ICON Public Limited (ICLR) 6-K25 Apr 242024 Q1 Third Amendment to Credit AgreementFinancial data

Company Profile

Cover

Cover	3 Months Ended
Cover	Mar. 31, 2024
Cover [Abstract]
Document Type	6-K
Document Fiscal Year Focus	2024
Entity Registrant Name	ICON plc
Amendment Flag	false
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001060955
Current Fiscal Year End Date	--12-31
Document Period End Date	Mar. 31, 2024

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 396,082	$ 378,102
Available for sale investments	1,955	1,954
Accounts receivable, net of allowance for credit losses	1,862,468	1,790,322
Unbilled revenue	993,141	951,936
Other receivables	76,638	65,797
Prepayments and other current assets	149,127	132,105
Income taxes receivable	75,544	91,254
Total current assets	3,554,955	3,411,470
Non-current assets:
Property, plant and equipment	353,302	361,184
Goodwill	9,018,301	9,022,075
Intangible assets	3,748,939	3,855,865
Operating right-of-use assets	159,620	140,333
Other receivables	87,193	78,470
Deferred tax asset	74,777	73,662
Investments in equity- long term	51,164	46,804
Total Assets	17,048,251	16,989,863
Current liabilities:
Accounts payable	113,579	131,584
Unearned revenue	1,709,938	1,654,507
Other liabilities	1,000,760	915,399
Income taxes payable	41,905	13,968
Current bank credit lines and loan facilities	79,762	110,150
Total current liabilities	2,945,944	2,825,608
Non-current liabilities:
Non-current bank credit lines and loan facilities	3,419,734	3,665,439
Lease liabilities	145,542	126,321
Non-current other liabilities	45,988	45,998
Non-current income taxes payable	188,876	186,654
Deferred tax liability	869,520	899,100
Commitments and contingencies	0	0
Total Liabilities	7,615,604	7,749,120
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 82,683,805 shares issued and outstanding at March 31, 2024 and 82,495,086 shares issued and outstanding at December 31, 2023	6,711	6,699
Additional paid-in capital	6,970,079	6,942,669
Other undenominated capital	1,162	1,162
Accumulated other comprehensive loss	(166,462)	(143,506)
Retained earnings	2,621,157	2,433,719
Total Shareholders' Equity	9,432,647	9,240,743
Total Liabilities and Shareholders' Equity	$ 17,048,251	$ 16,989,863
Ordinary shares, shares outstanding (in shares)	82,683,805	82,495,086
Common Stock, Shares, Issued	82,683,805	82,495,086

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - € / shares	Mar. 31, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Ordinary shares, par value (in EUR cents per share)	€ 0.06	€ 0.06
Ordinary shares, shares authorized (in shares)	100,000,000	100,000,000
Ordinary shares, shares issued (in shares)	82,683,805	82,495,086
Ordinary shares, shares outstanding (in shares)	82,683,805	82,495,086

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Income Statement [Abstract]
Revenue	$ 2,090,386	$ 1,978,578
Costs and expenses:
Direct costs (excluding depreciation and amortization)	1,471,367	1,395,546
Selling, general and administrative	177,350	200,006
Depreciation and amortization	149,181	145,126
Transaction and integration related	6,991	11,382
Restructuring	0	9,729
Total costs and expenses	1,804,889	1,761,789
Income from operations	285,497	216,789
Interest income	1,930	1,072
Interest expense	(71,665)	(86,551)
Income before income tax expense	215,762	131,310
Income tax expense	(28,324)	(14,273)
Income before share of losses from equity method investments	187,438	117,037
Share of losses from equity method investments	0	(383)
Net income	$ 187,438	$ 116,654
Net income per Ordinary Share (note 13):
Basic (in USD per share)	$ 2.27	$ 1.43
Diluted (in USD per share)	$ 2.25	$ 1.41
Weighted average number of Ordinary Shares outstanding (note 13):
Basic (in shares)	82,579,203	81,784,389
Diluted (in shares)	83,249,303	82,605,659

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Income (Loss) Attributable to Parent [Abstract]
Net income	$ 187,438	$ 116,654
Other comprehensive income, net of tax:
Currency translation adjustment	(32,040)	15,500
Gain / (loss) on cash flow hedge	9,084	(3,858)
Total comprehensive income	$ 164,482	$ 128,296

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) - 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, 2022		81,723,555
Beginning 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	116,654					116,654
Currency translation adjustment	15,500				15,500
Gain / (loss) on cash flow hedge	(3,858)				(3,858)
Total comprehensive income	128,296
Exercise of share options (in shares)		136,649
Exercise of share options	12,936	$ 9	12,927
Issue of restricted share units / performance share units (in shares)		68,218
Issue of restricted share units / performance share units	4	$ 4
Shared based compensation expense	14,658		14,658
Share issuance costs	(4)		(4)
Ending balance (in shares) at Mar. 31, 2023		81,928,422
Ending balance at Mar. 31, 2023	$ 8,653,853	$ 6,662	6,867,887	1,162	(159,896)	1,938,038
Beginning balance (in shares) at Dec. 31, 2023	82,495,086	82,495,086
Beginning balance at Dec. 31, 2023	$ 9,240,743	$ 6,699	6,942,669	1,162	(143,506)	2,433,719
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	187,438
Currency translation adjustment	(32,040)				(32,040)
Gain / (loss) on cash flow hedge	9,084				9,084
Total comprehensive income	164,482
Exercise of share options (in shares)		156,527
Exercise of share options	16,303	$ 10	16,293
Issue of restricted share units / performance share units (in shares)		32,192
Issue of restricted share units / performance share units	2	$ 2
Shared based compensation expense	11,121		11,121
Share issuance costs	$ (4)		(4)
Ending balance (in shares) at Mar. 31, 2024	82,683,805	82,683,805
Ending balance at Mar. 31, 2024	$ 9,432,647	$ 6,711	$ 6,970,079	$ 1,162	$ (166,462)	$ 2,621,157

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Cash flows from operating activities:
Net income	$ 187,438	$ 116,654
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization expense	149,181	145,126
Impairment of long lived assets	0	6,933
Reduction in carrying value of operating right-of-use assets	11,472	11,304
Loss on equity method investments	0	383
Amortization of financing costs and debt discount	3,907	4,497
Stock compensation expense	13,181	14,759
Deferred tax benefit	(35,145)	(43,823)
Unrealized foreign exchange movements	(1,027)	(1,319)
Other non-cash items	4,629	17,534
Changes in operating assets and liabilities:
Accounts receivable	(80,431)	(93,259)
Unbilled revenue	(42,546)	(18,346)
Unearned revenue	56,485	80,990
Other net assets	59,932	(65,933)
Net cash provided by operating activities	327,076	175,500
Cash flows from investing activities:
Purchase of property, plant and equipment	(27,152)	(26,744)
Purchase of subsidiary undertakings (net of cash acquired)	(7,831)	0
Sale of available for sale investments	1,005	482
Purchase of available for sale investments	(1,006)	(422)
Purchase of investments in equity - long term	(4,381)	(1,358)
Net cash used in investing activities	(39,365)	(28,042)
Cash flows from financing activities:
Drawdown of credit lines and loan facilities	50,000	180,000
Repayment of credit lines and loan facilities	(330,000)	(350,000)
Proceeds from exercise of equity compensation	16,305	12,940
Share issue costs	(4)	(4)
Net cash used in financing activities	(263,699)	(157,064)
Effect of exchange rate movements on cash	(6,032)	718
Net increase / (decrease) in cash and cash equivalents	17,980	(8,888)
Cash and cash equivalents at beginning of period	378,102	288,768	$ 288,768
Cash and cash equivalents at end of period	$ 396,082	$ 279,880	$ 378,102

Basis of presentation

Basis of presentation	3 Months Ended
Basis of presentation	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation These condensed consolidated financial statements which have been prepared in accordance with United States Generally Accepted Accounting Principles (“US GAAP”) have not been audited. The condensed consolidated financial statements reflect all adjustments, which are, in the opinion of management, necessary to present a fair statement of the operating results and financial position for the periods presented. The preparation of the condensed consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect reported amounts and disclosures in the condensed consolidated financial statements. Actual results could differ from those estimates. The condensed consolidated financial statements should be read in conjunction with the accounting policies and notes to the consolidated financial statements included in ICON’s Form 20-F for the year ended December 31, 2023 (see note 2 - Summary of significant accounting policies ). Operating results for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the fiscal period ending December 31, 2024.

