of December 31, 2023:

Operating Leases Operating Leases - Operating Lease Maturity (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Leases [Abstract]
2024	$ 40,894
2025	34,585
2026	29,172
2027	23,254
2028	15,023
Thereafter	33,998
Total future minimum lease payments	176,926
Lease imputed interest	(14,191)
Total	$ 162,735

Non-current other liabilities (Tables)

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

Non-current other liabilities

The carrying amount of Non-current other liabilities for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Defined benefit pension obligations, net (note 21)	$	14,738	$	13,033
Other non-current liabilities	31,260		25,227
	$	45,998	$	38,260

Share Capital	12 Months Ended
Share Capital	Dec. 31, 2023
Equity [Abstract]
Share Capital	Share Capital Holders of ordinary shares are entitled to receive such dividends as may be recommended by the Board of Directors of the Company and approved by the shareholders and/or such interim dividends as the Board of Directors of the Company may decide. On liquidation or a winding up of the Company, the par value of the ordinary shares are repaid out of the assets available for distribution among the holders of the ordinary shares of the Company. Holders of ordinary shares have no conversion or redemption rights. On a show of hands, every holder of an ordinary share present in person or proxy at a general meeting of shareholders shall have one vote, for each ordinary share held with no individual having more than one vote. (a)Employee share based payments During the year ended December 31, 2023, 535,705 options were exercised by employees at an average exercise price of $95.12 per share for total proceeds of $51.0 million. During the year ended December 31, 2023, 188,800 ordinary shares were issued in respect of certain RSUs and 47,026 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2022, 348,286 options were exercised by employees at an average exercise price of $102.87 per share for total proceeds of $35.8 million. During the year ended December 31, 2022, 195,029 ordinary shares were issued in respect of certain RSUs and 46,087 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2021, 1,065,529 options were exercised by employees at an average exercise price of $111.29 per share for total proceeds of $118.6 million. During the year ended December 31, 2021, 446,404 ordinary shares were issued in respect of certain RSUs and 44,132 ordinary shares were issued in respect of PSUs previously awarded by the Company. On July 1, 2021, the Company completed the Acquisition of PRA. In accordance with the terms of the Merger Agreement, the Company issued 27,372,427 shares of the Company’s ordinary share capital at par value in exchange for all outstanding PRA shares of common stock. (b)Share Repurchase Program A resolution was passed at the Company’s Annual General Meeting (“AGM”) on July 22, 2016, which authorized the Directors to purchase (buyback) up to 10% of the outstanding shares in the Company. This resolution has been renewed annually thereafter. On February 18, 2022, the Company commenced a share buyback program which was fully complete at March 31, 2022. Under this buyback program, 420,530 ordinary shares were redeemed by the Company for total consideration of $100.0 million. All ordinary shares that were redeemed under the buyback program were canceled in accordance with the Constitution of the Company and the nominal value of these shares transferred to other undenominated capital as required under Irish Company law. Under the repurchase program, a broker purchased or may purchase the Company's shares from time to time on the open market or in privately negotiated transactions in accordance with agreed terms and limitations. The program was and may be in the future designed to allow share repurchases during periods when the Company would ordinarily not be permitted to do so because it may be in possession of material non-public or price-sensitive information or due to applicable insider trading laws or self-imposed trading blackout periods. The Company's instructions to the broker in such cases were or may in the future be irrevocable and the trading decisions in respect of the repurchase program were made or will be made independently of and uninfluenced by the Company. The Company confirms that on entering the share repurchase plans it had no material non-public, price-sensitive or inside information regarding the Company or its securities. Furthermore, the Company will not enter into additional plans whilst in possession of such information. The timing and actual number of shares acquired by way of the redemption will be dependent on market conditions, legal and regulatory requirements and the other terms and limitations contained in the program. In addition, acquisitions under the program may be suspended or discontinued in certain circumstances in accordance with the agreed terms. Therefore, there can be no assurance as to the timing or number of shares that may be acquired under the program.

- Definition

The entire disclosure for equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 235
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481062/946-235-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481004/946-505-50-6

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480237/815-40-50-6

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//505/tableOfContent

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-13

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-14

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-16

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481112/505-10-50-18

+ Details

Goodwill - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Goodwill - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment charge	$ 0.0	$ 0.0

Derivatives (Tables)

12 Months Ended

Dec. 31, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of Derivative Instruments in Statement of Financial Position, Fair Value

The fair values of the Company’s derivative financial instruments, on a gross basis, are summarized in the following table:


	December 31, 2023						December 31, 2022
	Asset		Liability		Notional		Asset		Liability		Notional
	(in thousands)						(in thousands)
Derivatives designated as hedging instruments:

Interest Rate Caps	$	—	$	1,871	$	1,600,606	$	12	$	3,363	$	2,100,606
Interest Rate Swap	—		540		1,100,606		—		307		1,100,606

Total derivatives designated as hedging instruments	$	—	$	2,411	$	2,701,212	$	12	$	3,670	$	3,201,212

Property, Plant and Equipment, net - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation	$ 126.1	$ 106.4	$ 75.5

Income Taxes - Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at start of year	$ 217,584	$ 202,065	$ 19,078
Increase related to acquired tax positions	0	0	170,047
Increase related to prior year tax positions	1,161	16,098	204
Decrease related to prior year tax positions	(918)	(5,442)	(1,695)
Increase related to current year tax positions	4,552	5,701	18,613
Settlements	0	0	(844)
Lapse of statute of limitations	(62,363)	(838)	(3,338)
Unrecognized tax benefits at end of year	$ 160,016	$ 217,584	$ 202,065

Restructuring charges - Schedule of Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring and Related Activities [Abstract]
Restructuring charges	$ 45,390	$ 31,143	$ 31,105

Employee Benefits

12 Months Ended

Dec. 31, 2023

Retirement Benefits [Abstract]

Employee Benefits

Defined Contribution Plans

Defined contribution or profit sharing style plans ("the Plans") are offered in a number of countries. In some cases, these plans are required by local laws or regulations. Certain employees are eligible to participate in the Plans and participants in the Plans may elect to defer a portion of their pre-tax earnings into a pension plan, which is run by an independent party. The Company matches participant's contributions up to certain levels of the participant's annual compensation.

The Company's United States operations maintain retirement plans (the "U.S. Plans") that qualify as a deferred salary arrangement under Section 401(k) of the Internal Revenue Code. Participants in the U.S. Plans may elect to defer a portion of their earnings, up to the Internal Revenue Service annual contribution limit. The Company matches participant's contributions at varying amounts, subject to a maximum of 4.5% of the participant's annual compensation. Contributions to this U.S. Plan are recorded, in the year contributed, as an expense in the Consolidated Statement of Operations. Contributions for the years ended December 31, 2023, December 31, 2022 and December 31, 2021 were $40.4 million, $45.8 million and $23.7 million respectively.

Pension and Postretirement Benefit Plans

The Company maintains various retirement plans across the Group, many of which are required by local employment laws. The balances recorded to the balance sheet are as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Other receivables	$	6,321	$	6,492
Non-current other liabilities (note 16)	(14,738)		(13,033)

In addition to the specific defined benefit schemes shown separately below, the Company maintains several other retirement plans which have a cumulative total net obligation of $12.3 million and $11.6 million recorded to non-current other liabilities as of December 31, 2023 and December 31, 2022, respectively.

ICON Development Solutions Limited pension plan

ICON Development Solutions Limited, a subsidiary of ICON Plc, operates a defined benefit pension plan in the United Kingdom for its employees. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2023 and December 31, 2022 consist of units held in independently administered funds. Pension plan disclosures are presented in the following tables in accordance with the requirements of ASC 715-20, Defined Benefit Plans - General. The plan has been closed to new entrants with effect from July 1, 2003.


Funded surplus	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(20,999)	$	(19,558)
Fair value of plan assets	27,320		26,050
Funded surplus	$	6,321	$	6,492

The funded status as at December 31, 2023 and December 31, 2022 is included in other long-term receivables on the Consolidated Balance Sheet.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of year	$	19,558	$	41,813
Service cost	42		117
Interest cost	975		672
Plan participants' contributions	19		19

Benefits paid	(318)		(514)
Actuarial gain	(335)		(18,636)
Foreign currency exchange rate changes	1,058		(3,913)
Benefit obligation at end of year	$	20,999	$	19,558


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of year	$	26,050	$	36,198
Expected return on plan assets	1,015		1,258
Actuarial loss	(910)		(7,305)
Employer contributions	70		70
Plan participants' contributions	19		19
Benefits paid	(318)		(514)
Foreign currency exchange rate changes	1,394		(3,676)
Fair value of plan assets at end of year	$	27,320	$	26,050

The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company.

The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit (credit)/cost:


	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Service cost	$	42	$	117	$	134
Interest cost	975		672		665
Expected return on plan assets	(1,015)		(1,258)		(1,171)
Amortization of net (gain)/loss	(274)		228		625

Net periodic benefit (credit)/cost	$	(272)	$	(241)	$	253


	December 31, 2023		December 31, 2022		December 31, 2021
Discount rate	4.9	%	1.8	%	1.5	%
Rate of compensation increase	3.6	%	3.7	%	3.4	%
Expected rate of return on plan assets	3.8	%	3.8	%	3.4	%


Other comprehensive income/(loss)	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Actuarial gain - benefit obligation	$	(335)	$	(18,636)	$	(2,097)
Actuarial loss/(gain) - plan assets	910		7,305		(1,176)
Amortization of net gain/(loss) recognized in net periodic benefit (credit)/cost	274		(228)		(625)
Total	$	849	$	(11,559)	$	(3,898)

The estimated net loss and prior service cost for the defined benefit pension plan that will be amortized from accumulated other comprehensive income/(loss) into net periodic benefit cost over the next year are $0.2 million and $nil respectively.

