ICON Public Limited (ICLR) 20-F 24 Feb 21 2020 FY Annual report (foreign) Financial data

Cover

Cover	12 Months Ended
	Dec. 31, 2020shares
Document Information [Line Items]
Document Type	20-F
Document Registration Statement	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Document Shell Company Report	false
Entity File Number	333-08704
Entity Registrant Name	ICON PLC
Entity Incorporation, State or Country Code	L2
Entity Address, Address Line One	South County Business Park,
Entity Address, Address Line Two	Leopardstown,
Entity Address, City or Town	Dublin 18,
Entity Address, Country	IE
Title of 12(b) Security	ORDINARY SHARES, PAR VALUE €0.06 EACH
Trading Symbol	ICLR
Security Exchange Name	NASDAQ
Entity common stock, shares outstanding (in shares)	52,788,093
Entity Well Known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filler Category	Large Accelerated Filer
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Document Accounting Standard	U.S. GAAP
Entity Shell Company	false
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001060955
Entity Address, Postal Zip Code	D18 X5R3
Business Contact
Document Information [Line Items]
Entity Address, Address Line One	South County Business Park
Entity Address, Address Line Two	Leopardstown
Entity Address, City or Town	Dublin 18
Entity Address, Country	IE
Contact Personnel Name	Brendan Brennan
Contact Personnel Email Address	Brendan.Brennan@iconplc.com
Country Region	353
City Area Code	1
Local Phone Number	291-2000
Entity Address, Postal Zip Code	D18 X5R3

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current Assets:
Cash and cash equivalents	$ 840,305	$ 520,309
Available for sale investments (Note 3a)	1,729	49,628
Accounts receivable, net (Note 17)	715,271	527,708
Unbilled revenue (Note 17)	428,684	422,769
Other receivables	35,394	39,290
Prepayments and other current assets	53,477	41,517
Income taxes receivable (Note 13)	28,118	23,759
Total current assets	2,102,978	1,624,980
Other Assets:
Property, plant and equipment, net (Note 6)	174,343	165,087
Goodwill (Note 4)	936,257	883,170
Operating right-of-use assets (Note 22)	84,561	104,977
Other non-current assets	20,773	17,439
Non-current income taxes receivable (Note 13)	17,230	17,230
Non-current deferred tax asset (Note 13)	12,705	16,682
Equity method investments (Note 3c)	4,534	0
Investments in equity-long term (Note 3b)	15,765	10,053
Intangible assets (Note 5)	66,460	67,894
Total Assets	3,435,606	2,907,512
Current Liabilities:
Accounts payable	51,113	24,050
Unearned revenue (Note 17)	660,883	366,988
Other liabilities (Note 7)	399,769	378,543
Income taxes payable (Note 13)	12,178	12,031
Current bank credit lines and loan facilities (Note 23)	0	349,640
Total current liabilities	1,123,943	1,131,252
Other Liabilities:
Non-current bank credit lines and loan facilities (Note 23)	348,477	0
Non-current operating lease liabilities (Note 22)	60,801	76,593
Non-current other liabilities (Note 8)	26,366	17,512
Non-current government grants (Note 11)	838	813
Non-current income taxes payable (Note 13)	14,539	14,301
Non-current deferred tax liability (Note 13)	10,406	9,476
Commitments and contingencies (Note 15)	0	0
Total Liabilities	1,585,370	1,249,947
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, (Note 12), 52,788,093 shares issued and outstanding at December 31, 2020 and 53,622,206 shares issued and outstanding at December 31, 2019.	4,580	4,635
Additional paid-in capital	617,104	577,961
Other undenominated capital (Note 12 (a))	1,134	1,052
Accumulated other comprehensive income (Note 21)	(35,477)	(75,819)
Retained earnings	1,262,895	1,110,226
Total Shareholders’ Equity	1,850,236	1,618,055
Redeemable noncontrolling interest	0	39,510
Total Shareholders' Equity and Redeemable Noncontrolling Interest	1,850,236	1,657,565
Total Liabilities and Shareholders’ Equity and Redeemable Noncontrolling Interest	$ 3,435,606	$ 2,907,512

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - € / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Ordinary shares, par value (in EUR per share)	€ 6	€ 6
Ordinary shares, shares authorized (in shares)	100,000,000	100,000,000
Ordinary shares, shares issued (in shares)	52,788,093	53,622,206
Ordinary shares, shares outstanding (in shares)	52,788,093	53,622,206

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Revenue	$ 2,797,288	$ 2,805,839	$ 2,595,777
Costs and expenses:
Direct costs	1,979,883	1,974,135	1,818,220
Selling, general and administrative	341,690	336,748	325,794
Depreciation and amortization	66,126	61,550	65,916
Restructuring (Note 14)	18,089	0	12,490
Total costs and expenses	2,405,788	2,372,433	2,222,420
Income from operations	391,500	433,406	373,357
Interest income	2,724	6,859	4,759
Interest expense	(13,019)	(13,276)	(13,502)
Income before income taxes expense	381,205	426,989	364,614
Income tax expense (Note 13)	(47,875)	(51,133)	(41,958)
Income before share of earnings from equity method investments	333,330	375,856	322,656
Share of equity method investments	(366)	0	0
Net Income	332,964	375,856	322,656
Net income attributable to noncontrolling interest	(633)	(1,870)	0
Net income attributable to the Group	$ 332,331	$ 373,986	$ 322,656
Net income per Ordinary Share attributable to the Group (Note 26):
Basic (USD per share)	$ 6.20	$ 6.85	$ 5.96
Diluted (USD per share)	$ 6.15	$ 6.79	$ 5.89
Weighted average number of ordinary shares outstanding:
Basic (in shares)	52,859,911	53,859,537	54,118,764
Diluted (in shares)	53,283,585	54,333,461	54,790,663

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 332,964	$ 375,856	$ 322,656
Other comprehensive income, net of tax
Currency translation adjustment	48,129	(1,313)	(26,522)
Currency impact of long-term funding	(1,603)	(2,710)	(4,834)
Unrealized capital (loss)/gain - investments	(231)	681	(155)
Actuarial (loss)/gain on defined benefit pension plan	(4,138)	(2,226)	2,855
Amortization of interest rate hedge	(910)	(923)	(923)
Loss on interest rate hedge	(905)	0	0
Fair value of cash flow hedge	0	0	(1,036)
Total comprehensive income	373,306	369,365	292,041
Less comprehensive income attributable to redeemable noncontrolling interest	(633)	(1,870)	0
Total comprehensive income attributable to the Group	$ 372,673	$ 367,495	$ 292,041