Summary of significant accounti

Summary of significant accounting policies	3 Months Ended
Summary of significant accounting policies	Mar. 31, 2024
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies 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. 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. 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.

Revenue

Revenue	3 Months Ended
Revenue	Mar. 31, 2024
Revenue from Contract with Customer [Abstract]
Revenue	Revenue Revenue disaggregated by customer concentration is as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Clients 1-5 $ 543,960 $ 570,260 Clients 6-10 321,259 271,696 Clients 11-25 430,323 415,381 Other 794,844 721,241 Total $ 2,090,386 $ 1,978,578 There was no revenue from individual customers greater than 10% of consolidated revenue in the respective periods. Accounts receivables and unbilled revenue are as follows: March 31, 2024 December 31, 2023 (in thousands) Billed services (accounts receivable) $ 1,897,771 $ 1,821,855 Allowance for credit losses (35,303) (31,533) Accounts receivable (net) 1,862,468 1,790,322 Unbilled services (unbilled revenue) 993,141 951,936 Accounts receivable and unbilled revenue, net $ 2,855,609 $ 2,742,258 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) March 31, 2024 December 31, 2023 $ Change % Change Unbilled services (unbilled revenue) $ 993,141 $ 951,936 $ 41,205 4.3 % Unearned revenue (payments on account) (1,709,938) (1,654,507) (55,431) 3.4 % Net balance $ (716,797) $ (702,571) $ (14,226) 2.0 % 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 services and therefore contract assets rather than accounts receivables 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 to set-off between unbilled services (contract assets) and unearned revenue (contract liabilities), each are separately presented gross on the Condensed Consolidated Balance Sheet. Unbilled services as at March 31, 2024 increased by $41.2 million compared to December 31, 2023. Unearned revenue increased by $55.4 million over the same period resulting in an increase of $14.2 million in the net balance of unbilled services and unearned revenue or payments on account between December 31, 2023 and March 31, 2024. These fluctuations are primarily due to timing of payments and invoicing related to the Group'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. As of March 31, 2024 approximately $15.3 billion (March 31, 2023: $13.8 billion) of revenue is expected to be recognized in the future in respect of unsatisfied performance obligations. The Company expects to recognize revenue on approximately 51% of the unsatisfied performan

Accounts receivable, unbilled r

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities)	3 Months Ended
	Mar. 31, 2024
Revenue from Contract with Customer [Abstract]
Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities)	Revenue Revenue disaggregated by customer concentration is as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Clients 1-5 $ 543,960 $ 570,260 Clients 6-10 321,259 271,696 Clients 11-25 430,323 415,381 Other 794,844 721,241 Total $ 2,090,386 $ 1,978,578 There was no revenue from individual customers greater than 10% of consolidated revenue in the respective periods. Accounts receivables and unbilled revenue are as follows: March 31, 2024 December 31, 2023 (in thousands) Billed services (accounts receivable) $ 1,897,771 $ 1,821,855 Allowance for credit losses (35,303) (31,533) Accounts receivable (net) 1,862,468 1,790,322 Unbilled services (unbilled revenue) 993,141 951,936 Accounts receivable and unbilled revenue, net $ 2,855,609 $ 2,742,258 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) March 31, 2024 December 31, 2023 $ Change % Change Unbilled services (unbilled revenue) $ 993,141 $ 951,936 $ 41,205 4.3 % Unearned revenue (payments on account) (1,709,938) (1,654,507) (55,431) 3.4 % Net balance $ (716,797) $ (702,571) $ (14,226) 2.0 % 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 services and therefore contract assets rather than accounts receivables 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 to set-off between unbilled services (contract assets) and unearned revenue (contract liabilities), each are separately presented gross on the Condensed Consolidated Balance Sheet. Unbilled services as at March 31, 2024 increased by $41.2 million compared to December 31, 2023. Unearned revenue increased by $55.4 million over the same period resulting in an increase of $14.2 million in the net balance of unbilled services and unearned revenue or payments on account between December 31, 2023 and March 31, 2024. These fluctuations are primarily due to timing of payments and invoicing related to the Group'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. As of March 31, 2024 approximately $15.3 billion (March 31, 2023: $13.8 billion) of revenue is expected to be recognized in the future in respect of unsatisfied performance obligations. The Company expects to recognize revenue on approximately 51% of the unsatisfied performan

Goodwill

Goodwill	3 Months Ended
Goodwill	Mar. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill March 31, 2024 December 31, 2023 (in thousands) Opening balance $ 9,022,075 $ 8,971,670 Current period acquisition 2,732 36,750 Foreign exchange movement (6,506) 13,655 Closing balance $ 9,018,301 $ 9,022,075 On January 9, 2024, the Company acquired HumanFirst Inc. ("HumanFirst"), a life sciences technology company in exchange for consideration of $13.3 million. The net cash outflow was $7.8 million comprising initial cash payments of $11.8 million net of cash acquired of $4.0 million. $1.5 million of deferred consideration remains unpaid as of March 31, 2024. The purchase price allocation, as of the date of acquisition, was based on a preliminary valuation and may be subject to revision. Preliminarily, the HumanFirst acquisition resulted in the initial recognition of goodwill of $2.7 million and a developed technology intangible asset of $9.9 million . Preliminary goodwill arising in connection with the acquisition is primarily attributable to the assembled workforce of Humanfirst. The intangible asset will be amortized over its useful life of 5 years and in total, $0.5 million has been amortized in the period since the date of acquisition. There were no impairment charges for the three months ended March 31, 2024 or the year ended December 31, 2023.

Intangible assets

Intangible assets	3 Months Ended
Intangible assets	Mar. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets The carrying amount of Intangible Assets for the periods ended March 31, 2024 and December 31, 2023 is as follows: March 31, 2024 December 31, 2023 (in thousands) Cost Customer relationships $ 4,089,517 $ 4,090,393 Order backlog 540,921 541,302 Trade names & brands 204,629 204,653 Patient database 170,345 170,366 Technology assets 151,000 141,257 Total cost 5,156,412 5,147,971 Accumulated amortization (1,407,473) (1,292,106) Net book value $ 3,748,939 $ 3,855,865 In the three months ended March 31, 2024, the amortization expense recognized by the Company was $116.5 million (year ended December 31, 2023: $459.9 million ) . On January 9, 2024, the Company acquired HumanFirst. The acquisition resulted in the recognition of a developed technology intangible asset of $9.9 million . Refer to note 5. Goodwill for further details. The identifiable intangible assets are amortized over their estimated useful lives

Fair value measurements

Fair value measurements	3 Months Ended
Fair value measurements	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Fair value measurements	Fair value measurements 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 March 31, 2024, 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) $ 1,955 $ — $ — $ — $ 1,955 Available for sale investments (long-term) (b) — — — 51,164 51,164 Derivative instruments (c) — 8,225 — — 8,225 Total assets $ 1,955 $ 8,225 $ — $ 51,164 $ 61,344 Liabilities: Derivative instruments (c) — 270 — — 270 Total liabilities $ — $ 270 $ — $ — $ 270 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 Total assets $ 1,954 $ — $ — $ 46,804 $ 48,758 Liabilities: Derivative instruments (c) — 2,411 — — 2,411 Total liabilities $ — $ 2,411 $ — $ — $ 2,411 (a) Represents the fair value of investments in highly liquid investments with maturities of greater than three months and a minimum "A-" rated fixed term deposits and are based on quoted market prices. (b) To determine the classification of its 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 meet the definition of equity securities without readily determinable fair values, which qualify for the Net Asset Value (NAV) practical expedient in ASC 820 'Fair value measurements'. Any increases or decreases in fair value are recognized in net income in the period. (c) R epresents the fair value of the interest rate caps and the interest rate swap. Non-recurring Fair Value Measurements Certain assets and liabilities are carried on the accompanying Condensed 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 March 31, 2024, assets carried on the balance sheet at cost and not re-measured to fair value on a recurring basis totaled $12,767.2 million (December 31, 2023: $12,878.0 million ) an d are identified as Level 3 assets. These assets are comprised of goodwi ll of $9,018.3 million (December 31, 2023: $9,022.1 million) and net identifiable intangible assets of $3,748.9 million (December 31, 2023: $3,855.9 million). The estimated fair value of the Company’s debt was $3,501.4 million a t March 31, 2024 (December 31, 2023: $3,793.5 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.