Benefit Obligation

The following assumptions were used in determining the benefit obligation at December 31, 2023 and December 31, 2022:


	December 31, 2023		December 31, 2022
Discount rate	4.8	%	4.9	%
Rate of compensation increase	3.0	%	3.6	%

A single discount rate is used which, when used to discount the projected benefit cash flows underlying a pension scheme with a 19 year duration, gives the same result as a full AA corporate bond yield curve.

Actuarial gains on the benefit obligation during 2023 resulted from changes in the assumptions compared to those adopted at December 2022. Changes in the assumptions reflect the changes in market conditions from December 2022 to December 2023.

Plan Assets

The Company's pension plan asset allocation is as follows:


Asset Category	December 31, 2023		December 31, 2022
Government Bonds	78	%	88	%
Diversified Bonds	22	%	12	%


	100	%	100	%

During 2022, the scheme's asset strategy changed to align the plan assets more closely with government and diversified bonds. There is no self-investment in employer related assets. The Company’s assumption for the expected return on plan assets was determined by the weighted average of the long-term expected rate of return on each of the asset classes invested as of the balance sheet date. The expected long-term rate of return on assets is 3.8% at December 31, 2023 and December 31, 2022, respectively.

Plan Asset Fair Value Measurements


	December 31, 2023		December 31, 2022
	(in thousands)
Government Bonds	$	21,312	$	22,887
Diversified Bonds	$	6,008	$	3,163

	$	27,320	$	26,050

The value of assets held by the plan are represented by quoted prices in active markets for identical assets and are therefore classified as level 1 investments.

Cash Flows

The Company expects to contribute $0.1 million to the pension fund in the year ending December 31, 2024. The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid.


	(in thousands)
2024	$	379
2025	420
2026	679
2027	662
2028	612
Years 2029 - 2033	4,028

The expected cash flows are estimated figures based on the members expected to retire over the next 10 years assuming no early retirements, withdrawals or commutation of pension for cash. At the present time it is not clear whether annuities will be purchased when members reach retirement or whether pensions will be paid each month out of scheme assets. The cash flows above have been estimated on the assumption that pensions will be paid monthly out of scheme assets. If annuities are purchased, then the expected benefit payments will be significantly different from those shown above.

Aptiv Solutions pension plan

On May 7, 2014 the Company acquired 100% of the common stock of Aptiv Solutions ("Aptiv"). The Company has a defined benefit plan covering its employees in Switzerland as mandated by the Swiss government. Benefits are based on the employee's years of service and compensation. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2023 and December 31, 2022 consist of units held in independently administered funds. Pension plan disclosures are presented in the following tables in accordance with the requirements of ASC 715-20, Defined Benefit Plans - General.


Funded deficit	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(6,441)	$	(5,806)
Fair value of plan assets	6,261		5,681
Funded deficit	$	(180)	$	(125)

The funded deficit at December 31, 2023 and December 31, 2022 are included in non-current other liabilities on the Consolidated Balance Sheet.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of year	$	5,806	$	7,643
Service cost	109		146
Interest cost	136		30
Plan participants' contributions	93		82
Settlement	(645)		(218)
Prior service cost	(13)		(23)
Benefits paid and transferred balances	(134)		(182)
Actuarial loss/(gain)	511		(1,527)
Foreign currency exchange rate changes	578		(145)
Benefit obligation at end of year	$	6,441	$	5,806


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of year	$	5,681	$	6,964
Expected return on plan assets	133		29
Actuarial gain/(loss)	356		(987)
Scheme contributions	215		114
Plan participants' contributions	93		82
Benefits paid and transferred balances	(134)		(182)
Settlement	(645)		(218)
Foreign currency exchange rate changes	562		(121)
Fair value of plan assets at end of year	$	6,261	$	5,681

The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company.

PRA Switzerland AG pension plan

On July 1, 2021, the Company completed the Acquisition of PRA. PRA Switzerland AG, a subsidiary of the Company has a defined benefit plan covering its employees in Switzerland as mandated by the Swiss government. Benefits are based on the employee's years of service and compensation. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2023 consist of units held in independently administered funds. Pension plan disclosures are presented in the following tables in accordance with the requirements of ASC 715-20, Defined Benefit Plans - General.


Funded deficit	December 31, 2023		December 31, 2022
	(in thousands)
Projected benefit obligation	$	(7,747)	$	(5,345)
Fair value of plan assets	5,529		4,059
Funded deficit	$	(2,218)	$	(1,286)

The funded deficit at December 31, 2023 and December 31, 2022 are included in non-current other liabilities on the Consolidated Balance Sheet.


Change in benefit obligation	December 31, 2023		December 31, 2022
	(in thousands)
Benefit obligation at beginning of period	$	5,345	$	4,990
Service cost	417		404
Interest cost	132		20
Plan participants’ contributions	456		325


Settlement	(753)		(844)
Benefits paid and transferred balances	795		1,125
Actuarial loss/(gain)	711		(627)
Foreign currency exchange rate changes	644		(48)
Benefit obligation at end of year	$	7,747	$	5,345


Change in plan assets	December 31, 2023		December 31, 2022
	(in thousands)
Fair value of plan assets at beginning of period	$	4,059	$	3,017
Expected return on plan assets	101		29

Actuarial (loss)/gain	(52)		87
Scheme contributions	456		325
Plan participants’ contributions	456		325
Benefits paid and transferred balances	795		1,125

Settlement	(753)		(844)
Foreign currency exchange rate changes	467		(5)
Fair value of plan assets at end of year	$	5,529	$	4,059

The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480794/715-70-50-1

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480482/715-20-55-17

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(03)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (q)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
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-Name Accounting Standards Codification
-Section 50
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-Subparagraph (l)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//715/tableOfContent

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)(iii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
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-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (o)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (p)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
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-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480126/715-20-S99-2

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 60
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480266/715-60-50-3

+ Details

Summary of Significant accounting policies - Estimated Useful Lives of Intangible Assets (Details)	Dec. 31, 2023
Customer relationships \| Minimum
Indefinite-lived Intangible Assets [Line Items]
Amortization period	16 years
Customer relationships \| Maximum
Indefinite-lived Intangible Assets [Line Items]
Amortization period	23 years
Order backlog
Indefinite-lived Intangible Assets [Line Items]
Amortization period	3 years
Trade names & brands
Indefinite-lived Intangible Assets [Line Items]
Amortization period	3 years
Patient database
Indefinite-lived Intangible Assets [Line Items]
Amortization period	7 years
Technology assets
Indefinite-lived Intangible Assets [Line Items]
Amortization period	5 years

Bank Credit Lines and Loan Facilities - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Jul. 01, 2021
Debt Instrument [Line Items]
Total debt	$ 3,806,213	$ 4,701,213
Less current portion of debt	(110,150)	(55,150)
Total long-term debt	3,696,063	4,646,063
Less debt issuance costs and debt discount	(30,624)	(47,026)
Total long-term debt, net	$ 3,665,439	$ 4,599,037
Senior Secured Term Loan \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	7.86%	7.092%	1.00%
Total debt	$ 3,251,213	$ 4,201,213
Senior Secured Notes \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	2.875%	2.875%	2.875%
Total debt	$ 500,000	$ 500,000
Senior Secured Revolving Loan Facility \| Senior Secured Revolving Loan Facility
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	6.72%	0.00%
Total debt	$ 55,000	$ 0

Intangible Assets - Future Intangible Asset Amortization Expense (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2024	$ 344,518
2025	229,106
2026	214,869
2027	202,539
2028	189,539
Total	$ 1,180,571

Bank Credit Lines and Loan Facilities - Schedule of Maturities of Long-term Debt (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Debt Disclosure [Abstract]
2024	$ 110,150
2025	55,150
2026	555,150
2027	55,150
2028 and thereafter	3,030,613
Total long-term debt, net	$ 3,806,213

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Accumulated Other Comprehensive Loss

12 Months Ended

Dec. 31, 2023

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Accumulated Other Comprehensive Loss


	Year ended
	December 31, 2023		December 31, 2022
	(in thousands)
Currency translation adjustments	$	(149,148)	$	(175,369)
Actuarial gain on defined benefit pension plan	7,803		7,559
Loss on cash flow hedge	(2,161)		(3,728)
Total	$	(143,506)	$	(171,538)

Restructuring Charges

12 Months Ended

Dec. 31, 2023

Restructuring and Related Activities [Abstract]

Restructuring Charges

In the year ended, December 31, 2023, a restructuring charge of $45.4 million was recognized in the Consolidated Statements of Operations under a restructuring plan adopted following a review of operations. The restructuring plan reflected a workforce reduction of $34.1 million and an office consolidation program to optimize the Company's office footprint of $11.3 million.


	Year Ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Restructuring charges	$	45,390	$	31,143	$	31,105
Net charge	$	45,390	$	31,143	$	31,105

At December 31, 2023, a total liability of $7.0 million was recorded on the Consolidated Balance Sheet relating to restructuring activities. The total liability included $3.0 million of facilities related liabilities of which $1.0 million is included within other liabilities and $2.0 million is included within non-current other liabilities. The remaining provision of $4.0 million relates to workforce reduction and is included within other liabilities.


	Year Ended
	December 31, 2023		December 31, 2022
	(in thousands)
Opening provision	$	6,022	$	10,311
Additional provision in the year	36,704		4,364
Utilization	(35,727)		(8,653)
Ending provision	$	6,999	$	6,022

Derivatives

12 Months Ended

Dec. 31, 2023

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivatives

The Company has entered into interest rate cap and swap agreements for purposes of managing its exposure to interest rate fluctuations. These financial derivative agreements are designated as cash flow hedges.