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Ordinary Shares	Ordinary SharesCumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in CapitalCumulative Effect, Period of Adoption, Adjusted Balance	Other Undenominated Capital	Other Undenominated CapitalCumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income	Accumulated Other Comprehensive IncomeCumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained EarningsCumulative Effect, Period of Adoption, Adjustment	Retained EarningsCumulative Effect, Period of Adoption, Adjusted Balance	Redeemable Noncontrolling Interest	Redeemable Noncontrolling InterestCumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance (in shares) at Dec. 31, 2017				54,081,601	54,081,601
Beginning balance at Dec. 31, 2017	$ 1,191,000	$ (48,104)	$ 1,142,896	$ 4,664	$ 4,664	$ 481,337	$ 481,337	$ 912	$ 912	$ (38,713)	$ (38,713)	$ 742,800	$ (48,104)	$ 694,696
Comprehensive Income (net of tax):
Net income	322,656											322,656
Currency translation adjustment	(26,522)									(26,522)
Currency impact of long-term funding	(4,834)									(4,834)
Unrealized capital loss - investments	(155)									(155)
Actuarial gain (loss) on defined benefit pension plan	2,855									2,855
Amortization of interest rate hedge	(923)									(923)
Loss on interest rate hedge	0
Fair value of cash flow hedge	(1,036)									(1,036)
Total comprehensive income attributable to the Group	292,041
Exercise of share options (in shares)				408,699
Exercise of share options	16,806			$ 29		16,777
Issue of restricted share units / performance share units (in shares)				489,568
Issue of restricted share units / performance share units	36			$ 36
Share based compensation expense	31,544					31,544
Share issue costs	(16)					(16)
Repurchase of ordinary shares (in shares)				(1,008,162)
Repurchase of ordinary shares	(128,960)			$ (71)				71				(128,960)
Share repurchase costs	(66)											(66)
Ending balance (in shares) at Dec. 31, 2018				53,971,706
Ending balance at Dec. 31, 2018	1,354,281			$ 4,658		529,642		983		(69,328)		888,326
Beginning balance at Dec. 31, 2017															$ 0	$ 0
Ending balance at Dec. 31, 2018															0
Comprehensive Income (net of tax):
Net income	373,986											373,986
Currency translation adjustment	(1,313)									(1,313)
Currency impact of long-term funding	(2,710)									2,710
Unrealized capital loss - investments	681									681
Actuarial gain (loss) on defined benefit pension plan	(2,226)									(2,226)
Amortization of interest rate hedge	(923)									(923)
Loss on interest rate hedge	0
Total comprehensive income attributable to the Group	367,495
Exercise of share options (in shares)				329,870
Exercise of share options	21,621			$ 22		21,599
Issue of restricted share units / performance share units (in shares)				355,730
Issue of restricted share units / performance share units	24			$ 24
Share based compensation expense	26,733					26,733
Share issue costs	(13)					(13)
Repurchase of ordinary shares (in shares)				(1,035,100)
Repurchase of ordinary shares	(146,931)			$ (69)				69				(146,931)
Share repurchase costs	(107)											(107)
Exercise of call option on noncontrolling interest shares	$ (5,048)
Ending balance (in shares) at Dec. 31, 2019	53,622,206			53,622,206
Ending balance at Dec. 31, 2019	$ 1,618,055			$ 4,635		577,961		1,052		(75,819)		1,110,226
Increase (Decrease) in Temporary Equity [Roll Forward]
Net income															1,870
Noncontrolling interest adjustment to redemption amount															32,592
Exercise of call option on noncontrolling interest shares															5,048
Ending balance at Dec. 31, 2019	39,510														39,510
Comprehensive Income (net of tax):
Net income	332,331											332,331
Currency translation adjustment	48,129									48,129
Currency impact of long-term funding	(1,603)									(1,603)
Unrealized capital loss - investments	(231)									(231)
Actuarial gain (loss) on defined benefit pension plan	(4,138)									(4,138)
Amortization of interest rate hedge	(910)									(910)
Loss on interest rate hedge	(905)									(905)
Total comprehensive income attributable to the Group	372,673
Exercise of share options (in shares)				193,417
Exercise of share options	13,189			$ 13		13,176
Issue of restricted share units / performance share units (in shares)				207,688
Issue of restricted share units / performance share units	14			$ 14
Share based compensation expense	25,981					25,981
Share issue costs	(14)					(14)
Repurchase of ordinary shares (in shares)				(1,235,218)
Repurchase of ordinary shares	(175,000)			$ (82)				82				(175,000)
Share repurchase costs	$ (140)											(140)
Ending balance (in shares) at Dec. 31, 2020	52,788,093			52,788,093
Ending balance at Dec. 31, 2020	$ 1,850,236			$ 4,580		$ 617,104		$ 1,134		$ (35,477)		1,262,895
Increase (Decrease) in Temporary Equity [Roll Forward]
Net income															633
Noncontrolling interest adjustment to redemption amount	(4,522)											$ (4,522)			4,522
Exercise of call option on noncontrolling interest shares															(44,665)
Ending balance at Dec. 31, 2020	$ 0														$ 0

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended			24 Months Ended	36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2020
Cash flows from operating activities:
Net income	$ 332,964	$ 375,856	$ 322,656
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	141	346	70
Depreciation expense	46,892	45,603	50,565
Impairment of right-of-use assets	5,411	0	0
Reduction in carrying value of operating right-of-use assets	28,480	30,372	0
Amortization of intangibles	19,234	15,947	15,351
Amortization of government grants	(45)	(44)	(47)
Interest on short term investments	0	(1,065)	(1,329)
Realized gain on sale of short term investments	(234)	(55)	(56)
Interest on operating lease liability	1,668	2,626	0
(Gain)/loss on re-measurement of financial assets	(2,500)	800	(800)
Loss on equity method investments	366	0	0
Amortization of gain on interest rate hedge	(910)	(923)	(923)
Amortization of financing costs	523	540	812
Stock compensation expense	26,271	26,819	31,594
Deferred tax expense / (benefit)	927	(1,537)	1,652
Changes in assets and liabilities:
Increase in accounts receivable	(175,040)	(101,545)	(37,557)
Increase in unbilled revenue	(5,748)	(55,790)	(98,510)
Decrease in other receivables	3,041	2,513	3,107
Increase in prepayments and other current assets	(11,468)	(3,583)	(3,237)
(Increase)/decrease in other non-current assets	(3,225)	(2,912)	856
Increase/(decrease) in unearned revenue	291,844	86,567	(6,253)
Increase in other current liabilities	14,968	19,864	2,009
Decrease in operating lease liabilities	(30,504)	(33,437)	0
Increase/(decrease) in other non-current liabilities	256	1,405	(1,034)
(Decrease)/increase in income taxes payable	(1,765)	734	(5,220)
Increase/(decrease) in accounts payable	26,488	3,440	(5,067)
Net cash provided by operating activities	568,035	412,541	268,639
Cash flows from investing activities:
Purchase of property, plant and equipment	(51,055)	(50,645)	(48,397)
Purchase of subsidiary undertakings	(47,931)	(131,272)	(1,645)
Investment in equity method investments	(2,450)	0	0
Cash acquired from subsidiary undertakings	10,170	11,697	0
Sale of available for sale investments	47,902	21,686	99,865
Purchase of available for sale investments	0	(9,603)	(80,956)
Purchase of investments in equity - long term	(3,212)	(3,890)	(6,163)	$ (10,100)	$ (13,300)
Net cash used in investing activities	(46,576)	(162,027)	(37,296)
Cash flows from financing activities:
Financing costs	(1,554)	0	(823)
Drawdown of credit lines and facilities	350,000	0	0
Repayment of credit lines and facilities	(350,000)	0	0
Purchase of noncontrolling interest	(43,923)	0	0
Proceeds from the exercise of equity compensation	13,203	21,645	16,842
Share issue costs	(14)	(13)	(16)
Repurchase of ordinary shares	(175,000)	(146,931)	(128,960)
Share repurchase costs	(140)	(107)	(66)
Loss on settlement of interest rate hedge	(905)	0	0
Net cash used in financing activities	(208,333)	(125,406)	(113,023)
Effect of exchange rate movements on cash	6,870	(650)	(5,328)
Net increase in cash and cash equivalents	319,996	124,458	112,992
Cash and cash equivalents at beginning of year	520,309	395,851	282,859	282,859	282,859
Cash and cash equivalents at end of year	$ 840,305	$ 520,309	$ 395,851	$ 520,309	$ 840,305