Restructuring

Restructuring	3 Months Ended
Restructuring	Mar. 31, 2024
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring The Company did not incur a restructuring charge in the three months ended March 31, 2024 . In the three months ended March 31, 2023, a restructuring charge of $9.7 million was recorded in the Condensed Consolidated Statement of Operations under a restructuring plan adopted following a review of operations. The restructuring plan reflected an office consolidation program to optimize the Company's office footprint. Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Restructuring charges $ — $ 9,729 Total $ — $ 9,729 At March 31, 2024, a total liability of $3.1 million was recorded on the Consolidated Balance Sheet relating to restructuring activities. The total liability included $0.5 million of personnel related liabilities as a result of the workforce reduction; all of which have been classified as short-term. The total liability also included $2.6 million of facilities related liabilities of which $0.8 million is included within other liabilities and $1.8 million is included within non-current other liabilities. Three Months Ended Year March 31, 2024 December 31, 2023 (in thousands) Opening provision $ 6,999 $ 6,022 Charge during the period — 36,704 Utilization (3,853) (35,727) Ending provision $ 3,146 $ 6,999

Operating leases

Operating leases	3 Months Ended
Operating leases	Mar. 31, 2024
Leases [Abstract]
Operating leases	Operating leases Lease costs recorded under operating leases for three months ended March 31, 2024 and March 31, 2023 were as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Operating lease costs 13,163 12,871 Income from sub-leases (336) (278) Net operating lease costs 12,827 12,593 Of the total cost of $12.8 million incurred in the three months ended March 31, 2024 (March 31, 2023: $12.6 million), $10.0 million (March 31, 2023: $10.0 million) is recorded within selling, general and administration costs and $2.8 million (March 31, 2023: $2.6 million) is recorded within direct costs. During the three months ended March 31, 2024 and March 31, 2023, costs incurred by the Group related to variable lease payments was de minimis. Right-of-use assets obtained, in exchange for lease obligations during the three months ended March 31, 2024, totaled $35.6 million (March 31, 2023: $12.3 million). The weighted average remaining lease term and weighted-average discount rate at March 31, 2024 were 7.17 years and 3.73%, respectively. The weighted average remaining lease term and weighted-average discount rate at December 31, 2023 was 6.72 years and 3.29%, respectively . Future minimum lease payments under non-cancelable leases as of March 31, 2024 were as follows: (in thousands) Year 1 $ 41,837 Year 2 36,980 Year 3 31,766 Year 4 25,788 Year 5 17,496 Thereafter 49,864 Total future minimum lease payments 203,731 Lease imputed interest (22,135) Total $ 181,596 other liabilities

Bank credit lines and loan faci

Bank credit lines and loan facilities	3 Months Ended
Bank credit lines and loan facilities	Mar. 31, 2024
Debt Disclosure [Abstract]
Bank credit lines and loan facilities	Bank credit lines and loan facilities The Company had the following debt outstanding as of March 31, 2024 and December 31, 2023: Interest rate as of Principal amount Maturity Date March 31, 2024 December 31, 2023 March 31, 2024 December 31, 2023 (in thousands) Senior Secured Term Loan July 2028 7.302 % 7.860 % $ 2,976,213 $ 3,251,213 Senior Secured Notes July 2026 2.875 % 2.875 % 500,000 500,000 Senior Secured Revolving Loan* April 2024 6.179 % 6.720 % 50,000 55,000 Total debt 3,526,213 3,806,213 Less current portion of debt (79,762) (110,150) Total long-term debt 3,446,451 3,696,063 Less debt issuance costs and debt discount (26,717) (30,624) Total long-term debt, net $ 3,419,734 $ 3,665,439 * The final maturity date of the Senior Secured Revolving Loan facility is July 2026. As of March 31, 2024, the contractual maturities of the Company's debt obligations were as follows: Contractual maturities of debt (in thousands) 2024 (remaining) $ 79,762 2025 29,762 2026 529,762 2027 29,762 2028 and thereafter 2,857,165 Total $ 3,526,213 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. At March 31, 2024, the applicable margin is 2.0% (which reflects the Third Amendment). The senior secured term loan facility is subject to a floor of 0.50%. Reflecting the Third Amendment, 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 0.45%, 0.10% or –% based on the Company’s current corporate family rating assigned by S&P of BB (or lower), BB+ or BBB- (or higher), respectively, or (ii) Term SOFR plus a Term SOFR Adjustment on the interest period chosen plus an applicable margin of 1.45%, 1.10%, 0.85%, 0.65%, or 0.50% based on the Company’s current corporate family rating assigned by S&P of BB (or lower), BB+, BBB-, BBB or BBB+ (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. 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 quarter ended March 31, 2024 and year ended December 31, 2023 were as follows: Principal repayments (in thousands) Quarter 1, 2023 $ 250,000 Quarter 2, 2023 150,000 Quarter 3, 2023 300,000 Quarter 4, 2023 250,000 Total repayments in 2023 950,000 Quarter 1, 2024 275,000 Total repayments in 2024 $ 275,000 The voluntary repayment made during the quarter resulted in an accelerated charge associated with previously capitalized fees of $2.0 million ( March 31, 2023: $2.2 million). During the quarter ended March 31, 2024, the Company drew down $50.0 million of the senior secured revolving loan facility and repaid $55.0 million as shown below. As at March 31, 2024, $50.0 million (December 31, 2023: $55.0 million ) was drawn under the senior secured revolving loan facility. Drawdown Repayment Closing Balance (in thousands) 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 Total drawdown/repayments in 2023 370,000 315,000 Quarter 1, 2024 50,000 55,000 50,000 Total drawdown/repayments in 2024 $ 50,000 $ 55,000 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 (the "Senior Secured Notes") 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 $3,501.4 million a

Derivatives

Derivatives	3 Months Ended
Derivatives	Mar. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	Derivatives The Company has entered into interest rate cap and swap agreements for purposes of managing its exposure to interest rate fluctuations. 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 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 value of the Company’s derivative financial instruments, on a gross basis, are summarized in the following table: March 31, 2024 December 31, 2023 Asset Liability Notional Asset Liability Notional (in thousands) (in thousands) Interest Rate Caps $ — $ 270 $ 1,463,106 $ — $ 1,871 $ 1,600,606 Interest Rate Swap 8,225 — 1,100,606 — 540 1,100,606 Total derivatives designated as hedging instruments $ 8,225 $ 270 $ 2,563,712 $ — $ 2,411 $ 2,701,212 As of March 31, 2024, the Company recognized a current derivative asset of $2.9 million within other receivables non-current other receivables other liabilities other liabilities non-current other liabilities During the three months ended March 31, 2024, the Company recognized a gain of $9.1 million within other comprehensive income/loss (March 31, 2023: $3.9 million loss) after a reclassification of $0.7 million gain from other comprehensive income/loss to the income statement (March 31, 2023: $2.3 million expense). During the next 12 months, the Company estimates that an additional $2.0 million will be reflected as interest income in the consolidated statements.