On November 29, 2022, the Company entered into two interest rate cap agreements ("2022 Caps") with an initial total notional value of $2,101 million to limit its exposure to changes in the variable interest rate on its Senior Secured Credit Facilities. Interest on the 2022 Caps began accruing on December 30, 2022 and the interest rate caps expire on December 31, 2024. The Company pays a fixed rate of 0.42% and receives a variable rate equal to the amount that the three-month SOFR rate exceeds 4.75%.

On November 29, 2022, the Company entered into an interest rate swap agreement ("2022 Swap") with an initial notional value of $1,101 million to limit its exposure to changes in the variable interest rate on its Senior Secured Credit Facilities. Interest on the 2022 Swap begins accruing on December 31, 2024 and the interest rate swap expires on September 30, 2026. The Company pays a fixed rate of 3.4% and receives a variable rate of interest equal to the three-month SOFR on the 2022 Swap.

The 2022 Caps and the 2022 Swap are designated as cash flow hedges. Gains and losses are initially reported as a component of other comprehensive income/loss and subsequently recognized in net income.

The fair values of the Company’s derivative financial instruments, on a gross basis, are summarized in the following table:


	December 31, 2023						December 31, 2022
	Asset		Liability		Notional		Asset		Liability		Notional
	(in thousands)						(in thousands)
Derivatives designated as hedging instruments:

Interest Rate Caps	$	—	$	1,871	$	1,600,606	$	12	$	3,363	$	2,100,606
Interest Rate Swap	—		540		1,100,606		—		307		1,100,606

Total derivatives designated as hedging instruments	$	—	$	2,411	$	2,701,212	$	12	$	3,670	$	3,201,212

As of December 31, 2023, the Company recognized a current derivative liability of $1.9 million within other liabilities (December 31, 2022: $3.3 million within other liabilities and $0.01 million within other receivables) and a non current derivative liability of $0.5 million (December 31, 2022: $0.4 million) within non-current other liabilities.

During the year ended December 31, 2023, the Company recognized a gain of $1.6 million within other comprehensive income/loss (December 31, 2022: $3.7 million loss) after a reclassification of $2.4 million from other comprehensive income/loss to the income statement (December 31, 2022: $0.1 million).

During the next 12 months, the Company estimates that an additional $3.2 million will be reflected as interest expense in the consolidated statements.

Commitments and Contingencies - Narrative (Details)	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Non-cancelable operating leases for facilities expiration period	16 years

Intangible Assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	$ 5,147,971	$ 5,109,212
Accumulated amortization	(1,292,106)	(830,553)
Net book value	3,855,865	4,278,659
Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	4,090,393	4,076,435
Order backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	541,302	536,934
Trade names & brands
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	204,653	204,621
Patient database
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	170,366	170,238
Technology assets
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	$ 141,257	$ 120,984

Derivatives - Effect of Derivative Financial Instruments on Statement of Financial Position (Details) - Total derivatives designated as hedging instruments - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Interest Rate Caps
Derivative [Line Items]
Derivative Asset	$ 0	$ 12,000
Derivative Liability	1,871,000	3,363,000
Notional	1,600,606,000	2,100,606,000
Interest Rate Swap
Derivative [Line Items]
Derivative Asset	0	0
Derivative Liability	540,000	307,000
Notional	1,100,606,000	1,100,606,000
Total derivatives designated as hedging instruments
Derivative [Line Items]
Derivative Asset	0	12,000
Derivative Liability	2,411,000	3,670,000
Notional	2,701,212,000	3,201,212,000
Total derivatives designated as hedging instruments \| Level 2
Derivative [Line Items]
Derivative Asset	0	12,000
Derivative Liability	$ 2,411,000	$ 3,670,000

- Definition

Nominal or face amount used to calculate payments on the derivative asset.

+ References + Details

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Name Accounting Standards Codification
-Section 50
-Paragraph 12
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-URI https://asc.fasb.org//1943274/2147480524/946-210-50-12

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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+ Details

Intangible Assets - Narrative (Details) - USD ($) $ in Thousands			3 Months Ended	12 Months Ended			30 Months Ended
	Oct. 02, 2023	Jul. 01, 2021	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2023
Indefinite-lived Intangible Assets [Line Items]
Amortization of intangible assets				$ 459,900	$ 463,100	$ 239,500
Fair value of cash consideration				$ 71,766	$ 0	$ 5,914,475
Customer relationships \| Minimum
Indefinite-lived Intangible Assets [Line Items]
Amortization period			16 years	16 years			16 years
Customer relationships \| Maximum
Indefinite-lived Intangible Assets [Line Items]
Amortization period			23 years	23 years			23 years
Order backlog
Indefinite-lived Intangible Assets [Line Items]
Amortization period			3 years	3 years			3 years
Trade names & brands
Indefinite-lived Intangible Assets [Line Items]
Amortization period			3 years	3 years			3 years
Patient database
Indefinite-lived Intangible Assets [Line Items]
Amortization period			7 years	7 years			7 years
Technology assets
Indefinite-lived Intangible Assets [Line Items]
Amortization period			5 years	5 years			5 years
Biotel Research LLC
Indefinite-lived Intangible Assets [Line Items]
Amortization of intangible assets			$ 1,500
Fair value of cash consideration	$ 68,100
Value of intangible assets acquired	$ 36,400
Biotel Research LLC \| Minimum
Indefinite-lived Intangible Assets [Line Items]
Amortization period	3 years
Biotel Research LLC \| Maximum
Indefinite-lived Intangible Assets [Line Items]
Amortization period	16 years
Biotel Research LLC \| Customer relationships
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired	$ 12,500
Biotel Research LLC \| Order backlog
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired	3,900
Biotel Research LLC \| Technology assets
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired	$ 20,000
PRA Health Sciences, Inc
Indefinite-lived Intangible Assets [Line Items]
Amortization of intangible assets				$ 451,400			$ 1,128,500
Fair value of cash consideration		$ 5,308,646
Value of intangible assets acquired		$ 4,919,000
PRA Health Sciences, Inc \| Minimum
Indefinite-lived Intangible Assets [Line Items]
Amortization period		3 years
PRA Health Sciences, Inc \| Maximum
Indefinite-lived Intangible Assets [Line Items]
Amortization period		23 years
PRA Health Sciences, Inc \| Customer relationships
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired		$ 3,938,000
PRA Health Sciences, Inc \| Order backlog
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired		500,000
PRA Health Sciences, Inc \| Trade names & brands
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired		202,000
PRA Health Sciences, Inc \| Patient database
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired		168,000
PRA Health Sciences, Inc \| Technology assets
Indefinite-lived Intangible Assets [Line Items]
Value of intangible assets acquired		$ 111,000

Non-current other liabilities

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

Non-current other liabilities

The carrying amount of Non-current other liabilities for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Defined benefit pension obligations, net (note 21)	$	14,738	$	13,033
Other non-current liabilities	31,260		25,227
	$	45,998	$	38,260

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 612,335	$ 505,304	$ 153,185
Other comprehensive income, net of tax
Currency translation adjustment	26,221	(89,530)	(60,617)
Actuarial gain on defined benefit pension plan	244	12,657	4,266
Amortization of cash flow hedge	0	0	113
Gain/(loss) on cash flow hedge	1,567	(3,728)	778
Total comprehensive income	$ 640,367	$ 424,703	$ 97,725

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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+ Details

Business Combinations - Narrative, Oncacare Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Apr. 20, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Goodwill		$ 9,022,075	$ 8,971,670	$ 9,037,931
Acquisition related costs		$ 6,160	$ 0	$ 0
Oncacare
Business Acquisition [Line Items]
Percentage of share capital acquired	51.00%
Business combination, consideration transferred	$ 5,100
Goodwill	13,400
Acquisition related costs	$ 6,200

- Definition

+ References + Details

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current Assets:
Cash and cash equivalents	$ 378,102	$ 288,768
Available for sale investments (Note 8a)	1,954	1,713
Accounts receivable, net of allowance for credit losses (Note 4)	1,790,322	1,731,388
Unbilled revenue (Note 4)	951,936	957,655
Other receivables	65,797	63,658
Prepayments and other current assets	132,105	137,094
Income taxes receivable	91,254	48,790
Total current assets	3,411,470	3,229,066
Non-current Assets:
Property, plant and equipment, net (Note 9)	361,184	350,320
Goodwill (Note 10)	9,022,075	8,971,670
Intangible assets, net (Note 11)	3,855,865	4,278,659
Operating right-of-use assets (Note 15)	140,333	153,832
Other receivables	78,470	70,790
Income taxes receivable	0	21,380
Deferred tax asset (Note 20)	73,662	76,930
Investments in equity-long term (Note 8b)	46,804	32,631
Total Assets	16,989,863	17,185,278
Current Liabilities:
Accounts payable	131,584	81,194
Unearned revenue (Note 4)	1,654,507	1,507,449
Other liabilities (Note 12)	915,399	1,005,025
Income taxes payable	13,968	41,783
Current bank credit lines and loan facilities (Note 13)	110,150	55,150
Total current liabilities	2,825,608	2,690,601
Non-current Liabilities:
Non-current bank credit lines and loan facilities (Note 13)	3,665,439	4,599,037
Lease liabilities (Note 15)	126,321	131,644
Non-current other liabilities (Note 16)	45,998	38,260
Non-current income taxes payable	186,654	239,188
Deferred tax liability (Note 20)	899,100	988,585
Commitments and contingencies (Note 17)	0	0
Total Liabilities	7,749,120	8,687,315
Shareholders' Equity:
Ordinary shares par value 6 euro cents per share; 100,000,000 shares authorized, (Note 18). 82,495,086 shares issued and outstanding at December 31, 2023 and 81,723,555 shares issued and outstanding at December 31, 2022.	6,699	6,649
Additional paid-in capital	6,942,669	6,840,306
Other undenominated capital (Note 18b)	1,162	1,162
Accumulated other comprehensive loss (Note 25)	(143,506)	(171,538)
Retained earnings	2,433,719	1,821,384
Total Shareholders' Equity	9,240,743	8,497,963
Total Liabilities and Shareholders’ Equity	$ 16,989,863	$ 17,185,278

Intangible Assets

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets

The carrying amount of Intangible Assets for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
Cost	(in thousands)
Customer relationships	$	4,090,393	$	4,076,435
Order backlog	541,302		536,934
Trade names & brands	204,653		204,621
Patient database	170,366		170,238
Technology assets	141,257		120,984
Total cost	5,147,971		5,109,212

Accumulated amortization	(1,292,106)		(830,553)
Net book value	$	3,855,865	$	4,278,659

The amortization expense recognized by the Company was $459.9 million, $463.1 million, and $239.5 million for the twelve months ended December 31, 2023, 2022, and 2021, respectively.