Description of business

Description of business	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of business	Description of business ICON plc and its subsidiaries ("the Company" or "ICON") is a clinical research organization ("CRO"), providing outsourced development services on a global basis to the pharmaceutical, biotechnology and medical device industries. We specialize in the strategic development, management and analysis of programs that support all stages of the clinical development process from compound selection to Phase I-IV clinical studies. Our vision is to be the global CRO partner of choice in drug development by delivering best in class information, solutions and performance in clinical and outcomes research. We believe that we are one of a select group of CROs with the expertise and capability to conduct clinical trials in most major therapeutic areas on a global basis and have the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full-service" solution. At December 31, 2020 we had approximate ly 15,730 employees, in 93 locations in 41 countries. During the year ended December 31, 2020, we derived approximately 33.1%, 57.1% and 9.8% of our revenue in the United States, Europe and Rest of World, respectively. ICON’s ordinary shares are traded on the NASDAQ Global Select Market under the symbol “ICLR”. We began operations in 1990 and have expanded our business predominately through internal growth, together with a number of strategic acquisitions to enhance our capabilities and expertise in certain areas of the clinical development process. We are incorporated in Ireland and our principal executive office is located at: South County Business Park, Leopardstown, Dublin 18, Republic of Ireland. The contact telephone number of this office is +353 1 2912000.