Income taxes

Income taxes	3 Months Ended
Income taxes	Mar. 31, 2024
Income Tax Disclosure [Abstract]
Income taxes	Income taxes Income taxes recognized during three months ended March 31, 2024 and March 31, 2023, comprise: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Income Tax expense $ 28,324 $ 14,273 As at March 31, 2024 the Com pany maintains a $189.2 million liability (December 31, 2023: $187.1 million) for unrecognized tax benefit, which is comprised of $160.5 million (December 31, 2023: $160.0 million) related to items generating unrecognized tax benefits and $28.7 million (December 31, 2023: $27.1 million) for interest and penalties related to such items. The Company recognizes interest accrued on unrecognized tax benefits as an additional income tax ex pense. The Company has analyzed the filing positions in all of the significant federal, state and foreign jurisdictions where it is required to file income tax returns, as well as open tax years in these jurisdictions. The only periods subject to audit by the major tax jurisdictions where the Company does business are the 2016 through 2023 tax years. During such audits, local tax authorities may challenge the positions taken by us in our tax returns.

Net income per ordinary share

Net income per ordinary share	3 Months Ended
Net income per ordinary share	Mar. 31, 2024
Earnings Per Share [Abstract]
Net income per ordinary share	Net income per ordinary share Basic net income per ordinary share attributable to the Group 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: Three Months Ended March 31, 2024 March 31, 2023 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 82,579,203 81,784,389 Effect of dilutive share options and other awards outstanding under share based compensation programs 670,100 821,270 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 83,249,303 82,605,659 Three Months Ended March 31, 2024 March 31, 2023 Net income per Ordinary Share: Basic $ 2.27 $ 1.43 Diluted $ 2.25 $ 1.41

Share-based awards

Share-based awards	3 Months Ended
Share-based awards	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Share-based awards	Share-based awards Share Options The following table summarizes option activity for the three months ended March 31, 2024: Options Weighted Weighted Outstanding at December 31, 2023 902,806 $ 142.96 4.42 Granted 68,380 $ 325.51 Exercised (156,527) $ 104.48 Outstanding at March 31, 2024 814,659 $ 165.67 4.73 Exercisable at March 31, 2024 575,646 $ 132.89 4.00 The Company issues ordinary shares for all options exercised. The total amount of fully vested share options which remained outstanding at March 31, 2024 was 575,646. Fully vested share options at March 31, 2024 have an average remaining contractual term of 4.00 years and an average exercise price of $132.89. Fair value of Stock Options Assumptions The weighted average fair value of options granted during the three months ended March 31, 2024 and March 31, 2023 was calculated using the Black-Scholes option pricing model. The weighted average grant date fair values and assumptions used were as follows: Three Months Ended March 31, 2024 March 31, 2023 Weighted average grant date fair value $ 115.76 $ 85.75 Assumptions: Expected volatility 36 % 33 % Dividend yield — % — % Risk-free interest rate 4.20 % 4.22 % Expected life 4.3 years 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 2013 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. No awards may be granted under the 2019 Consultants RSU Plan after May 16, 2029. The Company has awarded RSUs and PSUs to certain key individuals of the Group. The following table summarizes RSU and PSU activity for the three months ended March 31, 2024: PSU PSU RSU RSU Outstanding at December 31, 2023 105,256 $ 226.29 621,011 $ 218.27 Granted 48,626 $ 325.51 42,826 $ 325.51 Shares vested (9,544) $ 174.96 (22,648) $ 214.55 Forfeited (50,194) $ 231.36 (22,751) $ 215.86 Outstanding at March 31, 2024 94,144 $ 280.04 618,438 $ 225.92 The fair value of PSUs vested for the three months ended March 31, 2024 totaled $1.7 million (2023: $7.2 million). The fair value of RSUs vested for the three months ended March 31, 2024 totaled $4.9 million (2023: $4.4 million). The PSUs vest based on service and specified EPS targets over the periods 2022 - 2024, 2023 - 2025 and 2024 - 2026. Depending on the amount of EPS from 2022 to 2026, up to an additional 63,089 PSUs may also be granted. Stock compensation expense Stock compensation expense for the three months ended March 31, 2024 and March 31, 2023 has been allocated as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Direct costs $ 6,721 $ 5,668 Selling, general and administrative 6,460 9,091 Total $ 13,181 $ 14,759

Share capital

Share capital	3 Months Ended
Share capital	Mar. 31, 2024
Equity [Abstract]
Share capital	Share capital The Company can acquire up to 10% of its outstanding ordinary shares (by way of redemption), in accordance with Irish law and the Company’s constitutional documents through open market share acquisitions. O n 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

Business Segment and Geographic

Business Segment and Geographical Information	3 Months Ended
Business Segment and Geographical Information	Mar. 31, 2024
Segment Reporting [Abstract]
Business Segment and Geographical Information	Business Segment and Geographical Information The Company is a CRO providing outsourced services on a global basis to pharmaceutical, biotechnology, medical device and government and public health organizations. It specializes 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. The Company has the expertise and capability to conduct clinical trials in the 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 pharmaceutical, biotechnology, medical device and government and public health organizations. 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 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. The geographical distribution of the Company’s segment measures for the three months ended March 31, 2024 and March 31, 2023 and as at March 31, 2024 and December 31, 2023 is as follows: a) The distribution of revenue by geographical area was as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Ireland $ 652,685 $ 511,834 Rest of Europe 390,215 414,223 U.S. 809,485 844,404 Rest of World 238,001 208,117 Total $ 2,090,386 $ 1,978,578 b) The distribution of income from operations by geographical area was as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Ireland * $ 136,730 $ 51,327 Rest of Europe 53,511 59,766 U.S. 76,899 88,089 Rest of World 18,357 17,607 Total $ 285,497 $ 216,789 * 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: Three Months Ended Year March 31, 2024 December 31, 2023 (in thousands) Ireland $ 198,339 $ 199,051 Rest of Europe 98,040 94,046 U.S. 170,078 159,245 Rest of World 46,465 49,175 Total $ 512,922 $ 501,517

Subsequent events

Subsequent events	3 Months Ended
Subsequent events	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	Subsequent events The Company has evaluated subsequent events from the balance sheet date through to the date at which the financial statements were available to be issued. The Company has determined that there are no items to disclose.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	3 Months Ended
Summary of significant accounting policies (Policies)	Mar. 31, 2024
Accounting Policies [Abstract]
Revenue recognition	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.
Intangible Assets	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.
Income taxes	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.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	3 Months Ended
Summary of significant accounting policies (Tables)	Mar. 31, 2024
Accounting Policies [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 periods ended March 31, 2024 and December 31, 2023 is as follows: March 31, 2024 December 31, 2023 (in thousands) Cost Customer relationships $ 4,089,517 $ 4,090,393 Order backlog 540,921 541,302 Trade names & brands 204,629 204,653 Patient database 170,345 170,366 Technology assets 151,000 141,257 Total cost 5,156,412 5,147,971 Accumulated amortization (1,407,473) (1,292,106) Net book value $ 3,748,939 $ 3,855,865

Revenue (Tables)

Revenue (Tables)	3 Months Ended
Revenue (Tables)	Mar. 31, 2024
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Revenue disaggregated by customer concentration is as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Clients 1-5 $ 543,960 $ 570,260 Clients 6-10 321,259 271,696 Clients 11-25 430,323 415,381 Other 794,844 721,241 Total $ 2,090,386 $ 1,978,578

Accounts receivable, unbilled_2

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) (Tables)	3 Months Ended
	Mar. 31, 2024
Revenue from Contract with Customer [Abstract]
Schedule of accounts receivables and unbilled revenue, and unbilled services and unearned revenue	Accounts receivables and unbilled revenue are as follows: March 31, 2024 December 31, 2023 (in thousands) Billed services (accounts receivable) $ 1,897,771 $ 1,821,855 Allowance for credit losses (35,303) (31,533) Accounts receivable (net) 1,862,468 1,790,322 Unbilled services (unbilled revenue) 993,141 951,936 Accounts receivable and unbilled revenue, net $ 2,855,609 $ 2,742,258 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) March 31, 2024 December 31, 2023 $ Change % Change Unbilled services (unbilled revenue) $ 993,141 $ 951,936 $ 41,205 4.3 % Unearned revenue (payments on account) (1,709,938) (1,654,507) (55,431) 3.4 % Net balance $ (716,797) $ (702,571) $ (14,226) 2.0 %