On October 2, 2023, the Company acquired the entire outstanding equity interests of BioTel Research LLC (“BioTel”), a leading provider of medical imaging and cardiac safety monitoring services, from BioTelemetry Inc. in exchange for initial cash consideration of $68.1 million. The acquisition resulted in the recognition of intangible assets of $36.4 million, comprising customer relationships of $12.5 million, order backlog of $3.9 million and technology assets of $20.0 million. These assets will be amortized over their expected useful lives of between 3 and 16 years. In total, $1.5 million has been amortized in the period since the date of acquisition.

On July 1, 2021, ICON plc announced the completion of its Merger with PRA Health Sciences, Inc. The Merger resulted in the recognition of Customer relationships of $3,938.0 million, Order backlog of $500.0 million, Trade names of $202.0 million, Patient database of $168.0 million and Technology assets of $111.0 million. These assets are being amortized over their expected useful lives of between 3 and 23 years. The valuation of these assets was finalized in June 30, 2022. In total, $1,128.5 million has been amortized in the period since the date of acquisition, $451.4 million has been amortized in the year ended December 31, 2023.

Future intangible asset amortization expense for the years ended December 31, 2024 to December 31, 2028 is as follows:


	(in thousands)
2024	$	344,518
2025	229,106
2026	214,869
2027	202,539
2028	189,539
Total	$	1,180,571

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Valuation allowance for deferred tax assets	$ 42,967,000	$ 43,379,000	$ 45,500,000
Net change in the total valuation allowance	(400,000)	(2,100,000)
Investments in foreign subsidiaries	52,408,000	1,587,000
Unrecognized tax benefit, potentially expire in 2021	65,000,000
Total unrecognized tax benefits net of potential benefits	160,000,000	217,600,000	202,100,000
Interest and penalties recognized as an expense	4,200,000	7,100,000	$ 1,900,000
Total accrued interest and penalties	27,100,000	22,600,000
Provision for income taxes
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	(1,100,000)	(800,000)
Other Comprehensive Income
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	700,000	$ (1,300,000)
Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Other non-U.S subsidiaries operating loss carryforwards for income tax	42,900,000
Foreign Country \| Tax year 2024 to 2030
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	12,900,000
Ireland \| Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Ireland subsidiaries additional tax credit carryforward for income tax	16,900,000
United States
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. Federal net operating loss carry forwards currently available for offset	1,200,000
Alternative minimum tax credit carry forwards	800,000
United States \| Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net operating loss carryforwards	1,744,000
Tax credit carry forward	204,000,000
Other non-U.S subsidiaries operating loss carryforwards for income tax	0
United States \| State NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net operating loss carryforwards	331,370,000
Other non-U.S subsidiaries operating loss carryforwards for income tax	9,251,000
Other \| Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Ireland subsidiaries additional tax credit carryforward for income tax	$ 4,300,000

Employee Benefits - Annual Benefit Payments (Details) - United Kingdom - ICON Development Solutions Limited pension plan $ in Thousands	Dec. 31, 2023 USD ($)
Defined Benefit Plan Disclosure [Line Items]
2024	$ 379
2025	420
2026	679
2027	662
2028	612
Years 2029 - 2033	$ 4,028

Bank Credit Lines and Loan Facilities - Schedule of Movement on Debt Instrument (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Drawdown of credit lines and facilities					$ 370,000	$ 75,000	$ 5,905,100
Repayment of credit lines and facilities					1,265,000	875,000	$ 877,780
Total long-term debt, net	$ 3,806,213				3,806,213
Senior Secured Revolving Loan Facility \| Senior secured revolving loan facility \| Line of credit
Debt Instrument [Line Items]
Drawdown of credit lines and facilities	65,000	$ 75,000	$ 50,000	$ 180,000	370,000	0
Repayment of credit lines and facilities	85,000	50,000	80,000	100,000	315,000	0
Total long-term debt, net	$ 55,000	$ 75,000	$ 50,000	$ 80,000	$ 55,000	$ 0

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References + Details

Income Taxes - Components of Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	$ 624,467	$ 567,851	$ 196,680
Ireland
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	624,642	432,963	231,893
United States
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	(352,532)	(270,440)	(278,413)
Other
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	$ 352,357	$ 405,328	$ 243,200

Fair Value

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value

The Company records certain assets and liabilities at fair value. Fair value is defined as the price that would be received to sell an asset, or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. A three-level fair value hierarchy that prioritizes the inputs used to measure fair value is described below. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows:

•Level 1 — Quoted prices in active markets for identical assets or liabilities.

•Level 2 — Observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets; quoted prices for identical or similar assets and liabilities in markets that are not active; or other inputs that are observable or can be corroborated by observable market data.

•Level 3 — Unobservable inputs that are supported by little or no market activity. This includes certain pricing models, discounted cash flow methodologies, and similar techniques that use significant unobservable inputs.

The carrying amounts of financial instruments, including cash and cash equivalents, accounts receivable and accounts payable approximate fair value due to the short maturities of these instruments.

As of December 31, 2023, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,954	$	—	$	—	$	—	$	1,954
Available for sale investments (long-term) (b)	—		—		—		46,804		46,804
Derivative instruments (c)	—		—		—		—		—
Total assets	$	1,954	$	—	$	—	$	46,804	$	48,758

Liabilities:
Derivative instruments (c)	—		2,411		—		—		2,411
Total Liabilities	$	—	$	2,411	$	—	$	—	$	2,411

As of December 31, 2022, the fair value of the major classes of the Company's assets and liabilities measured at fair value on a recurring basis were as follows:


	Level 1		Level 2		Level 3		Investments Measured at Net Asset Value		Total
	(in thousands)
Assets:
Available for sale securities (short-term) (a)	$	1,713	$	—	$	—	$	—	$	1,713
Available for sale investments (long-term) (b)	—		—		—		32,631		32,631
Derivative instruments (c)			12						12
Total assets	$	1,713	$	12	$	—	$	32,631	$	34,356

Liabilities:
Derivative instruments (c)	—		3,670		—		—		3,670
Total liabilities	$	—	$	3,670	$	—	$	—	$	3,670

(a) Represents the fair value of highly liquid investments with maturities greater than three months, a minimum "A-" rated fixed term deposit and are based on quoted market prices.

Non-recurring Fair Value Measurements

Certain assets and liabilities are carried on the Consolidated Balance Sheet at cost and are not re-measured to fair value on a recurring basis. These assets include finite-lived intangible assets that are tested for impairment when a triggering event occurs and goodwill that is tested for impairment annually or when a triggering event occurs. As of December 31, 2023, assets carried on the balance sheet and not re-measured to fair value on a recurring basis totaled approximately $12,878.0 million (December 31, 2022: $13,250.4 million) and are identified as Level 3 assets. These assets are comprised of goodwill of $9,022.1 million (December 31, 2022: $8,971.7 million) and intangible assets of $3,855.9 million (December 31, 2022: $4,278.7 million).

The estimated fair value of the Company’s debt was $3,793.5 million at December 31, 2023 (December 31, 2022: $4,650.3 million). The fair values of the Senior Secured Credit Facilities and Senior Secured Notes were determined based on Level 2 inputs, which are based on rates at which the debt is traded among financial institutions. The fair value of the senior secured revolving loan facility is recorded as its carrying value, due to the short term duration.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor firm ID	1116
Auditor name	KPMG
Auditor location	Dublin, Ireland

Business Combinations - Fair Value of Consideration Transferred, PRA Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 01, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Fair value of cash consideration		$ 71,766	$ 0	$ 5,914,475
PRA Health Sciences, Inc
Business Acquisition [Line Items]
Fair value of cash consideration	$ 5,308,646
Fair value of ordinary shares issued to acquiree stockholders	5,658,126
Fair value of replacement share-based awards issued to acquiree employees	209,399
Repayment of term loan obligations and accrued interest	865,800
Consideration transferred	12,041,971
Payments to settle debt obligations that existed prior to acquisition date	$ 401,600

Operating Leases (Tables)

12 Months Ended

Dec. 31, 2023

Leases [Abstract]

Lease Cost

Lease costs recorded under operating leases for the years ended December 31, 2023, 2022 and 2021 were as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)

Operating lease costs	$	46,820	$	53,880	$	51,200
Income from sub-leases	(1,103)		(1,165)		(1,338)
Net operating lease costs	$	45,717	$	52,715	$	49,862

Schedule of Operating Lease Maturity

Future minimum lease payments under non-cancelable leases as of December 31, 2023 were as follows:


	Minimum rental payments
	(in thousands)
2024	$	40,894
2025	34,585
2026	29,172
2027	23,254
2028	15,023
Thereafter	33,998
Total future minimum lease payments	176,926
Lease imputed interest	(14,191)
Total	$	162,735

Business Segment and Geographical Information (Tables)

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

Schedule of Distribution of Net Revenue by Geographical Area

Reportable geographic information at December 31, 2023 and December 31, 2022 and for the years ended December 31, 2023, December 31, 2022 and December 31, 2021 is as follows:

a) The distribution of revenue by geographical area was as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021

	(in thousands)
Ireland	$	2,377,104	$	1,984,567	$	1,365,909
Rest of Europe	1,574,783		1,618,350		1,175,515
U.S.	3,283,790		3,574,610		2,581,007
Other	884,499		563,859		358,395
Total	$	8,120,176	$	7,741,386	$	5,480,826

Schedule of Distribution of Income from Operations by Geographical Area

The distribution of income from operations (including Restructuring) by geographical area was as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Ireland *	$	432,026	$	218,088	$	131,961
Rest of Europe	153,450		253,799		177,863
U.S.	310,343		254,849		39,132
Other	60,333		68,501		29,573
Total	$	956,152	$	795,237	$	378,529

* Includes the full amount of the amortization charge associated with the intangible asset acquired in the Merger.