Significant accounting policies

Significant accounting policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Significant accounting policies	Significant accounting policies The accounting policies noted below were applied in the preparation of the accompanying financial statements of the Company and are in conformity with accounting principles generally accepted in the United States. (a) Basis of consolidation The consolidated financial statements include the financial statements of the Company and all of its subsidiaries. All significant intercompany profits, transactions and account balances have been eliminated. The results of subsidiary undertakings acquired in the period are included in the Consolidated Statement of Operations from the date of acquisition. (b) Use of estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reported period. Actual results could differ from those estimates. The principal management estimates and judgments used in preparing the financial statements relate to revenue recognition and taxation. (c) Revenue recognition The Company primarily 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, and laboratory 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). 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. We have concluded that 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 project. 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 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 third party costs) at each reporting period as a percentage of forecasted total project costs. 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 We have concluded that 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 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. 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 amounts corresponds to the value of the Company's performance and the transfer of value to the customer. 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. (d) Third party costs (Reimbursable expenses) Reimbursable expenses comprise investigator payments and certain other costs which are reimbursed by clients under terms specific to each contract to the investigators. See note 2 (e) Direct costs below. (e) Direct costs Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs. Reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time i.e. the full-service obligation is in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred. Investigator payment costs are recorded and reported reflecting investigator activity over the life of the contract. Investigator payments are made based on predetermined contractual arrangements. Payments may differ from the recording and reporting of the expense which is based on activity. (f) Advertising costs All costs associated with advertising and promotion are expensed as incurred. The advertising and promotion costs were $7,020,328, $8,088,451 and $6,516,637 for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 respectively. (g) Foreign currencies and translation of subsidiaries ICON plc's financial statements are prepared in United States dollars. Transactions in currencies other than the functional currency of the individual entities within the ICON Group are recorded at the rate ruling at the date of the transaction. Monetary assets and liabilities denominated in currencies other than the functional currency of the individual entities within the ICON Group are translated into the functional currency of that entity at exchange rates prevailing at the Balance Sheet date. Adjustments resulting from these translations are charged or credited to income. Amounts charged or credited to the Consolidated Statements of Operations for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 were as follows: Year ended 2020 2019 2018 (in thousands) Amounts charged/(credited) $ 5,979 $ 590 $ (3,876) The financial statements of subsidiaries with other functional currencies are translated at period end rates for the Consolidated Balance Sheets and average rates for the Consolidated Statements of Operations. Translation gains and losses arising are reported as a movement on accumulated other comprehensive income. Foreign currency transaction gains and losses are reported in other comprehensive income rather than through income where the foreign currency transaction is 'long-term investment' in nature i.e. settlement is not planned or anticipated in the foreseeable future. (h) Disclosure of fair value of financial instruments Cash, cash equivalents, other receivables, available for sale investments, accounts receivable, accounts payable, investigator payments and income taxes payable have carrying amounts that approximate fair value due to the short term maturities of these instruments. Other liabilities' carrying amounts approximate fair value based on net present value of estimated future cash flows. Debt is measured at historical cost. Financial instruments are measured in the Consolidated Balance Sheets at amortized cost or fair value using a fair value hierarchy of valuation inputs. For financial instruments measured at amortized cost, the fair value of these financial instruments closely approximates their fair value. The fair value hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. These levels are: Level 1: Level 2: Inputs are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. The Group's Senior notes (private placement debt) are carried at $350.0 million (prior to related financing costs). The carrying value at December 31, 2020, closely approximates fair value as the Group's Senior notes were entered into close to the end of the reporting period. The Company classifies its investments in short term debt or equity investments as available for sale, as it does not actively trade such securities nor does it intend to hold them to maturity. The fair value of short term investments are represented by level 1 fair value measurements – quoted prices in active markets for identical assets. The unrealized movements in fair value are recognized in equity until disposal or sale, at which time, those unrealized movements from prior periods are recognized in the Consolidated Statement of Operations. Losses other than temporary, which reduce the carrying amount below cost are recognized in Consolidated Statement of Operations. (i) Business combinations The cost of a business combination is measured as the aggregate of the fair values at the date of exchange of assets given, liabilities incurred or assumed and equity instruments issued in exchange for control. Where a business combination agreement provides for an adjustment to the cost of the acquisition which is contingent upon future events, the amount of the estimated adjustment is recognized at the acquisition date at the fair value of the contingent consideration. Any changes to this estimate outside the measurement period will depend on the classification of the contingent consideration. If the contingent consideration is classified as equity it shall not be re-measured and the settlement shall be accounted for within equity. If the contingent consideration is classified as a liability any adjustments will be accounted for through the Consolidated Statement of Operations or Other Comprehensive Income depending on whether the liability is considered a financial instrument. The assets, liabilities and contingent liabilities of businesses acquired are measured at their fair values at the date of acquisition. In the case of a business combination which is completed in stages, the fair values of the identifiable assets, liabilities and contingent liabilities are determined at the date of each exchange transaction. When the initial accounting for a business combination is determined provisionally, any subsequent adjustments to the provisional values allocated to the identifiable assets, liabilities and contingent liabilities are made within twelve months of the acquisition date and presented as adjustments to goodwill in the reporting period in which the adjustments are determined. The Company allocates a share of net income to the noncontrolling interest holders based on percentage ownership. (j) Goodwill and Impairment Goodwill represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill primarily comprises acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. Goodwill is stated net of any provision for impairment. The Company tests goodwill annually for any impairments or whenever events occur which may indicate impairment. The Company applied the provisions of ASU 2017-04 'Intangibles - Goodwill and Other: Simplifying the test for goodwill impairment' with effect from January 1, 2018. Under the amendment, the Company was required to perform its annual goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An impairment charge would be recognized for any amount by which the carrying amount exceeds the reporting unit's fair value up to the amount of existing goodwill. The amendment allows an entity to perform a qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. No impairment was recognized as a result of the impairment testing carried out for the years ended December 31, 2020, December 31, 2019 and December 31, 2018. (k) Intangible assets Intangible assets are amortized on a straight line basis over their estimated useful life. (l) Cash and cash equivalents Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value. (m) Investments in debt, equity and other Available for sale investments The Company classifies short-term investments as available for sale in accordance with the terms of ASC 320 'Investments - Debt and Equity Securities' . Realized gains and losses are determined using specific identification. The investments are reported at fair value, with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of the investments are represented by accrued interest and unrealized gains/losses. Long term investments The Company classifies its interests in funds having considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2020 meet the definition of equity securities without readily determinable fair values. Effective from January 1, 2018, the Company concluded that the interests held at December 31, 2019 and December 31, 2020 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. Any increases or decreases in fair value are recognized in net income in the period. These are therefore measured at Level 3 of the fair value hierarchy. Equity method investments The Company’s investments that are not consolidated are accounted for under the equity method if the Company exercises significant influence that is considered to be greater than minor. These investments are classified as equity method investments on the accompanying Consolidated Balance Sheet. The Company records its pro rata share of the earnings/losses of these investments in Share of equity method investments in the Consolidated Statement of Operations. The Company reviews these for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. (n) Accounts receivable, net and unbilled revenue Accounts receivable and unbilled revenue are recorded at fair value less an estimate of the credit losses expected to be incurred on the Company's accounts receivable portfolio. The Company's estimate of expected credit losses considers historical credit loss information that is adjusted, where necessary, for current conditions and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. The Company's receivables and unbilled services are predominantly due from large and mid-tier pharmaceutical and biotechnology companies that share similar risk characteristics. The Company monitors their portfolio of receivables and unbilled services for any deterioration in current or expected credit quality (for example, expected delinquency level), and adjusts the allowance for credit losses as required. Changes in the allowance for credit losses are recorded as a provision for (or reversal of) credit loss expense in the Consolidated Statement of Operations. Losses are charged against the allowance when management believes the uncollectibility of a previously provisioned amount is confirmed. Accounts receivable factoring Where the Company enters into an agreement to sell certain portfolios of its accounts receivable balances, the sale is accounted for in accordance with ASC Topic 860 'Transfers and Servicing' (ASC 860). Agreements which result in true sales of the transferred receivables, as defined in ASC 860, which occur when receivables are transferred without recourse to ICON, are excluded from amounts reported in the Consolidated Balance Sheet. Cash proceeds received from such sales are included in operating cash flows. The associated finance costs are presented as interest expense. (o) Inventory Inventory is valued at the lower of cost and net realizable value and after provisions for obsolescence. The cost of inventories comprises the purchase price and attributable costs, less trade discounts. At December 31, 2020 the carrying value of inventory, included within prepayments and other current assets on the Consolidated Balance Sheet, was $4.8 million (2019: $3.2 million). (p) Property, plant and equipment Property, plant and equipment is stated at cost less accumulated depreciation. Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5 Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter. (q) Leases The Company adopted ASC 842 'Leases' ( ASC 842), with a date of initial application of January 1, 2019. The lease accounting policy applied in preparation of the results for the year ended December 31, 2019 and December 31, 2020 therefore reflect application of ASC 842. The Company adopted the standard using the cumulative-effect adjustment approach. Under this transition method, the Company applied the ASC 842 as at the date of initial application (i.e. January 1, 2019), without restatement of comparative period amounts. The cumulative effect of applying the standard is recorded as an adjustment to the opening consolidated balance sheet as at the date of initial application (see note 24 - Impact of new accounting pronouncements for further details). The comparative information for the year ended December 31, 2018 has not been adjusted and therefore continues to be reported under ASC 840 'Leases' (ASC 840) . ASC 842 requires lessees to recognize the rights and obligations resulting from virtually all leases on the Consolidated Balance Sheet as right-of-use (ROU) assets with corresponding lease liabilities. The most significant impact of application of the standard for the Company related to the recognition of right-of-use assets and lease liabilities on the Consolidated Balance Sheet for operating leases for certain property, vehicles and equipment. Prior to application of ASC 842, costs in respect of operating leases were charged to the Consolidated Statements of Operations on a straight-line basis over the lease term. Pursuant to certain practical expedients available as part of adopting ASC 842, ICON did not reassess whether existing or expired supplier contracts are or contain leases, the classification of existing or expired leases, or whether unamortized initial direct costs meet the new definition of initial direct costs under ASC 842. Additionally, the Company elected to use hindsight in determining the lease term and in assessing impairment of ROU assets, if any. The Company determines if an arrangement is a lease at inception. Finance leases, if any, are depreciated on the same basis as property, plant and equipment. At December 31, 2020 and December 31, 2019, the Company did not account for any leases as finance leases. Operating leases are included in operating right-of-use assets, other liabilities and non-current operating lease liabilities on our Consolidated Balance Sheet with the lease charge recognized on a straight-line basis over the lease term. ROU assets and lease liabilities are recognized based on the present value of future minimum lease payments over the lease term at commencement date or date of transition. Our lease terms may also include options to extend or terminate. The Company actively reviews options to extend or terminate leases and adjusts the ROU asset and lease liability when it is reasonably certain the option will be exercised. The ROU asset is adjusted for any prepayments made at the date of commencement and any initial direct costs incurred. As most of the Company's leases do not provide an implicit rate, the discount rate used is based on the rate of traded corporate bonds available at the commencement date adjusted for country risk, liquidity and lease term. Leasehold improvements are amortized over the shorter of the depreciable lives of the corresponding fixed assets or the lease term including any applicable renewals. Certain property leases include variable lease payments resulting from periodic rent increases based on an index which cannot be reasonably estimated at the lease commencement date. These costs are expensed as incurred on the Consolidated Statements of Operations. The Company accounts for lease and non-lease components separately with lease components flowing through the Consolidated Balance Sheet and non-lease components expensed directly to the Consolidated Statements of Operations. In some cases, the Company enters into sublease agreements and becomes both a lessee and a lessor for the same underlying asset. Although subleases are accounted for separately from the lease they relate to, subleases are accounted for in the same way as other leases. (r) 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 amounts of existing assets and liabilities and their respective tax bases and for operating loss and tax credit carry-forwards. 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 income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance to the amount 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 GILTI (“global intangible low-taxed income”) as a period item in the period it arises and has therefore not provided for deferred taxes in respect of this item. (s) Government grants Government grants received relating to capital expenditures are shown by deducting the grant from the asset's carrying amount and crediting them to income on a basis consistent with the depreciation policy of the relevant assets. Grants relating to categories of operating expenditures are shown as deferred income and credited to income in the period in which the expenditure to which they relate is charged. Under the grant agreements amounts received may become repayable in full should certain circumstances specified within the grant agreements occur, including downsizing by the Company, disposing of the related assets, ceasing to carry on its business or the appointment of a receiver over any of its assets. The Company has not recognized any loss contingency having assessed as remote the likelihood of these events arising. (t) Research and development credits Research and development credits are available to the Company under the tax laws in certain jurisdictions, based on qualifying research and development spend as defined under those tax laws. Research and development credits are generally recognized as a reduction of income tax expense. However, certain tax jurisdictions provide refundable credits that are not wholly dependent on the Company's ongoing income tax status or income tax position. In these circumstances the benefit of these credits is not recorded as a reduction to income tax expense, but rather as a reduction of operating expenditure. (u) Pension costs The Company contributes to defined contribution plans covering all eligible employees. The Company contributes to these plans based upon various fixed percentages of employee compensation and such contributions are expensed as incurred. The Company operates, through two subsidiaries, a defined benefit plan for certain of its United Kingdom and Swiss employees. The Company accounts for the costs of these plans in accordance with ASC 715-30 'Defined Benefit Plans – Pension' . These plans are presented in accordance with the requirements of ASC 715-60 'Defined Benefit Plans – Other Postretirement' . (v) Redeemable noncontrolling interests and equity The Company acquired a majority ownership interest in MeDiNova during the year ended December 31, 2019. Included in the purchase agreement were put and call option arrangements with the noncontrolling interest holders that required (put option) or enabled (call option) the Company to purchase the remaining minority ownership at a future date. The option was accounted for as temporary equity, which is presented separately as redeemable noncontrolling interest on the Consolidated Balance Sheet. This classification reflects the assessment that the instruments are contingently redeemable in accordance with ASC 480-10-S99 'Distinguishing Liabilities from Equity'. On March 9, 2020, ICON exercised its option to call the remaining shares and took 100% ownership of MeDiNova. Redeemable noncontrolling interests are accreted to their redemption value over the period from the date of issuance to the first date on which the option is exercisable. The change in the option's redemption value is recorded against retained earnings. In a computation of earnings per share, the accretion of redeemable noncontrolling interests to their redemption value is a reduction of net income attributable to the Group. Basic and diluted net income per ordinary share attributable to the Group includes the adjustment to reflect the accretion of the noncontrolling interest to its redemption value until the redemption of the noncontrolling interest on March 9, 2020. (w) Net income per ordinary share Basic net income per ordinary share attributable to the Company has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares. There is no difference in net income used for basic and diluted net income per ordinary share. Basic and diluted net income per ordinary share attributable to the Company includes the adjustment to reflect the accretion of the noncontrolling interest in MeDiNova to its redemption value (see note 26 - Net income per ordinary share ). (x) Share-based compensation The Company accounts for its share options, Restricted Share Units ("RSUs") and Performance Share Units ("PSUs") in accordance with the provisions of ASC 718 'Compensation – Stock Compensation'. Share-based compensation expense for equity-settled awards made to employees and directors is measured and recognized based on estimated grant date fair values. These equity-settled awards include employee share options, RSUs and PSUs. Share-based compensation expense for share options awarded to employees and directors is estimated at the grant date based on each option's fair value as calculated using the Black-Scholes option-pricing model. Share-based compensation for RSUs and PSUs awarded to employees and directors is calculated based on the market value of the Company's shares on the date of award of the RSUs and PSUs. The value of awards expected to vest is recognized as an expense over the requisite service periods. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates. Estimating the grant date fair value of share options as of the grant date using an option-pricing model, such as the Black-Scholes model, is affected by the Company's share price as well as assumptions regarding a number of complex variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards. Liability classified awards are measured at the fair value of the award on the grant date and remeasured at each reporting period at fair value until the award is settled. (y) Impairment of long-lived assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured at the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount of the asset or fair value less selling costs. (z) Derivative financial instruments We enter into transactions in the normal course of business using various financial instruments in order to hedge against exposure to fluctuating exchange and interest rates. We use derivative financial instruments to reduce exposure to fluctuations in interest rates. A derivative is a financial instrument or other contract whose value changes in response to some underlying variable, which has an initial net investment smaller than would be required for other instruments that have a similar response to the variable and that will be settled at a future date. We do not enter into derivative financial instruments