Goodwill (Tables)

Goodwill (Tables)	3 Months Ended
Goodwill (Tables)	Mar. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	March 31, 2024 December 31, 2023 (in thousands) Opening balance $ 9,022,075 $ 8,971,670 Current period acquisition 2,732 36,750 Foreign exchange movement (6,506) 13,655 Closing balance $ 9,018,301 $ 9,022,075

Intangible assets (Tables)

Intangible assets (Tables)	3 Months Ended
Intangible assets (Tables)	Mar. 31, 2024
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 periods ended March 31, 2024 and December 31, 2023 is as follows: March 31, 2024 December 31, 2023 (in thousands) Cost Customer relationships $ 4,089,517 $ 4,090,393 Order backlog 540,921 541,302 Trade names & brands 204,629 204,653 Patient database 170,345 170,366 Technology assets 151,000 141,257 Total cost 5,156,412 5,147,971 Accumulated amortization (1,407,473) (1,292,106) Net book value $ 3,748,939 $ 3,855,865

Fair Value Measures and Disclos

Fair Value Measures and Disclosures (Tables)	3 Months Ended
Fair Value Measures and Disclosures (Tables)	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Fair Value, Liabilities Measured on Recurring Basis	As of March 31, 2024, 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) $ 1,955 $ — $ — $ — $ 1,955 Available for sale investments (long-term) (b) — — — 51,164 51,164 Derivative instruments (c) — 8,225 — — 8,225 Total assets $ 1,955 $ 8,225 $ — $ 51,164 $ 61,344 Liabilities: Derivative instruments (c) — 270 — — 270 Total liabilities $ — $ 270 $ — $ — $ 270 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 Total assets $ 1,954 $ — $ — $ 46,804 $ 48,758 Liabilities: Derivative instruments (c) — 2,411 — — 2,411 Total liabilities $ — $ 2,411 $ — $ — $ 2,411 (a) Represents the fair value of investments in highly liquid investments with maturities of greater than three months and a minimum "A-" rated fixed term deposits and are based on quoted market prices. (b) To determine the classification of its 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 meet the definition of equity securities without readily determinable fair values, which qualify for the Net Asset Value (NAV) practical expedient in ASC 820 'Fair value measurements'. Any increases or decreases in fair value are recognized in net income in the period. (c) R
Fair Value, Assets Measured on Recurring Basis	As of March 31, 2024, 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) $ 1,955 $ — $ — $ — $ 1,955 Available for sale investments (long-term) (b) — — — 51,164 51,164 Derivative instruments (c) — 8,225 — — 8,225 Total assets $ 1,955 $ 8,225 $ — $ 51,164 $ 61,344 Liabilities: Derivative instruments (c) — 270 — — 270 Total liabilities $ — $ 270 $ — $ — $ 270 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 Total assets $ 1,954 $ — $ — $ 46,804 $ 48,758 Liabilities: Derivative instruments (c) — 2,411 — — 2,411 Total liabilities $ — $ 2,411 $ — $ — $ 2,411 (a) Represents the fair value of investments in highly liquid investments with maturities of greater than three months and a minimum "A-" rated fixed term deposits and are based on quoted market prices. (b) To determine the classification of its 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 meet the definition of equity securities without readily determinable fair values, which qualify for the Net Asset Value (NAV) practical expedient in ASC 820 'Fair value measurements'. Any increases or decreases in fair value are recognized in net income in the period. (c) R

Restructuring (Tables)

Restructuring (Tables)	3 Months Ended
Restructuring (Tables)	Mar. 31, 2024
Restructuring and Related Activities [Abstract]
Summary of Restructuring Charges	Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Restructuring charges $ — $ 9,729 Total $ — $ 9,729
Summary of details of movement in restructuring charges	Three Months Ended Year March 31, 2024 December 31, 2023 (in thousands) Opening provision $ 6,999 $ 6,022 Charge during the period — 36,704 Utilization (3,853) (35,727) Ending provision $ 3,146 $ 6,999

Operating leases (Tables)

Operating leases (Tables)	3 Months Ended
Operating leases (Tables)	Mar. 31, 2024
Leases [Abstract]
Schedule of Lease Costs	Lease costs recorded under operating leases for three months ended March 31, 2024 and March 31, 2023 were as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Operating lease costs 13,163 12,871 Income from sub-leases (336) (278) Net operating lease costs 12,827 12,593
Schedule of Operating Lease Maturity	Future minimum lease payments under non-cancelable leases as of March 31, 2024 were as follows: (in thousands) Year 1 $ 41,837 Year 2 36,980 Year 3 31,766 Year 4 25,788 Year 5 17,496 Thereafter 49,864 Total future minimum lease payments 203,731 Lease imputed interest (22,135) Total $ 181,596

Bank credit lines and loan fa_2

Bank credit lines and loan facilities (Tables)	3 Months Ended
Bank credit lines and loan facilities (Tables)	Mar. 31, 2024
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments	The Company had the following debt outstanding as of March 31, 2024 and December 31, 2023: Interest rate as of Principal amount Maturity Date March 31, 2024 December 31, 2023 March 31, 2024 December 31, 2023 (in thousands) Senior Secured Term Loan July 2028 7.302 % 7.860 % $ 2,976,213 $ 3,251,213 Senior Secured Notes July 2026 2.875 % 2.875 % 500,000 500,000 Senior Secured Revolving Loan* April 2024 6.179 % 6.720 % 50,000 55,000 Total debt 3,526,213 3,806,213 Less current portion of debt (79,762) (110,150) Total long-term debt 3,446,451 3,696,063 Less debt issuance costs and debt discount (26,717) (30,624) Total long-term debt, net $ 3,419,734 $ 3,665,439 * The final maturity date of the Senior Secured Revolving Loan facility is July 2026. Drawdown Repayment Closing Balance (in thousands) 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 Total drawdown/repayments in 2023 370,000 315,000 Quarter 1, 2024 50,000 55,000 50,000 Total drawdown/repayments in 2024 $ 50,000 $ 55,000
Schedule of Contractual Maturities of Debt	As of March 31, 2024, the contractual maturities of the Company's debt obligations were as follows: Contractual maturities of debt (in thousands) 2024 (remaining) $ 79,762 2025 29,762 2026 529,762 2027 29,762 2028 and thereafter 2,857,165 Total $ 3,526,213
Schedule of Principal Payments on Debt Instrument	Principal repayments, comprising mandatory and voluntary repayments, during the quarter ended March 31, 2024 and year ended December 31, 2023 were as follows: Principal repayments (in thousands) Quarter 1, 2023 $ 250,000 Quarter 2, 2023 150,000 Quarter 3, 2023 300,000 Quarter 4, 2023 250,000 Total repayments in 2023 950,000 Quarter 1, 2024 275,000 Total repayments in 2024 $ 275,000

Derivatives (Tables)

Derivatives (Tables)	3 Months Ended
Derivatives (Tables)	Mar. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value	The fair value of the Company’s derivative financial instruments, on a gross basis, are summarized in the following table: March 31, 2024 December 31, 2023 Asset Liability Notional Asset Liability Notional (in thousands) (in thousands) Interest Rate Caps $ — $ 270 $ 1,463,106 $ — $ 1,871 $ 1,600,606 Interest Rate Swap 8,225 — 1,100,606 — 540 1,100,606 Total derivatives designated as hedging instruments $ 8,225 $ 270 $ 2,563,712 $ — $ 2,411 $ 2,701,212

Income taxes (Tables)

Income taxes (Tables)	3 Months Ended
Income taxes (Tables)	Mar. 31, 2024
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense	Income taxes recognized during three months ended March 31, 2024 and March 31, 2023, comprise: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Income Tax expense $ 28,324 $ 14,273