Schedule of Distribution of Net Revenue by Geographical Area

The distribution of long-lived assets (property, plant and equipment and operating right-of-use assets), net, by geographical area was as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Ireland	$	199,051	$	143,025
Rest of Europe	94,046		99,721
U.S.	159,245		213,311
Other	49,175		48,095
Total	$	501,517	$	504,152

Employee Benefits - Narrative (Details) - USD ($)	12 Months Ended
Employee Benefits - Narrative (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	May 07, 2014
Other retirement plans
Defined Benefit Plan Disclosure [Line Items]
Non-current other liabilities (note 16)	$ 12,300,000	$ 11,600,000
ICON Development Solutions Limited pension plan
Defined Benefit Plan Disclosure [Line Items]
Expected rate of return on plan assets	3.80%	3.80%
PRA Switzerland AG pension plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.50%	2.30%
United States
Defined Benefit Plan Disclosure [Line Items]
Participant's contributions percentage of annual compensation (in percent)	4.50%
Defined contribution plan, cost	$ 40,400,000	$ 45,800,000	$ 23,700,000
United Kingdom \| ICON Development Solutions Limited pension plan
Defined Benefit Plan Disclosure [Line Items]
Non-current other liabilities (note 16)	20,999,000	$ 19,558,000	$ 41,813,000
Net loss that will be amortized	200,000
Net prior service cost for defined benefit pension plan that will be amortized	$ 0
Pension plan period	19 years
Expected rate of return on plan assets	3.80%	3.80%	3.40%
Contribution to pension fund in the year ending December 31, 2021	$ 100,000
Retirement period used as a basis to estimate expected cash flows	10 years
Discount rate	4.80%	4.90%
Switzerland \| Aptiv Solutions pension plan
Defined Benefit Plan Disclosure [Line Items]
Non-current other liabilities (note 16)	$ 6,441,000	$ 5,806,000	$ 7,643,000
Discount rate	1.50%	2.30%
Switzerland \| Aptiv Solutions pension plan \| Aptiv Solutions
Defined Benefit Plan Disclosure [Line Items]
Percentage of share capital acquired (in percent)				100.00%
Switzerland \| PRA Switzerland AG pension plan
Defined Benefit Plan Disclosure [Line Items]
Non-current other liabilities (note 16)	$ 7,747,000	$ 5,345,000	$ 4,990,000

Investments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Investments - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Apr. 20, 2023	Jul. 24, 2020
Related Party Transaction [Line Items]
Available for sale investments	$ 1,954	$ 1,713
Short term investments average maturity period	3 years 7 months 6 days	2 years 9 months 18 days
Debt securities, available for sale, cumulative amount advanced under subscription agreements	$ 14,000	$ 3,700
Increase (decrease) in equity securities, FV-NI	200	6,300
Carrying value of long-term investments in equity	46,800	32,600
Commitment to acquire future long-term investments	66,400
Equity method investments					$ 4,900
Loss on equity method investments	$ 383	3,136	$ 2,161
Oncacare
Related Party Transaction [Line Items]
Percentage of share capital acquired				51.00%
Oncacare
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	49.00%				49.00%
Equity method investments					$ 4,900
Loss on equity method investments	$ 400	$ 3,100	$ 2,200

Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Finite-Lived Intangible Assets

Intangible assets are amortized on a straight-line basis over their estimated useful lives, as set forth in the table below:


			Estimated Useful Life
Customer relationships			16 - 23 years
Order backlog			3 years
Trade names			3 years
Patient database			7 years
Technology assets			5 years

The carrying amount of Intangible Assets for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
Cost	(in thousands)
Customer relationships	$	4,090,393	$	4,076,435
Order backlog	541,302		536,934
Trade names & brands	204,653		204,621
Patient database	170,366		170,238
Technology assets	141,257		120,984
Total cost	5,147,971		5,109,212

Accumulated amortization	(1,292,106)		(830,553)
Net book value	$	3,855,865	$	4,278,659

Schedule of Future Intangible Asset Amortization Expense

Future intangible asset amortization expense for the years ended December 31, 2024 to December 31, 2028 is as follows:


	(in thousands)
2024	$	344,518
2025	229,106
2026	214,869
2027	202,539
2028	189,539
Total	$	1,180,571

Contract Balances

12 Months Ended

Dec. 31, 2023

Revenue from Contract with Customer [Abstract]

Contract Balances

Disaggregation of Revenue

Revenue disaggregated by customer profile is as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Top client	$	721,309	$	683,546	$	441,173
Clients 2-5	1,453,508		1,506,087		1,291,946
Clients 6-10	1,188,943		1,112,636		752,325
Clients 11-25	1,743,539		1,585,739		1,077,073
Other	3,012,877		2,853,378		1,918,309
Total	$8,120,176		$7,741,386		$5,480,826

Our customers have similar profiles and economic characteristics, and therefore have similar degrees of risk and growth opportunities.Contract Balances

Accounts receivable and unbilled revenue are as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Billed services (accounts receivable)	$	1,821,855	$	1,751,950
Allowance for credit losses (note 5)	(31,533)		(20,562)
Accounts receivable (net)	1,790,322		1,731,388
Unbilled services (unbilled revenue)	951,936		957,655
Accounts receivable and unbilled revenue, net	$	2,742,258	$	2,689,043

Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows:


(in thousands, except percentages)	December 31, 2023		December 31, 2022		$ Change		% Change

Unbilled services (unbilled revenue)	$	951,936	$	957,655	$	(5,719)	(0.6)	%
Unearned revenue (payments on account)	(1,654,507)		(1,507,449)		(147,058)		9.8	%
Net balance	$	(702,571)	$	(549,794)	$	(152,777)	27.8	%

The credit loss expense recognized on the Company's receivables and unbilled services was $24.6 million and $17.8 million for the year ended December 31, 2023 and 2022, respectively.

- Definition

+ References

+ Details

Description of business	12 Months Ended
Description of business	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of business	Description of Business ICON plc and its subsidiaries ("the Company" or "ICON") is a clinical research organization ("CRO"), providing outsourced development services on a global basis to the pharmaceutical, biotechnology and medical device industries. We specialize in the strategic development, management and analysis of programs that support all stages of the clinical development process from compound selection to Phase I-IV clinical studies. Our mission is to improve the lives of patients by accelerating the development of our customers' drugs and devices through innovative solutions. We believe that we are one of a select group of CROs with the expertise and capability to conduct clinical trials in most major therapeutic areas on a global basis and have the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full-service" solution. At December 31, 2023 we had approximately 41,100 employees, in 106 locations in 53 countries. During the year ended December 31, 2023, we derived approximately 40.4%, 48.7% and 10.9% of our revenue in the United States, Europe and Rest of World, respectively. ICON’s ordinary shares are traded on the NASDAQ Global Select Market under the symbol “ICLR”. We began operations in 1990 and have expanded our business through internal growth, together with a number of strategic acquisitions to enhance our capabilities and expertise in certain areas of the clinical development process. We are incorporated in Ireland and our principal executive office is located at: South County Business Park, Leopardstown, Dublin 18, Republic of Ireland. The contact telephone number of this office is +353 1 2912000.

Fair value - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 48,758	$ 34,356
Long-term debt at fair value	3,793,500	4,650,300
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Fair value, nonrecurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	12,878,000	13,250,400
Goodwill	9,022,100	8,971,700
Identifiable intangible assets, net	$ 3,855,900	$ 4,278,700

Bank Credit Lines and Loan Facilities

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Bank Credit Lines and Loan Facilities

The Company had the following debt outstanding as of December 31, 2023 and 2022:


		Interest rate as of				Principal amount as of
	Maturity Date	December 31, 2023		December 31, 2022		December 31, 2023		December 31, 2022
						(in thousands)
Senior Secured Term Loan	July 2028	7.860	%	7.092	%	$	3,251,213	$	4,201,213
Senior Secured Notes	July 2026	2.875	%	2.875	%	500,000		500,000
Senior Secured Revolving Loan Facility	January 2024	6.720	%	—		55,000		—



Total debt						3,806,213		4,701,213

Less current portion of debt						(110,150)		(55,150)
Total long-term debt						3,696,063		4,646,063
Less debt issuance costs and debt discount						(30,624)		(47,026)
Total long-term debt, net						$	3,665,439	$	4,599,037

The Company incurred interest costs from various financing arrangements during the years ended December 31, 2023, December 31, 2022 and December 31, 2021 as set out in the table below. These costs have been charged in the interest expense line of the Consolidated Statement of Operations. In the years ending December 31, 2023, December 31, 2022 and December 31, 2021, the Company expensed $16.4 million, $17.7 million and $86.7 million of transaction related financing costs, inclusive of the amortization of financing fees which were previously capitalized, associated with the debt facilities used to finance the Merger.