Investments

Investments	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Investments	Investments (a) Available for sale investments December 31, 2020 December 31, 2019 (in thousands) Available for sale investments at start of year $ 49,628 $ 59,910 Purchases — 9,603 Sales and maturities (47,902) (21,686) Interest on short term investments — 1,065 Realized gain on sale of short term investments 234 55 Unrealized capital (loss)/gain – investments (231) 681 $ 1,729 $ 49,628 The Company classifies its investment in short term investments as available for sale. Short term investments comprise highly liquid investments with maturities of greater than three months and minimum "A-" rated fixed term deposits. Short term investments at December 31, 2020 have an average maturity of 3.4 years compared to 0.65 years at December 31, 2019. The following table represents our available for sale short term investments by major security type as of December 31, 2020: Maturity by period Cost Fair Value Less than 1 1 to 5 (in millions) Term deposits $ 1.73 $ 1.73 $ 0.50 $ 1.23 Total ($ in millions) $ 1.73 $ 1.73 $ 0.50 $ 1.23 The contractual maturity of certain investments in the portfolio is greater than 12 months; however, classification as short-term investments reflects the Company practice and intention in respect of these investments. The Company recognizes the unrealized losses at fair value in equity as these unrealized losses on short term investments have been considered as temporary. (b) Investments in equity - long term The Company entered into subscription agreements with a number of funds. Capital totaling $13.3 million had been advanced under the terms of the subscription agreements at December 31, 2020 (December 31, 2019: $10.1 million). The Company determined that the interests in the funds meet the definition of equity securities without readily determinable fair values. Effective from January 1, 2018, the Company concluded that the interests held at December 31, 2020 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. There was an increase in fair value of $2.5 million (December 31, 2019: decrease in fair value of $0.8 million) recognized in net income during the year bringing the carrying value of the subscriptions to $15.8 million at December 31, 2020 (December 31, 2019: $10.1 million). At (c) Equity method investments The Company has invested $4.9 million to obtain a 49% interest in the voting share capital of Oncacare. The Company’s investment in Oncacare is accounted for under the equity method due to the Company's ability to exercise significant influence over Oncacare that is considered to be greater than minor. The Company records its pro rata share of the earnings/losses of this investment in 'Share of equity method investments' in the Consolidated Statement of Operations. See additional details in note 2 - Significant accounting policies. The majority investor has the right to sell the 51% majority voting share capital exclusively to the Company in an eighteen month period, commencing January 1, 2023 and ICON also has the right to acquire the 51% majority voting share capital from August 1, 2025. The following table represents our equity method investments at December 31, 2020: Ownership Percentage Carrying Value Carrying Value At date of investment December 31, 2020 December 31, 2020 July 24, 2020 (in thousands) Oncacare Limited 49 % $ 4,534 $ 4,900 The Company has invested $4.9 million in Oncacare, of which, $2.5 million has been paid with the remainder to be paid in the next twelve months.