Net income per ordinary share (

Net income per ordinary share (Tables)	3 Months Ended
Net income per ordinary share (Tables)	Mar. 31, 2024
Earnings Per Share [Abstract]
Schedule of 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: Three Months Ended March 31, 2024 March 31, 2023 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 82,579,203 81,784,389 Effect of dilutive share options and other awards outstanding under share based compensation programs 670,100 821,270 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 83,249,303 82,605,659 Three Months Ended March 31, 2024 March 31, 2023 Net income per Ordinary Share: Basic $ 2.27 $ 1.43 Diluted $ 2.25 $ 1.41

Share-based awards (Tables)

Share-based awards (Tables)	3 Months Ended
Share-based awards (Tables)	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Summary of stock option activity	The following table summarizes option activity for the three months ended March 31, 2024: Options Weighted Weighted Outstanding at December 31, 2023 902,806 $ 142.96 4.42 Granted 68,380 $ 325.51 Exercised (156,527) $ 104.48 Outstanding at March 31, 2024 814,659 $ 165.67 4.73 Exercisable at March 31, 2024 575,646 $ 132.89 4.00
Schedule of weighted average fair values and assumptions used	The weighted average grant date fair values and assumptions used were as follows: Three Months Ended March 31, 2024 March 31, 2023 Weighted average grant date fair value $ 115.76 $ 85.75 Assumptions: Expected volatility 36 % 33 % Dividend yield — % — % Risk-free interest rate 4.20 % 4.22 % Expected life 4.3 years 5 years
Summary of RSU and PSU activity	The following table summarizes RSU and PSU activity for the three months ended March 31, 2024: PSU PSU RSU RSU Outstanding at December 31, 2023 105,256 $ 226.29 621,011 $ 218.27 Granted 48,626 $ 325.51 42,826 $ 325.51 Shares vested (9,544) $ 174.96 (22,648) $ 214.55 Forfeited (50,194) $ 231.36 (22,751) $ 215.86 Outstanding at March 31, 2024 94,144 $ 280.04 618,438 $ 225.92
Schedule of non-cash stock compensation expense	Stock compensation expense for the three months ended March 31, 2024 and March 31, 2023 has been allocated as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Direct costs $ 6,721 $ 5,668 Selling, general and administrative 6,460 9,091 Total $ 13,181 $ 14,759

Business Segment and Geograph_2

Business Segment and Geographical Information (Tables)	3 Months Ended
Business Segment and Geographical Information (Tables)	Mar. 31, 2024
Segment Reporting [Abstract]
Schedule of Distribution of Revenue by Geographical Area	The distribution of revenue by geographical area was as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Ireland $ 652,685 $ 511,834 Rest of Europe 390,215 414,223 U.S. 809,485 844,404 Rest of World 238,001 208,117 Total $ 2,090,386 $ 1,978,578
Schedule of Distribution of Income from Operations by Geographical Area	The distribution of income from operations by geographical area was as follows: Three Months Ended March 31, 2024 March 31, 2023 (in thousands) Ireland * $ 136,730 $ 51,327 Rest of Europe 53,511 59,766 U.S. 76,899 88,089 Rest of World 18,357 17,607 Total $ 285,497 $ 216,789
Schedule of Distribution of Depreciation and Amortization 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: Three Months Ended Year March 31, 2024 December 31, 2023 (in thousands) Ireland $ 198,339 $ 199,051 Rest of Europe 98,040 94,046 U.S. 170,078 159,245 Rest of World 46,465 49,175 Total $ 512,922 $ 501,517

Summary of significant accoun_4

Summary of significant accounting policies - Estimated Useful Lives of Intangible Assets (Details)	Mar. 31, 2024
Customer relationships \| Minimum
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	16 years
Customer relationships \| Maximum
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	23 years
Order backlog
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	3 years
Trade names
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	3 years
Patient database
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	7 years
Technology assets
Indefinite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years

Revenue (Details)

Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Revenue (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 2,090,386	$ 1,978,578
Clients 1-5
Disaggregation of Revenue [Line Items]
Revenue	543,960	570,260
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue	321,259	271,696
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue	430,323	415,381
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 794,844	$ 721,241
No customer \| Revenue benchmark \| Customer concentration risk
Disaggregation of Revenue [Line Items]
Customer concentration risk (in percent)	10%

Accounts receivable, unbilled_3

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Accounts Receivables and Unbilled Revenue (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Contract Assets [Abstract]
Billed services (accounts receivable)	$ 1,897,771	$ 1,821,855
Allowance for credit losses	(35,303)	(31,533)
Accounts receivable (net)	1,862,468	1,790,322
Unbilled services (unbilled revenue)	993,141	951,936
Accounts receivable and unbilled revenue, net	$ 2,855,609	$ 2,742,258

Accounts receivable, unbilled_4

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Schedule of Unbilled Services and Unearned Revenue or Payments on Account (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Unbilled services (unbilled revenue)	$ 993,141	$ 951,936
Change in unbilled services (unbilled revenue)	$ 41,205
Percentage change in unbilled services (unbilled revenue)	4.30%
Unearned revenue (payments on account)	$ (1,709,938)	(1,654,507)
Change in unearned revenue (payments on account)	$ (55,431)
Percentage change in unearned revenue (payments on account)	3.40%
Net balance	$ (716,797)	$ (702,571)
Change in net balance	$ 14,226
Percentage change in net balance	2%

Accounts receivable, unbilled_5

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Increase in unbilled services (unbilled revenue)	$ 41,205
Increase in unearned revenue (payments on account)	55,431
Increase in net balance	14,226
Revenue, remaining performance obligation	$ 15,300,000	$ 13,800,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation percentage		52%
Revenue, remaining performance obligation, expected timing of satisfaction, period		12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation percentage	51%
Revenue, remaining performance obligation, expected timing of satisfaction, period	12 months

Goodwill - Schedule of Goodwill

Goodwill - Schedule of Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Goodwill [Roll Forward]
Opening balance	$ 9,022,075	$ 8,971,670
Current period acquisition	2,732	36,750
Foreign exchange movement	(6,506)	13,655
Closing balance	$ 9,018,301	$ 9,022,075

Goodwill - Narrative (Details)

Goodwill - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
Goodwill - Narrative (Details) - USD ($) $ in Thousands	Jan. 09, 2024	Mar. 31, 2024	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to acquire businesses, gross		$ 7,831		$ 0
Goodwill		9,018,301	$ 9,018,301		$ 9,022,075	$ 8,971,670
Amortization of intangible assets		116,500			459,900
Goodwill, impairment loss		0			$ 0
HumanFirst Inc
Acquired Finite-Lived Intangible Assets [Line Items]
Business combination, consideration transferred	$ 13,300
Payments to acquire businesses, gross	11,800
Payments to acquire businesses, net of cash acquired	7,800
Cash acquired from acquisition	4,000
Deferred consideration		$ 1,500	1,500
Goodwill	2,700
Intangible assets acquired	$ 9,900
Finite-lived intangible asset, useful life	5 years
Amortization of intangible assets			$ 500

Intangible assets (Details)

Intangible assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Cost
Total cost	$ 5,156,412	$ 5,147,971
Accumulated amortization	(1,407,473)	(1,292,106)
Net book value	3,748,939	3,855,865
Customer relationships
Cost
Total cost	4,089,517	4,090,393
Order backlog
Cost
Total cost	540,921	541,302
Trade names & brands
Cost
Total cost	204,629	204,653
Patient database
Cost
Total cost	170,345	170,366
Technology assets
Cost
Total cost	$ 151,000	$ 141,257

Intangible assets - Narrative (

Intangible assets - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2024	Dec. 31, 2023	Jan. 09, 2024
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 116.5		$ 459.9
HumanFirst Inc
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		$ 0.5
Intangible assets acquired				$ 9.9