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Interest expense on drawn facilities	$	311,019	$	209,189	$	93,809
Amortization of merger related financing fees	16,402		17,749		12,890
Transaction and one time financing costs	—		—		75,391
Other financing costs*	9,278		2,793		333
Total financing costs	$	336,699	$	229,731	$	182,423

*Includes costs associated with the senior secured revolving loan facility.

As of December 31, 2023, the contractual maturities of the Company's debt obligations were as follows:


Current maturities of debt:	(in thousands)
2024	$	110,150
2025	55,150
2026	555,150
2027	55,150
2028 and thereafter	3,030,613
Total	$	3,806,213

The Company's primary financing arrangements are its senior secured credit facilities (the "Senior Secured Credit Facilities"), which consists of a senior secured term loan and a revolving credit facility, and the senior secured notes (the "Senior Secured Notes").

Senior Secured Credit Facilities

On July 1, 2021, the Company completed the acquisition of PRA Health Sciences, Inc. ("PRA") by means of a merger whereby Indigo Merger Sub, Inc., a Delaware corporation and subsidiary of ICON, merged with and into PRA, the parent of the PRA Health Sciences ("the Merger"). In conjunction with the completion of the Merger, on July 1, 2021, ICON entered into a credit agreement providing for a senior secured term loan facility of $5,515 million and a senior secured revolving loan facility in an initial aggregate principal amount of $300 million (the "Senior Secured Credit Facilities"). The Senior Secured Credit Facility and Senior Secured Notes were issued at a discount of $27.6 million. On May 2, 2023, the Company agreed with its lenders to increase the aggregate principal amount of the senior secured revolving loan facility from $300 million to $500 million.

Borrowings under the senior secured term loan facility amortize in equal quarterly installments in an amount equal to 1.00% per annum of the principal amount, with the remaining balance due at final maturity. The interest rate margin applicable to borrowings under the senior secured term loan facility is USD Term SOFR and a Term SOFR Adjustment depending on the interest period chosen plus an applicable margin which is dependent on the Company's net leverage ratio. As of December 31, 2023, the applicable margin is 2.25%. The senior secured term loan facility is subject to a floor of 0.50%.

The interest rate margin applicable to borrowings under the revolving loan facility will be, at the option of the borrower, either (i) the applicable base rate plus an applicable margin of 1.00%, 0.60% or 0.25% based on ICON’s current corporate family rating assigned by S&P of BB- (or lower), BB or BB+ (or higher), respectively, or (ii) Term SOFR plus a Term SOFR Adjustment on the interest period chosen plus an applicable margin of 2.00%, 1.60% or 1.25% based on ICON’s current corporate family rating assigned by S&P of BB- (or lower), BB or BB+ (or higher), respectively. In addition, lenders under the revolving loan facility are entitled to commitment fees as a percentage of the applicable margin at the time of drawing and utilization fees dependent on the proportion of the facility drawn. As at December 31, 2023, $445.0 million remained undrawn under the senior secured revolving loan facility.

The Borrowers’ (as defined in the Senior Secured Credit Facility) obligations under the Senior Secured Credit Facilities are guaranteed by ICON and the subsidiary guarantors. The Senior Secured Credit Facilities are secured by a lien on substantially all of ICON’s, the Borrowers’ and each of the subsidiary guarantor’s assets (subject to certain exceptions), and the Senior Secured Credit Facilities will have a first-priority lien on such assets, which will rank pari passu with the lien securing the Senior Secured Notes, subject to other permitted liens. The Company is permitted to make prepayments on the senior secured term loan without penalty.

Principal repayments, comprising mandatory and voluntary repayments, during the year ended December 31, 2023 and December 31, 2022 were as follows:


Principal repayments	December 31, 2023		December 31, 2022
	(in thousands)
Quarter 1	$	250,000	$	300,000
Quarter 2	150,000		100,000
Quarter 3	300,000		200,000
Quarter 4	250,000		200,000
Total	$	950,000	$	800,000

The voluntary repayments made during the year resulted in an accelerated charge associated with previously capitalized fees of $7.9 million (December 31, 2022: $7.8 million).

During the year ended December 31, 2023, the Company drew down $370.0 million (December 31, 2022: $75.0 million) of the senior secured revolving loan facility and repaid $315.0 million (December 31, 2022: $75.0 million) as shown below. As at December 31, 2023, $55.0 million (December 31, 2022: $nil) was drawn under the senior secured revolving loan facility.


	Drawdown		Repayment		Closing Balance
	(in thousands)
December 31, 2022	$	—	$	—	$	—
Quarter 1, 2023	180,000		100,000		80,000
Quarter 2, 2023	50,000		80,000		50,000
Quarter 3, 2023	75,000		50,000		75,000
Quarter 4, 2023	65,000		85,000		55,000
	$	370,000	$	315,000

Senior Secured Notes

In addition to the Senior Secured Credit Facilities, on July 1, 2021, a subsidiary of the Company issued $500 million in aggregate principal amount of 2.875% senior secured notes due 2026 in a private offering (the “Offering”). The Senior Secured Notes will mature on July 15, 2026.

Fair Value of Debt

The estimated fair value of the Company’s debt was $3,793.5 million and $4,650.3 million at December 31, 2023 and December 31, 2022, respectively. The fair values of the Senior Secured Credit Facilities and Senior Secured Notes were determined based on Level 2 inputs, which are based on rates at which the debt is traded among financial institutions. The fair value of the senior secured revolving loan facility is recorded as its carrying value due to the short term duration.

Property, Plant and Equipment, net (Tables)

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property Plant and Equipment, Net

The carrying amount of Property, Plant and Equipment for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Cost
Land	$	3,724	$	3,724
Buildings	70,072		70,880
Computer equipment and software	550,119		500,135
Office furniture and fixtures	45,856		50,600
Laboratory equipment	56,217		59,946
Leasehold improvements	55,000		65,167
Motor vehicles	79		41

	781,067		750,493
Less accumulated depreciation and asset write offs	(419,883)		(400,173)

Property, plant and equipment (net)	$	361,184	$	350,320

Other Liabilities (Tables)

12 Months Ended

Dec. 31, 2023

Other Liabilities Disclosure [Abstract]

Schedule of Other Liabilities

The carrying amount of Other Liabilities for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
General trade and overhead liabilities*	$	463,882	$	530,204
Personnel related liabilities	385,499		395,862
Operating lease liabilities	36,414		43,657
Facility related liabilities	11,078		16,896
Other liabilities	13,532		12,852
Restructuring liabilities	4,951		5,512
Short term government grants	43		42
	$	915,399	$	1,005,025

*includes amounts due to third parties in respect of accrued reimbursable investigator expenses of $333.0 million at December 31, 2023 and $406.3 million at December 31, 2022.

Business Combinations - Schedule of Acquisitions, PRA Acquisition (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jul. 01, 2021
Business Acquisition [Line Items]
Goodwill	$ 9,022,075	$ 8,971,670	$ 9,037,931
PRA Health Sciences, Inc
Business Acquisition [Line Items]
Cash and cash equivalents				$ 259,971
Accounts receivable and unbilled revenue				934,308
Other current assets				125,156
Fixed assets				156,851
Operating lease right-of-use assets				180,601
Goodwill				8,084,314
Intangible assets				4,919,000
Deferred tax assets				25,190
Other assets				33,928
Accounts payable				(50,259)
Accrued expenses and other current liabilities				(380,048)
Current portion of operating lease liabilities				(36,506)
Unearned revenue				(739,278)
Non-current portion of operating lease liabilities				(147,204)
Deferred tax liabilities				(1,119,762)
Other non-current liabilities				(204,291)
Net assets acquired				$ 12,041,971

Operating Leases - Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease costs	$ 46,820	$ 53,880	$ 51,200
Income from sub-leases	(1,103)	(1,165)	(1,338)
Net operating lease costs	$ 45,717	$ 52,715	$ 49,862

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Other Undenominated Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Beginning balance (in shares) at Dec. 31, 2020		52,788,093
Beginning balance at Dec. 31, 2020	$ 1,850,236	$ 4,580	$ 617,104	$ 1,134	$ (35,477)	$ 1,262,895
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	153,185					153,185
Currency translation adjustment	(60,617)				(60,617)
Actuarial gain on defined benefit pension plan	4,266				4,266
Amortization of cash flow hedge	113				113
Gain on cash flow hedge	778				778
Total comprehensive income	97,725
Exercise of share options (in shares)		1,065,529
Exercise of share options	118,589	$ 77	118,512
Issue of restricted share units / performance share units (in shares)		328,634
Issue of restricted share units / performance share units	23	$ 23
Share based compensation expense	133,553		133,553
Share issuance costs	(853)		(853)
Issue of shares associated with a business combination (in shares)		27,372,427
Repurchase of ordinary shares	5,658,155	$ 1,960	5,656,195
Replacement share-based awards issued to acquiree employees	209,399		209,399
Ending balance (in shares) at Dec. 31, 2021		81,554,683
Ending balance at Dec. 31, 2021	8,066,827	$ 6,640	6,733,910	1,134	(90,937)	1,416,080
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	505,304					505,304
Currency translation adjustment	(89,530)				(89,530)
Actuarial gain on defined benefit pension plan	12,657				12,657
Amortization of cash flow hedge	0
Gain on cash flow hedge	(3,728)				(3,728)
Total comprehensive income	424,703
Exercise of share options (in shares)		348,286
Exercise of share options	35,828	$ 21	35,807
Issue of restricted share units / performance share units (in shares)		241,116
Issue of restricted share units / performance share units	16	$ 16
Share based compensation expense	70,606		70,606
Share issuance costs	(17)		(17)
Repurchase of ordinary shares (in shares)		(420,530)
Repurchase of ordinary shares	(99,983)	$ (28)		28		(99,983)
Share repurchase costs	$ (17)					(17)
Ending balance (in shares) at Dec. 31, 2022	81,723,555	81,723,555
Ending balance at Dec. 31, 2022	$ 8,497,963	$ 6,649	6,840,306	1,162	(171,538)	1,821,384
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	612,335					612,335
Currency translation adjustment	26,221				26,221
Actuarial gain on defined benefit pension plan	244				244
Amortization of cash flow hedge	0
Gain on cash flow hedge	1,567				1,567
Total comprehensive income	640,367
Exercise of share options (in shares)		535,705
Exercise of share options	50,958	$ 35	50,923
Issue of restricted share units / performance share units (in shares)		235,826
Issue of restricted share units / performance share units	15	$ 15
Share based compensation expense	51,456		51,456
Share issuance costs	$ (16)		(16)
Ending balance (in shares) at Dec. 31, 2023	82,495,086	82,495,086
Ending balance at Dec. 31, 2023	$ 9,240,743	$ 6,699	$ 6,942,669	$ 1,162	$ (143,506)	$ 2,433,719