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill December 31, 2020 December 31, 2019 (in thousands) Opening goodwill $ 883,170 $ 756,260 Current year acquisitions 27,191 126,932 Prior period acquisition (note 4 (b),(c)) 123 — Foreign exchange movement 25,773 (22) Closing goodwill $ 936,257 $ 883,170 The Company has made a number of strategic acquisitions since inception to enhance its capabilities and experience in certain areas of the clinical development process. Goodwill arising on acquisition represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill is primarily comprised of the acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. The Company tests goodwill annually for impairment or whenever events occur which may indicate impairment. The results of the Company's goodwill impairment testing assessed at September 30, 2020 during the year ended December 31, 2020 provided no evidence of impairment and indicated the existence of sufficient headroom such that a reasonably possible change to the key assumptions used would be unlikely to result in an impairment of the related goodwill. Acquisitions (a) MedPass Group ("MedPass") On January 22, 2020 a subsidiary of the Company, ICON Investments Limited acquired 100% of the equity share capital of the MedPass Group. MedPass is the leading European medical device CRO, regulatory and reimbursement consultancy, that specializes in medical device development and market access. The acquisition of MedPass further enhances ICON’s Medical Device and Diagnostic Research services, through the addition of new regulatory and clinical capabilities in Europe. The integration of MedPass’s services brings noted expertise in complex class 3 medical devices, interventional cardiology and structural heart devices . Accounting for the acquisition of MedPass was finalized in the year ended December 31, 2020. The acquisition of MedPass has been accounted for as a business combination in accordance with ASC 805 ' Business Combinations' . The Company has made an assessment of the fair value of assets acquired and liabilities assumed as at that date. The following table summarizes the Company’s fair values of the assets acquired and liabilities assumed: January 22, 2020 (in thousands) Cash & cash equivalents $ 10,170 Property, plant and equipment 45 Operating right of use assets 539 Goodwill * 27,191 Customer relationships 11,725 Order backlog 2,883 Accounts receivable 3,033 Prepayments and other current assets 158 Accounts payable (368) Unearned revenue (989) Other liabilities (2,202) Current lease liabilities (219) Non-current lease liabilities (320) Non-current deferred tax liability (4,090) Net assets acquired $ 47,556 Cash outflows $ 46,992 Working capital adjustment paid 564 Contingent consideration ** — Total consideration $ 47,556 * Goodwill represents the acquisition of an established workforce that specializes in medical device development and market access. None of the goodwill recognized is expected to be deductible for income tax purposes. ** The fair value of the contingent consideration was estimated at the date of acquisition. Depending on performance of the company, the total consideration could have increased by a maximum of $6.7 million in contingent consideration. At the acquisition date and at December 31, 2020, the fair value of this contingent consideration payable to MedPass is $Nil. In finalizing the acquisition of MedPass in the twelve month period from acquisition, fair value adjustments were made which resulted in an increase in accounts receivable ($0.2 million) and unearned revenue ($0.8 million) and a decrease in operating right of use assets ($0.8 million), other liabilities ($0.8 million), current lease liabilities ($0.1 million), non-current lease liabilities ($0.7 million) and non-current deferred tax liability ($0.6 million). Customer relationship and order backlog assets were also finalized. Since January 22, 2020, MedPass has earned revenue of $13.2 million and net income of $2.5 million in the year ended December 31, 2020. The proforma effect of the MedPass acquisition if completed on January 1, 2019 would have resulted in revenue, net income and earnings per share for the fiscal years ended December 31, 2020 and December 31, 2019 as follows: Year Ended 2020 2019 (in thousands) Revenue $ 2,798,180 $ 2,820,796 Net income $ 332,521 $ 377,485 (in ones) Basic earnings per share $ 6.21 $ 6.91 Diluted earnings per share $ 6.16 $ 6.85 (b) Symphony On September 24, 2019 a subsidiary of the Company, ICON Clinical Research LLC, acquired a 100% interest in Symphony. Symphony is a leading provider of at-home trial services and site support services. The acquisition of Symphony further enhances our site and patient services offering. The acquisition resulted in the recognition of goodwill of $22.9 million. Accounting for the acquisition of Symphony was finalized in the year ended December 31, 2020. The acquisition of Symphony has been accounted for as a business combination in accordance with ASC 805 ' Business Combinations' . The Company has made an assessment of the fair value of assets acquired and liabilities assumed as at that date.The following table summarizes the Company’s fair values of the assets acquired and liabilities assumed: September 24, 2019 (in thousands) Cash & cash equivalents $ 3,292 Property, plant and equipment 564 Operating right of use assets 820 Goodwill * 22,865 Customer relationships 8,159 Order backlog 2,163 Accounts receivable 4,544 Unbilled revenue 186 Prepayments and other current assets 181 Other receivables 6 Accounts payable (799) Unearned revenue (2,411) Other liabilities (933) Current lease liabilities (289) Non-current lease liabilities (531) Net assets acquired $ 37,817 Cash outflows 34,976 Working capital adjustment paid 341 Contingent consideration 2,500 Total consideration $ 37,817 * Goodwill represents the acquisition of an established workforce and the capability to provide at-home trial services and site support solutions. The full amount of the goodwill recognized is expected to be deductible for income tax purposes. In finalizing the acquisition of Symphony in the twelve month period from acquisition, fair value adjustments were made which resulted in increases in accounts receivable ($0.6 million) and unearned revenue ($1.0 million) and decreases in unbilled revenue ($0.1 million), accounts payable ($0.3 million) and other liabilities ($0.1 million). Customer relationship and order backlog assets were also finalized. The contingent consideration was assessed at the acquisition date and was based on revenue targets set for the company to the period ending March 31, 2020. The fair value of the contingent consideration on acquisition, and at December 31, 2019, was $2.5 million based on an income approach. The value of the contractual contingent consideration is in the range of zero to a maximum of $2.5 million. Due to the short term nature of the contingent consideration, no discounting was applied. As the fair value measure is based on significant inputs that are not observable in the market, the contingent consideration meets the definition of a level 3 financial instrument under ASC 820. The inputs that are not observable in the market are the expected revenue of the company to the period ending March 31, 2020 and the probability of achievement of this revenue target by the company. A positive or negative movement of these inputs by 10% would not result in a material change to the contingent consideration balance. In the twelve month period ended December 31, 2020, the contingent consideration was settled at fair value in the amount of $0.5 million. The change in fair value has been recorded in the selling, general and administrative expense line of the Consolidated Statement of Operations. Since September 24, 2019, Symphony has earned revenue of $4.5 million and net income of $1.0 million in the year ended December 31, 2019. The proforma effect of the Symphony acquisition if completed on January 1, 2018 would have resulted in revenue, net income and earnings per share for the fiscal years ending December 31, 2019 and December 31, 2018 as follows: Year Ended 2019 2018 (in thousands) Revenue $ 2,818,280 $ 2,609,233 Net income $ 376,636 $ 324,671 (in ones) Basic earnings per share $ 6.90 $ 6.00 Diluted earnings per share $ 6.84 $ 5.93 (c) MeDiNova On May 23, 2019 a subsidiary of the Company, ICON Clinical Research (U.K.) Limited acquired a 60% majority shareholding in MeDiNova, a site network with research sites in key markets in Europe and Africa. On March 9, 2020 ICON exercised its option to call the outstanding shares in the noncontrolling interest to take 100% ownership of MeDiNova. The acquisition further enhances ICON's patient recruitment capabilities in EMEA and complements ICON's existing site network in the US, PMG Research. The acquisition resulted in the recognition of goodwill of $81.8 million. Accounting for the acquisition of MeDiNova was finalized in the year ended December 31, 2020. The acquisition of MeDiNova has been accounted for as a business combination in accordance with ASC 805 ' Business Combinations' . The Company made an assessment of the fair value of assets acquired and liabilities assumed as at that date. The following table summarizes the Company’s fair values of the assets acquired and liabilities assumed: May 23, 2019 (in thousands) Cash & cash equivalents $ 7,719 Property, plant and equipment 670 Operating right of use assets 1,558 Goodwill * 81,760 Customer relationships 3,887 Order backlog 171 Patient database 2,542 Accounts receivable 3,488 Unbilled revenue 4,272 Other receivables 819 Prepayments and other current assets 406 Accounts payable (5,484) Unearned revenue (5,796) Other liabilities (6,860) Current lease liabilities (430) Non-current lease liabilities (1,128) Non-current deferred tax liability (1,345) Net assets acquired $ 86,249 Cash outflows $ 54,123 Working capital adjustment received (466) Redeemable noncontrolling interest ** 32,592 Total consideration $ 86,249 *Goodwill represents the acquisition of an established workforce and access to a broad site network in Europe and Africa. None of the goodwill recognized is expected to be deductible for income tax purposes. **The fair value of the redeemable noncontrolling interest on May 23, 2019 was $32.6 million which was estimated by applying an income based approach. The valuation approach used was based on the future earnings of the company times an appropriate earnings multiple. On March 9, 2020 ICON exercised its option to call the outstanding shares in the noncontrolling interest to take 100% ownership of MeDiNova. Effective from this date, the noncontrolling interest was derecognized and a liability of $44.7 million was recognized, representing the assessment of the redemption value of the noncontrolling interest. This liability was settled on July 17, 2020 for $43.9 million. In finalizing the acquisition of MeDiNova in the twelve month period from acquisition, fair value adjustments were made which resulted in an increase in other liabilities ($1.6 million) and decreases in operating right of use assets ($0.3 million), current lease liabilities ($0.1 million), non-current lease liabilities ($0.3 million) and non-current deferred tax liability ($2.2 million). Cust omer relationship, order backlog and patient database assets were also finalized. Since May 23, 2019, MeDiNova has earned revenue of $6.5 million (after elimination of intercompany revenue from ICON) and net income of $4.7 million in the year ended December 31, 2019. The proforma effect of the MeDiNova acquisition if completed on January 1, 2018 would have resulted in revenue, net income and earnings per share for the fiscal years ending December 31, 2019 and December 31, 2018 as follows: Year Ended 2019 2018 (in thousands) Revenue $ 2,807,788 $ 2,599,091 Net income $ 375,979 $ 323,759 (in ones) Basic earnings per share $ 6.89 $ 5.98 Diluted earnings per share $ 6.83 $ 5.91 (d) MMD On January 25, 2019 a subsidiary of the Company, ICON Laboratory Services, Inc. acquired 100% of the share capital of MMD. MMD is a molecular diagnostic specialty laboratory that enables the development and commercialization of precision medicines in oncology. The acquisition resulted in the recognition of goodwill of $22.4 million. The acquisition of MMD has been accounted for as a business combination in accordance with ASC 805 ' Business Combinations' . The Company made an assessment of the fair value of assets acquired and liabilities assumed as at that date. The following table summarizes the Company’s fair values of the assets acquired and liabilities assumed: January, 25 2019 (in thousands) Cash & cash equivalent $ 686 Property, plant and equipment 1,697 Operating right of use assets 2,866 Goodwill * 22,430 Customer relationships * 10,708 Order backlog * 2,787 Accounts receivable 3,100 Unbilled revenue 2,421 Other receivables 43 Prepayments and other current assets 908 Deferred tax asset 1,568 Accounts payable (1,280) Unearned revenue (540) Other liabilities (1,232) Current lease liabilities (699) Non-current lease liabilities (2,167) Non-current other liabilities (1,123) Net assets acquired $ 42,173 Cash outflows $ 42,349 Working capital adjustment (176) Total consideration $ 42,173 *Goodwill represents the acquisition of an established workforce with experience in molecular diagnostic specialty laboratory services and commercialization of precision medicines in oncology. None of the goodwill recognized is expected to be deductible for income tax purposes. In finalizing the goodwill on acquisition of MMD in the twelve month period from acquisition, fair value adjustments were made which resulted in increases in unbilled revenue ($2.1 million), deferred tax asset ($3.7 million), accounts payable ($0.6 million) and other liabilities ($0.1 million) and decreases in property, plant and equipment ($0.1 million) and unearned revenue ($0.9 million). Customer relationship and order backlog assets were also finalized. Since January 25, 2019, MMD has earned revenue of $17.9 million and net income of $1.1 million in the year ended December 31, 2019. The proforma effect of the MMD acquisition if completed on January 1, 2018 would have resulted in revenue, net income and earnings per share for the fiscal years ending December 31, 2019 and December 31, 2018 as follows: Year Ended 2019 2018 (in thousands) Revenue $ 2,806,908 $ 2,612,346 Net income $ 373,930 $ 321,698 (in ones) Basic earnings per share $ 6.85 $ 5.94 Diluted earnings per share $ 6.79 $ 5.87