Fair value measurements - Measu

Fair value measurements - Measurement of Fair Value of Major Asset and Liability classed (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (short-term)	$ 1,955	$ 1,954
Available for sale investments (long-term)	51,164	46,804
Total assets	61,344	48,758
Total liabilities	270	2,411
Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	8,225
Derivative instruments	270	2,411
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (short-term)	1,955	1,954
Available for sale investments (long-term)	0	0
Total assets	1,955	1,954
Total liabilities	0	0
Level 1 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	0
Derivative instruments	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (short-term)	0	0
Available for sale investments (long-term)	0	0
Total assets	8,225	0
Total liabilities	270	2,411
Level 2 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	8,225	0
Derivative instruments	270	2,411
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (short-term)	0	0
Available for sale investments (long-term)	0	0
Total assets	0	0
Total liabilities	0	0
Level 3 \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	0
Derivative instruments	0	0
Investments Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities (short-term)	0	0
Available for sale investments (long-term)	51,164	46,804
Total assets	51,164	46,804
Total liabilities	0	0
Investments Measured at Net Asset Value \| Total derivatives designated as hedging instruments \| Total derivatives designated as hedging instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments	0
Derivative instruments	$ 0	$ 0

Fair value measurements - Narra

Fair value measurements - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 61,344	$ 48,758
Long-term debt at fair value	3,501,400	3,793,500
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Level 3 \| Fair value, nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	12,767,200	12,878,000
Goodwill at fair value	9,018,300	9,022,100
Net intangible assets at fair value	$ 3,748,900	$ 3,855,900

Restructuring - Narrative (Deta

Restructuring - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Restructuring Cost and Reserve [Line Items]
Restructuring	$ 0	$ 9,729
Restructuring reserve	3,146		$ 6,999	$ 6,022
Personnel related liabilities
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	500
Facility liability
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	2,600
Restructuring reserve, current	800
Restructuring reserve, noncurrent	$ 1,800

Restructuring Summary of Restru

Restructuring Summary of Restructuring Charges (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Restructuring and Related Activities [Abstract]
Restructuring	$ 0	$ 9,729

Restructuring - Summary of deta

Restructuring - Summary of details of movement in restructuring charges (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Opening provision	$ 6,999	$ 6,022
Charge during the period	0	36,704
Utilization	(3,853)	(35,727)
Ending provision	$ 3,146	$ 6,999

Operating leases - Schedule of

Operating leases - Schedule of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Leases [Abstract]
Operating lease costs	$ 13,163	$ 12,871
Income from sub-leases	(336)	(278)
Net operating lease costs	$ 12,827	$ 12,593

Operating leases - Narrative (D

Operating leases - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Lessee, Lease, Description [Line Items]
Total lease cost	$ 12,827	$ 12,593
Right-of-use assets obtained in exchange for lease obligations	$ 35,600	$ 12,300
Weighted average remaining lease term	7 years 2 months 1 day		6 years 8 months 19 days
Weighted average discount rate	3.73%		3.29%
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other Liabilities	Other Liabilities
Other liabilities
Lessee, Lease, Description [Line Items]
Operating lease liabilities	$ 36,100	$ 36,400
Selling, general and administration costs
Lessee, Lease, Description [Line Items]
Total lease cost	10,000	10,000
Direct costs
Lessee, Lease, Description [Line Items]
Total lease cost	$ 2,800	$ 2,600

Operating leases - Schedule o_2

Operating leases - Schedule of Operating Lease Maturity (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Leases [Abstract]
Year 1	$ 41,837
Year 2	36,980
Year 3	31,766
Year 4	25,788
Year 5	17,496
Thereafter	49,864
Total future minimum lease payments	203,731
Lease imputed interest	(22,135)
Total	$ 181,596

Bank credit lines and loan fa_3

Bank credit lines and loan facilities - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Jul. 01, 2021
Debt Instrument [Line Items]
Total debt	$ 3,526,213	$ 3,806,213
Less current portion of debt	(79,762)	(110,150)
Total long-term debt	3,446,451	3,696,063
Less debt issuance costs and debt discount	(26,717)	(30,624)
Total long-term debt, net	$ 3,419,734	$ 3,665,439
Senior Secured Term Loan \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	7.302%	7.86%	1%
Total debt	$ 2,976,213	$ 3,251,213
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 \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	6.179%	6.72%
Senior Secured Revolving Loan \| Senior Secured Revolving Loan \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Total debt	$ 50,000	$ 55,000

Bank credit lines and loan fa_4

Bank credit lines and loan facilities - Schedule of Contractual Maturities of Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Disclosure [Abstract]
2024 (remaining)	$ 79,762
2025	29,762
2026	529,762
2027	29,762
2028 and thereafter	2,857,165
Total debt	$ 3,526,213	$ 3,806,213

Bank credit lines and loan fa_5

Bank credit lines and loan facilities - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023	May 02, 2023	Jul. 01, 2021
Debt Instrument [Line Items]
Drawdown of credit lines and loan facilities	$ 50,000				$ 180,000
Repayments of senior debt	55,000
Line of credit facility, remaining borrowing capacity	50,000	$ 55,000				$ 55,000
Long-term debt at fair value	$ 3,501,400	$ 3,793,500				$ 3,793,500
Senior secured credit facility and senior secured notes \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt discount paid								$ 27,600
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%
Senior Secured Term Loan \| Senior Secured Term Loan
Debt Instrument [Line Items]
Debt instrument, principal amount								$ 5,515,000
Debt instrument, interest rate (in percent)	7.302%	7.86%				7.86%		1%
Accelerated charge on repayments	$ 2,000				2,200
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BB Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	1.45%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BB+ Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	1.10%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB- Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0.85%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB+ Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0.50%
Senior Secured Revolving Loan \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Variable rate component two \| Standard & Poor's, BBB Rating
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0.65%
Senior Secured Revolving Loan \| Base rate \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0.10%
Senior Secured Revolving Loan \| Base rate \| Maximum \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0.45%
Senior Secured Revolving Loan \| Base rate \| Minimum \| Variable rate component one
Debt Instrument [Line Items]
Basis spread on variable rate (in percent)	0%
Senior Secured Revolving Loan \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)	6.179%	6.72%				6.72%
Senior Secured Revolving Loan \| Senior Secured Revolving Loan \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Debt instrument, maximum borrowing capacity							$ 500,000	$ 300,000
Drawdown of credit lines and loan facilities	$ 50,000	$ 65,000	$ 75,000	$ 50,000	$ 180,000	$ 370,000
Senior Secured Notes \| Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, principal amount								$ 500,000
Debt instrument, interest rate (in percent)	2.875%	2.875%				2.875%		2.875%

Bank credit lines and loan fa_6

Bank credit lines and loan facilities - Schedule of Principal Payments on Debt Instruments (Details) - Senior Secured Term Loan - Senior Secured Term Loan - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023
Debt Instrument [Line Items]
Periodic principal payment	$ 275,000	$ 250,000	$ 300,000	$ 150,000	$ 250,000
Annual principal payment	$ 275,000	$ 950,000

Bank credit lines and loan fa_7

Bank credit lines and loan facilities - Schedule of Movement on Debt Instrument (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023
Debt Instrument [Line Items]
Drawdown of credit lines and loan facilities	$ 50,000				$ 180,000
Repayments of Lines of Credit	330,000				350,000
Senior Secured Revolving Loan \| Senior Secured Revolving Loan \| Senior Secured Revolving Loan
Debt Instrument [Line Items]
Drawdown of credit lines and loan facilities	50,000	$ 65,000	$ 75,000	$ 50,000	180,000	$ 370,000
Repayments of Lines of Credit	55,000	85,000	50,000	80,000	100,000	315,000
Total long-term debt, net	$ 50,000	$ 55,000	$ 75,000	$ 50,000	$ 80,000	$ 55,000

Derivatives - Narrative (Detail

Derivatives - Narrative (Details) $ in Thousands	3 Months Ended
Derivatives - Narrative (Details) $ in Thousands	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Nov. 29, 2022 USD ($) instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Asset, Current, Statement of Financial Position [Extensible Enumeration]	Other receivables
Derivative Asset, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other receivables
Derivative Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other liabilities		Other liabilities
Derivative Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]			Non-current other liabilities
Other comprehensive income (loss), cash flow hedge, gain (loss), before reclassification and tax	$ (9,100)	$ 3,900
Other comprehensive income (loss), cash flow hedge, gain (loss), reclassification, before tax	(700)	$ (2,300)
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net	2,000
Interest Rate Caps \| Total derivatives designated as hedging instruments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of interest rate derivatives held \| instrument				2
Derivative, notional amount				$ 2,101,000
Derivative, fixed interest rate				0.42%
Derivative liability, current	300
Interest Rate Caps \| Total derivatives designated as hedging instruments \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, variable interest rate, reference rate in excess of				4.75%
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [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 Instruments and Hedging Activities Disclosures [Line Items]
Derivative asset, current	2,900
Derivative asset, noncurrent	$ 5,300
Derivative liability, current			$ 1,900
Derivative liability, noncurrent			$ 500