Restructuring charges - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 45,390	$ 31,143	$ 31,105
Restructuring reserve	6,999	$ 6,022	$ 10,311
Lease Liability
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	3,000
Restructuring reserve, current	1,000
Restructuring reserve, noncurrent	2,000
Workforce reductions
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	4,000
2021 Restructuring Plan
Restructuring Cost and Reserve [Line Items]
Restructuring charges	45,400
Severance costs	34,100
Other restructuring costs	$ 11,300

Business Segment and Geographical Information

12 Months Ended

Dec. 31, 2023

Segment Reporting [Abstract]

Business Segment and Geographical Information

The Company has the expertise and capability to conduct clinical trials in most major therapeutic areas on a global basis and has the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full-service" solution. The Company has expanded through internal growth, together with a number of strategic acquisitions to enhance its expertise and capabilities in certain areas of the clinical development process.

The Company determines and presents operating segments based on the information that is internally provided to the chief operating decision maker, the (‘CODM’) in accordance with ASC 280, Segment Reporting. The Company determined that the CODM was comprised of the Chief Executive Officer and the Chief Financial Officer.

The Company operates as one reportable segment, which is the provision of outsourced development services on a global basis to the pharmaceutical, biotechnology and medical devices industries.

Revenues are allocated to individual entities based on where the work is performed in accordance with the Company's global transfer pricing model. Revenues and income from operations in Ireland are a function of our global contracting model and the Group’s transfer pricing model.

ICON Ireland acts as the Group entrepreneur under the Company’s global transfer pricing model given its role in the development and management of the Group, its ownership of key intellectual property and customer relationships, its key role in the mitigation of risks faced by the Group and its responsibility for maintaining the Company’s global network. ICON Ireland enters into the majority of the Company’s customer contracts.

ICON Ireland remunerates other operating entities in the Group on the basis of an arm’s length return for the services they perform in each of their local territories. The arm’s length return for each ICON entity is established to ensure that each of ICON Ireland and the ICON entities that are involved in the conduct of services for customers, earn an appropriate return having regard to the assets owned, risks borne, and functions performed by each entity from these intercompany transactions. The arm’s length return is reviewed annually to ensure that it is market appropriate. The integration of entities acquired through the Merger into this global network and global transfer pricing model has been completed.

The geographic split of revenue disclosed for each region outside Ireland is the arm’s length revenue attributable to these entities. The residual revenues of the Group, once each ICON entity has been paid its respective intercompany service fee, generally fall to be retained by ICON Ireland. As such, revenues and income from operations in Ireland are a function of this global transfer pricing model and comprise revenues of the Group after deducting the arm’s length revenues attributable to the activities performed outside Ireland.

There have been no changes to the overall basis of segmentation or the measurement basis for the segment results since the prior year.

Reportable geographic information at December 31, 2023 and December 31, 2022 and for the years ended December 31, 2023, December 31, 2022 and December 31, 2021 is as follows:

a) The distribution of revenue by geographical area was as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021

	(in thousands)
Ireland	$	2,377,104	$	1,984,567	$	1,365,909
Rest of Europe	1,574,783		1,618,350		1,175,515
U.S.	3,283,790		3,574,610		2,581,007
Other	884,499		563,859		358,395
Total	$	8,120,176	$	7,741,386	$	5,480,826

b) The distribution of income from operations (including Restructuring) by geographical area was as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Ireland *	$	432,026	$	218,088	$	131,961
Rest of Europe	153,450		253,799		177,863
U.S.	310,343		254,849		39,132
Other	60,333		68,501		29,573
Total	$	956,152	$	795,237	$	378,529

* Includes the full amount of the amortization charge associated with the intangible asset acquired in the Merger.

c) The distribution of long-lived assets (property, plant and equipment and operating right-of-use assets), net, by geographical area was as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Ireland	$	199,051	$	143,025
Rest of Europe	94,046		99,721
U.S.	159,245		213,311
Other	49,175		48,095
Total	$	501,517	$	504,152

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-15

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 31
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-31

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-42

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-40

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//280/tableOfContent

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 26
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-26

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-34

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-41

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-21

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-21

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

+ Details

Net Income Per Ordinary Share - Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares)	82,101,813	81,532,320	67,110,186
Effect of dilutive share options and other awards outstanding under share based compensation programs (in shares)	615,827	936,043	958,125
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares)	82,717,640	82,468,363	68,068,311

Employee Benefits - Funded Status (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
United Kingdom \| ICON Development Solutions Limited pension plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ (20,999)	$ (19,558)
Fair value of plan assets	27,320	26,050	$ 36,198
Funded surplus	6,321	6,492
Switzerland \| Aptiv Solutions pension plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	(6,441)	(5,806)
Fair value of plan assets	6,261	5,681	6,964
Funded surplus	(180)	(125)
Switzerland \| PRA Switzerland AG pension plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	(7,747)	(5,345)
Fair value of plan assets	5,529	4,059	$ 3,017
Funded surplus	$ (2,218)	$ (1,286)

- Definition

+ References + Details

Summary of Significant accounting policies - Adjustments Resulting from Foreign Currency Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Foreign Currency [Abstract]
Amounts charged/(credited)	$ 12,916	$ (25,997)	$ (14,316)

Restructuring charges - Schedule of Restructuring Reserve by Type of Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Restructuring Reserve [Roll Forward]
Opening provision	$ 6,022	$ 10,311
Additional provision in the year	36,704	4,364
Utilization	(35,727)	(8,653)
Ending provision	$ 6,999	$ 6,022

Accumulated Other Comprehensive Loss (Tables)

12 Months Ended

Dec. 31, 2023

Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]

Schedule of Accumulated Other Comprehensive Income


	Year ended
	December 31, 2023		December 31, 2022
	(in thousands)
Currency translation adjustments	$	(149,148)	$	(175,369)
Actuarial gain on defined benefit pension plan	7,803		7,559
Loss on cash flow hedge	(2,161)		(3,728)
Total	$	(143,506)	$	(171,538)

Other Liabilities

12 Months Ended

Dec. 31, 2023

Other Liabilities Disclosure [Abstract]

Other Liabilities

The carrying amount of Other Liabilities for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
General trade and overhead liabilities*	$	463,882	$	530,204
Personnel related liabilities	385,499		395,862
Operating lease liabilities	36,414		43,657
Facility related liabilities	11,078		16,896
Other liabilities	13,532		12,852
Restructuring liabilities	4,951		5,512
Short term government grants	43		42
	$	915,399	$	1,005,025

*includes amounts due to third parties in respect of accrued reimbursable investigator expenses of $333.0 million at December 31, 2023 and $406.3 million at December 31, 2022.

Net Income Per Ordinary Share (Tables)

12 Months Ended

Dec. 31, 2023

Earnings Per Share [Abstract]

Reconciliation of number of shares used in computation of basic and diluted net income per ordinary share

The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows:


	Year ended
	December 31, 2023	December 31, 2022	December 31, 2021
Weighted average number of ordinary shares outstanding for basic net income per ordinary share	82,101,813	81,532,320	67,110,186
Effect of dilutive share options and other awards outstanding under share based compensation programs	615,827	936,043	958,125
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share	82,717,640	82,468,363	68,068,311

Reconciliation of net income attributable to the group per the statement of operating income and net income used for net income per ordinary share








	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
Net income per ordinary share:
Basic	$	7.46	$	6.20	$	2.28
Diluted	$	7.40	$	6.13	$	2.25

Summary of Significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Adjustments resulting from foreign currency translations

Amounts charged or credited to the Consolidated Statements of Operations for the years ended December 31, 2023, December 31, 2022 and December 31, 2021 were as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Amounts charged/(credited)	$	12,916	$	(25,997)	$	(14,316)

Estimated useful lives of assets

Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below:


			Estimated Useful Life
Buildings			40 years
Computer equipment and software			2 - 8 years
Office furniture and fixtures			8 years
Laboratory equipment			5 years
Motor vehicles			5 years

Schedule of Finite-Lived Intangible Assets

Intangible assets are amortized on a straight-line basis over their estimated useful lives, as set forth in the table below:


			Estimated Useful Life
Customer relationships			16 - 23 years
Order backlog			3 years
Trade names			3 years
Patient database			7 years
Technology assets			5 years

The carrying amount of Intangible Assets for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
Cost	(in thousands)
Customer relationships	$	4,090,393	$	4,076,435
Order backlog	541,302		536,934
Trade names & brands	204,653		204,621
Patient database	170,366		170,238
Technology assets	141,257		120,984
Total cost	5,147,971		5,109,212

Accumulated amortization	(1,292,106)		(830,553)
Net book value	$	3,855,865	$	4,278,659

Summary of Significant accounting policies - Estimated Useful Lives of Property Plant and Equipment (Details)	Dec. 31, 2023
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Office furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Motor vehicles
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Minimum \| Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Maximum \| Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years

Allowances for Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Opening provision	$ 20,562	$ 7,081
Amounts used during the year	(13,358)	(3,913)
Amounts provided during the year	24,550	17,800
Foreign exchange	(221)	(406)
Closing provision	$ 31,533	$ 20,562

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company has evaluated subsequent events from the Balance Sheet date through February 23, 2024, the date at which the consolidated financial statements were available to be issued. On February 20, 2024, the Company's Board of Directors authorized a new buyback program of up to $500 million of the outstanding ordinary shares of the Company. All ordinary shares that are redeemed under the buyback program will be canceled in accordance with the constitutional documents of the Company and the nominal value of these shares transferred to an undenominated capital fund as required under Irish Company law. Repurchases under the share buyback program may be effected from time to time in open market or privately negotiated transactions in accordance with agreed terms and limitations. The timing and amount of the repurchase transactions under this program will depend on a variety of factors, including market conditions and corporate and regulatory considerations. Depending upon results of operations, market conditions and the development of the economy, as well as other factors, generally we will consider share repurchases on an opportunistic basis from time to time. The Company has determined that there are no other items to disclose.