Derivatives - Effect of Derivat

Derivatives - Effect of Derivative Financial Instruments on Statement of Financial Position (Details) - Total derivatives designated as hedging instruments - USD ($)	Mar. 31, 2024	Dec. 31, 2023
Interest Rate Caps
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Asset	$ 0	$ 0
Liability	270,000	1,871,000
Notional	1,463,106,000	1,600,606,000
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Asset	8,225,000	0
Liability	0	540,000
Notional	1,100,606,000	1,100,606,000
Total derivatives designated as hedging instruments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Asset	8,225,000
Liability	270,000	2,411,000
Notional	2,563,712,000	2,701,212,000
Total derivatives designated as hedging instruments \| Level 2
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Asset	8,225,000	0
Liability	$ 270,000	$ 2,411,000

Income taxes - Components of Ta

Income taxes - Components of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Income Tax Disclosure [Abstract]
Income Tax expense	$ 28,324	$ 14,273

Income taxes - Narrative (Detai

Income taxes - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Liability for unrecognized tax benefit	$ 189.2	$ 187.1
Items generating unrecognized tax benefits	160.5	160
Interest and related penalties	$ 28.7	$ 27.1

Net income per ordinary share -

Net income per ordinary share - Schedule of Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Details) - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares)	82,579,203	81,784,389
Effect of dilutive share options and other awards outstanding under share based compensation programs (in shares)	670,100	821,270
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares)	83,249,303	82,605,659

Net income per ordinary share_2

Net income per ordinary share - Schedule of Reconciliation of Net Income Attributable to the Group (Details) - $ / shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net income per Ordinary Share:
Basic (in USD per share)	$ 2.27	$ 1.43
Diluted (in USD per share)	$ 2.25	$ 1.41

Share-based awards - Summary of

Share-based awards - Summary of Stock Option Activity (Details) - Employee Stock Option - $ / shares			3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2024
Options Outstanding Number of Shares
Outstanding at beginning of period (in shares)			902,806
Granted (in shares)			68,380
Exercised (in shares)			(156,527)
Outstanding at end of period (in shares)	814,659	902,806	814,659
Exercisable at end of period (in shares)	575,646		575,646
Weighted Average Exercise Price
Outstanding at beginning of period (in USD per share)			$ 142.96
Granted (in USD per share)			325.51
Exercised (in USD per share)			104.48
Outstanding at end of period (in USD per share)	$ 165.67	$ 142.96	165.67
Exercisable at end of period (in USD per share)	$ 132.89		$ 132.89
Weighted Average Remaining Contractual Life
Weighted average contractual term of options outstanding	4 years 8 months 23 days	4 years 5 months 1 day
Weighted average contractual term of options exercisable	4 years

Share-based awards - Narrative

Share-based awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended	12 Months Ended	59 Months Ended
	Mar. 31, 2024	May 11, 2015	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2024	May 16, 2019
Employee Stock Option
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Number of shares exercisable (in shares)	575,646		575,646		575,646
Weighted average contractual term of options exercisable	4 years
Weighted average exercise price of options exercisble (USD per share)	$ 132.89		$ 132.89		$ 132.89
Restricted stock units 2013
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Additional ordinary shares which have been reserved for issuance (in shares)		2,500,000
Ordinary shares which have been reserved for issuance (in shares)		4,100,000
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 directors
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period					12 months
Performance Share Unit (PSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 1.7	$ 7.2
Restricted Stock Units (RSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 4.9	$ 4.4
PSUs based on service and EPS targets \| Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units to be granted (in shares)	63,089		63,089		63,089

Share-based awards - Schedule o

Share-based awards - Schedule of Weighted Average Fair Values and Assumptions Used (Details) - Employee Stock Option - $ / shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value	$ 115.76	$ 85.75
Assumptions:
Expected volatility	36%	33%
Dividend yield	0%	0%
Risk-free interest rate	4.20%	4.22%
Expected life	4 years 3 months 18 days	5 years

Share-based awards - Summary _2

Share-based awards - Summary of RSU and PSU Activity (Details)	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Performance Share Unit (PSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares) \| shares	105,256
Granted (in shares) \| shares	48,626
Shares vested (in shares) \| shares	(9,544)
Forfeited (in shares) \| shares	(50,194)
Outstanding at end of period (in shares) \| shares	94,144
Weighted Average Grant Date Fair Value
Outstanding at beginning of period (in USD per share) \| $ / shares	$ 226.29
Granted (in USD per share) \| $ / shares	325.51
Shares vested (in USD per share) \| $ / shares	174.96
Forfeited (in USD per share) \| $ / shares	231.36
Outstanding at end of period (in USD per share) \| $ / shares	$ 280.04
Restricted Stock Units (RSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares) \| shares	621,011
Granted (in shares) \| shares	42,826
Shares vested (in shares) \| shares	(22,648)
Forfeited (in shares) \| shares	(22,751)
Outstanding at end of period (in shares) \| shares	618,438
Weighted Average Grant Date Fair Value
Outstanding at beginning of period (in USD per share) \| $ / shares	$ 218.27
Granted (in USD per share) \| $ / shares	325.51
Shares vested (in USD per share) \| $ / shares	214.55
Forfeited (in USD per share) \| $ / shares	215.86
Outstanding at end of period (in USD per share) \| $ / shares	$ 225.92

Share-based awards - Schedule_2

Share-based awards - Schedule of Non-cash Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 13,181	$ 14,759
Direct costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	6,721	5,668
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total	$ 6,460	$ 9,091

Share capital (Details)

Share capital (Details) - USD ($) $ in Millions	3 Months Ended
Share capital (Details) - USD ($) $ in Millions	Mar. 31, 2024	Feb. 20, 2024
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount		$ 500
Buyback Program \| Maximum
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized percentage	10%

Business Segment and Geograph_3

Business Segment and Geographical Information - Narrative (Details)	3 Months Ended
	Mar. 31, 2024 segment
Segment Reporting [Abstract]
Number of operating segments	1

Business Segment and Geograph_4

Business Segment and Geographical Information - Schedule of Distribution of Revenue by Geographical Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenue	$ 2,090,386	$ 1,978,578
Ireland
Segment Reporting Information [Line Items]
Revenue	652,685	511,834
Rest of Europe
Segment Reporting Information [Line Items]
Revenue	390,215	414,223
U.S.
Segment Reporting Information [Line Items]
Revenue	809,485	844,404
Rest of World
Segment Reporting Information [Line Items]
Revenue	$ 238,001	$ 208,117

Business Segment and Geograph_5

Business Segment and Geographical Information - Schedule of Distribution of Income from Operations by Geographical Area (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Income from operations	$ 285,497	$ 216,789
Ireland
Segment Reporting Information [Line Items]
Income from operations	136,730	51,327
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations	53,511	59,766
U.S.
Segment Reporting Information [Line Items]
Income from operations	76,899	88,089
Rest of World
Segment Reporting Information [Line Items]
Income from operations	$ 18,357	$ 17,607

Business Segment and Geograph_6

Business Segment and Geographical Information - Schedule of Distribution of Depreciation and Amortization by Geographical Area (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Long-lived assets	$ 512,922	$ 501,517
Ireland
Segment Reporting Information [Line Items]
Long-lived assets	198,339	199,051
Rest of Europe
Segment Reporting Information [Line Items]
Long-lived assets	98,040	94,046
U.S.
Segment Reporting Information [Line Items]
Long-lived assets	170,078	159,245
Rest of World
Segment Reporting Information [Line Items]
Long-lived assets	$ 46,465	$ 49,175