Non-current other liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Defined benefit pension obligations, net (note 21)	$ 14,738	$ 13,033
Other non-current liabilities	31,260	25,227
Other Liabilities, Noncurrent	$ 45,998	$ 38,260

Business Combinations - Narrative, PRA Acquisition (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Jul. 01, 2021 Employee $ / shares shares	Dec. 31, 2023 USD ($) Employee	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Number of employees \| Employee	38,000	41,100
Integration related costs		$ 44,176	$ 39,695	$ 198,263
Total financing costs		336,699	229,731	182,423
PRA Health Sciences, Inc
Business Acquisition [Line Items]
Business acquisition, share price (in USD per share) \| $ / shares	$ 80
Business acquisition, number of shares issued (in shares) \| shares	0.4125
Total financing costs		$ 16,400	$ 17,700	$ 86,700

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References + Details

Fair Value - Measurement of Fair Value of Major Asset and Liability classed (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
ASSETS
Available for sale investments (Note 8a)	$ 1,954	$ 1,713
Available for sale investments (long-term)	46,804	32,631
Total assets	48,758	34,356
Liabilities:
Total Liabilities	2,411	3,670
Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
ASSETS
Derivative instruments	0	12
Liabilities:
Derivative instruments	2,411	3,670
Level 1
ASSETS
Available for sale investments (Note 8a)	1,954	1,713
Available for sale investments (long-term)	0	0
Total assets	1,954	1,713
Liabilities:
Total Liabilities	0	0
Level 1 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
ASSETS
Derivative instruments	0
Liabilities:
Derivative instruments	0	0
Level 2
ASSETS
Available for sale investments (Note 8a)	0	0
Available for sale investments (long-term)	0	0
Total assets	0	12
Liabilities:
Total Liabilities	2,411	3,670
Level 2 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
ASSETS
Derivative instruments	0	12
Liabilities:
Derivative instruments	2,411	3,670
Level 3
ASSETS
Available for sale investments (Note 8a)	0	0
Available for sale investments (long-term)	0	0
Total assets	0	0
Liabilities:
Total Liabilities	0	0
Level 3 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
ASSETS
Derivative instruments	0
Liabilities:
Derivative instruments	0	0
Investments Measured at Net Asset Value
ASSETS
Available for sale investments (Note 8a)	0	0
Available for sale investments (long-term)	46,804	32,631
Total assets	46,804	32,631
Liabilities:
Total Liabilities	0	0
Investments Measured at Net Asset Value \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
ASSETS
Derivative instruments	0
Liabilities:
Derivative instruments	$ 0	$ 0

Subsequent Events (Details) $ in Millions	Feb. 20, 2024 USD ($)
Subsequent event \| Buyback program
Subsequent Event [Line Items]
Stock repurchase program, authorized amount	$ 500

Business Segment and Geographical Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 segment
Segment Reporting [Abstract]
Number of reporting segments	1

Income Taxes - Expected Expiry Dates of NOL's (Details) - United States $ in Thousands	Dec. 31, 2023 USD ($)
State NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, not subject to expiration	$ 9,251
Net operating loss carryforwards	331,370
State NOL's \| 2024-2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	271,434
State NOL's \| 2038-2042
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	50,685
Federal NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, not subject to expiration	0
Net operating loss carryforwards	1,744
Federal NOL's \| 2024-2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	1,744
Federal NOL's \| 2038-2042
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	$ 0

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Summary of Components of Income Before Provision for Income Taxes

The components of income before income tax expense are as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Ireland	$	624,642	$	432,963	$	231,893
United States	(352,532)		(270,440)		(278,413)
Other	352,357		405,328		243,200
Income before income tax expense	$	624,467	$	567,851	$	196,680

Summary of Components of Provision for Income Taxes

The components of income tax expense are as follows:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Income tax expense:
Current tax (benefit)/expense:
Ireland	$	80,427	$	53,248	$	18,469
United States	(42,428)		60,753		35,478
Other	59,153		70,395		48,003
Total current tax expense	97,152		184,396		101,950

Deferred tax (benefit)/expense:
Ireland	45,253		(6,166)		553
United States	(143,323)		(118,475)		(52,717)
Other	12,667		(344)		(8,452)
Total deferred tax benefit	(85,403)		(124,985)		(60,616)

Income tax expense allocated to continuing operations	11,749		59,411		41,334

Income tax expense was allocated to the following components of other comprehensive income:

Currency impact on long term funding	(3,903)		7,211		1,776

Cash flow hedge	301		—		—
Total	$	8,147	$	66,622	$	43,110

Schedule of Reconciliation of Consolidated Reported Provision for Income Taxes and Statutory Rate

The Company's consolidated reported income tax expense differed from the amount that would result from applying the Irish statutory rate as set forth below:


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Taxes at Irish statutory rate of 12.5% (2022:12.5%; 2021:12.5%)	$	78,058	$	70,980	$	24,586
Rate differential from amortization of intangible assets	(71,223)		(59,330)		(31,228)
Foreign and other income taxed at higher rates	35,778		52,464		51,273
Research & development tax incentives	(3,868)		(2,608)		(3,120)
Movement in valuation allowance	(1,068)		(777)		3,101
Effects of change in tax rates	3,154		(300)		(128)
(Decrease)/ increase in unrecognized tax benefits	(54,347)		8,392		5,246
Investor tax expense on foreign subsidiary earnings	39,165		—		—
Impact of stock compensation	(11,487)		(8,756)		(9,083)

Other	(2,413)		(654)		687
Income tax expense	$	11,749	$	59,411	$	41,334

Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities

The tax effects of temporary differences and carryforwards that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below:


	December 31, 2023		December 31, 2022
	(in thousands)
Deferred tax liabilities:
Property, plant and equipment	$	7,547	$	10,927
Operating right-of-use-assets	16,108		23,260
Goodwill	39,014		37,150
Intangible assets	950,055		1,078,302
Investments in foreign subsidiaries	52,408		1,587
Other	7,982		7,467
Total deferred tax liabilities recognized	1,073,114		1,158,693

Deferred tax assets:
Operating loss and tax credits carryforwards	124,150		88,697
Property, plant and equipment	9,082		6,010
Operating lease liabilities	20,190		27,593
Intangible assets	2,166		3,602
Stock compensation	17,605		21,862
Other liabilities	84,928		66,933
Unearned revenue	23,748		66,565
Other	8,774		9,155
Total deferred tax assets	290,643		290,417
Valuation allowance for deferred tax assets	(42,967)		(43,379)
Deferred tax assets recognized	247,676		247,038
Overall net deferred tax liability	$	(825,438)	$	(911,655)

Schedule of Expected Expiry Dates of NOL's

The expected expiry dates of the US NOLs are as follows:


	Federal NOL's		State NOL's
	(in thousands)
2024-2037	$	1,744	$	271,434
2038-2042	—		50,685
Indefinite	—		9,251
	$	1,744	$	331,370

Schedule of Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows:


	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Unrecognized tax benefits at start of year	$	217,584	$	202,065	$	19,078
Increase related to acquired tax positions	—		—		170,047
Increase related to prior year tax positions	1,161		16,098		204
Decrease related to prior year tax positions	(918)		(5,442)		(1,695)
Increase related to current year tax positions	4,552		5,701		18,613
Settlements	—		—		(844)
Lapse of statute of limitations	(62,363)		(838)		(3,338)
Unrecognized tax benefits at end of year	$	160,016	$	217,584	$	202,065

Supplemental Disclosure of Cash Flow Information

12 Months Ended

Dec. 31, 2023

Supplemental Cash Flow Elements [Abstract]

Supplemental Disclosure of Cash Flow Information


	Year ended
	December 31, 2023		December 31, 2022		December 31, 2021
	(in thousands)
Cash paid for interest	$	317,975	$	210,918	$	106,205
Cash paid for income taxes (net of refunds)	$	163,778	$	116,322	$	55,105

Goodwill (Tables)

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Goodwill

The change in the carrying amount of goodwill for the years ended December 31, 2023 and 2022 is as follows:


	December 31, 2023		December 31, 2022
	(in thousands)
Opening goodwill	$	8,971,670	$	9,037,931
Current year acquisitions (note 6)	36,750		—
Prior period acquisition (note 6)	—		(35,692)
Foreign exchange movement	13,655		(30,569)
Closing goodwill	$	9,022,075	$	8,971,670

- Definition

Tabular disclosure of goodwill by reportable segment and in total which includes a rollforward schedule.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
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+ Details