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets December 31, 2020 December 31, 2019 Cost (in thousands) Customer relationships $ 144,101 $ 132,192 Technology asset 11,169 11,169 Order backlog 39,224 36,318 Trade names/ brands 2,766 2,766 Volunteer list 1,325 1,325 Non-compete arrangements 489 489 Patient database 2,542 2,542 Foreign exchange movement 209 (5,728) Total cost 201,825 181,073 Accumulated amortization (135,430) (116,196) Foreign exchange movement 65 3,017 Net book value $ 66,460 $ 67,894 On January 22, 2020 a subsidiary of the Company, ICON Investments Limited acquired 100% of the equity share capital of the MedPass Group. MedPass is the leading European medical device CRO, regulatory and reimbursement consultancy, that specializes in medical device development and market access. The acquisition of MedPass further enhances ICON’s Medical Device and Diagnostic Research services, through the addition of new regulatory and clinical capabilities in Europe. The value of certain customer relationships and order backlog identified of $11.7 million and $2.9 million respectively were recognized on acquisition and are being amortized over approximately 13 years and 3 years, the estimated period of benefit. In total, $1.8 million has been amortized in the period since the date of acquisition. On September 24, 2019 a subsidiary of the Company, ICON Clinical Research LLC, acquired a 100% interest in Symphony. Symphony is a leading provider of at-home trial services and site support services. The acquisition of Symphony further enhances our site & patient services offering. The value of certain customer relationships and order backlog identified of $8.2 million and $2.2 million respectively were recognized on acquisition and are being amortized over approximately 12 years and 2 years, the estimated period of benefit. In total, $2.2 million has been amortized in the period since the date of acquisition. On May 23, 2019 a subsidiary of the Company, ICON Clinical Research (U.K.) Limited acquired a majority shareholding in MeDiNova, a site network with research sites in key markets in Europe and Africa. ICON had the right to acquire the remaining shares in the company during 2020 and on March 9, 2020 ICON exercised its option to call the outstanding shares in the noncontrolling interest to take 100% ownership of MeDiNova. The acquisition further enhances ICON's patient recruitment capabilities in EMEA and complements ICON's existing site network in the US, PMG Research. The value of certain customer relationships, patient database and order backlog identified of $3.9 million, $2.5 million and $0.2 million respectively were recognized on acquisition and are being amortized over approximately 12 years, 7 years and 1 year, the estimated period of benefit. In total, $1.3 million has been amortized in the period since the date of acquisition. The order backlog was fully amortized at December 31, 2020. On January 25, 2019 a subsidiary of the Company, ICON Laboratory Services, Inc. acquired 100% of the share capital of MMD. MMD is a molecular diagnostic specialty laboratory that enables the development and commercialization of precision medicines in oncology. The value of certain customer relationships and order backlog identified of $10.7 million and $2.8 million respectively were recognized on acquisition and are being amortized over approximately over 16 years and 2 years, the estimated period of benefit. In total, $4.0 million has been amortized in the period since the date of acquisition. On July 27, 2017, a subsidiary of the Company, ICON Clinical Research Limited acquired Mapi Group. Mapi is a leading patient-centered health outcomes research and commercialization company. The acquisition of Mapi strengthens ICON’s existing commercialization and outcomes research business adding significant commercialization presence, analytics, real world evidence generation and strategic regulatory services. The value of certain customer relationships and order backlog identified of $18.4 million and $13.0 million respectively were recognized on acquisition and are being amortized over approximately over 8 years and 8.5 years, the estimated period of benefit. In total, $13.3 million has been amortized in the period since the date of acquisition. On September 15, 2016, a subsidiary of the Company, ICON US Holdings Inc., acquired ICON Government & Public Health Solutions (''GPHS'') (formerly Clinical Research Management (ClinicalRM)), a full-service CRO specializing in preclinical through Phase IV support of clinical research and clinical trial services for biologics, drugs and devices. The organization helps customers progress their products to market faster with a wide array of research, regulatory and sponsor services within the U.S. and around the globe. GPHS provides full-service and functional research solutions to a broad range of US government agencies. The value of certain customer relationship, order backlog and brand assets identified of $4.0 million, $1.7 million and $1.4 million respectively are being amortized over approximately 7 years, 2 years and 5 years respectively, the estimated period of benefit. In total, $5.6 million has been amortized in the period since the date of acquisition. The order backlog and brand assets were fully amortized at December 31, 2020. On December 4, 2015, a subsidiary of the Company, ICON Clinical Research LLC, acquired PMG, an integrated network of 52 clinical research sites in North Carolina, South Carolina, Tennessee, Illinois and Iowa. The site network includes wholly owned facilities and dedicated clinical research sites. PMG conducts clinical trials in all major therapeutic areas and has particular expertise in vaccine, gastroenterology, cardiovascular, neurology and endocrinology studies. The value of certain customer relationship and order backlog assets identified of $6.9 million and $3.0 million respectively are being amortized over approximately 7 years and 2 years respectively, the estimated period of benefit. In total, $8.0 million has been amortized in the period since the date of acquisition. The order backlog was fully amortized at December 31, 2020. On February 27, 2015, a subsidiary of the Company, ICON Holdings Unlimited Company (formerly ICON Holdings), acquired MediMedia Pharma Solutions. Headquartered in Yardley, Pennsylvania, MediMedia Pharma Solutions includes MediMedia Managed Markets and Complete Healthcare Communications. MediMedia Managed Markets is a leading provider of strategic payer-validated market access solutions. Complete Healthcare Communications is one of the leading medical and scientific communication agencies working with medical affairs, commercial and brand development teams within life science companies. The value of certain customer relationships and order backlog identified of $22.8 million and $2.5 million respectively are being amortized over approximately 7 years and 1 year, the estimated period of benefit. $21.5 million has been amortized in the period since the date of acquisition. The order backlog is fully amortized at December 31, 2020. On May 7, 2014, a subsidiary of the Company, ICON US Holdings Inc., acquired Aptiv Solutions, Inc. ("Aptiv"), a global biopharmaceutical and medical device development services company and leader in adaptive clinical trials. Aptiv offers full-service clinical trial consulting and regulatory support for drugs, medical devices and diagnostics with a specific focus on strategy to increase product development efficiency and productivity. The value of certain customer relationships and order backlog identified of $21.4 million and $7.9 million respectively are being amortized over approximately 7 years and 3 years, the estimated period of benefit. In total, $28.3 million has been amortized in the period since the date of acquisition. The order backlog is fully amortized at December 31, 2020. On February 28, 2012, a subsidiary of the Company, ICON Clinical Research LLC (formerly ICON Clinical Research, Inc.), acquired PriceSpective, a strategy consulting company. The value of certain customer relationships identified of $10.2 million are being amortized over approximately 10 years, the estimated period of benefit. The value of order backlog and certain non-compete arrangements identified of $0.4 million and $0.4 million respectively were amortized over approximately 0.8 years and 3 years, the estimated period of benefit and were fully amortized at December 31, 2020. In total, $9.8 million has been amortized in the period since the date of acquisition. On February 15, 2012, a subsidiary of the Company, ICON Clinical Research Limited, acquired BeijingWits Medical, a Chinese CRO. The value of certain customer relationships identified of $1.8 million are being amortized over approximately 10 years, the estimated period of benefit. The value of certain order backlog and non-compete arrangements identified of $0.4 million and $0.1 million respectively were amortized over approximately 4 years and 5 years, the estimated period of benefit and were fully amortized at December 31, 2020. In total, $2.0 million has been amortized in the period since the date of acquisition. On July 14, 2011, a subsidiary of the Company, ICON Clinical Research Limited, acquired Firecrest Clinical Limited, a provider of technology solutions that boost investigator site performance and study management. The value of certain technology assets and customer relationships identified of $11.2 million and $5.2 million respectively are being amortized over approximately 7.5 years, the estimated period of benefit and were fully amortized at December 31, 2020. The value of the Firecrest trade name and order backlog identified of $1.4 million and $1.2 million respectively were amortized over approximately 4.5 years and 1.2 years, the estimated period of benefit and were fully amortized at December 31, 2020. In total, $16.5 million has been amortized in the period since the date of acquisition. Future intangible asset amortization expense for the years ended December 31, 2021 to December 31, 2025 is as follows: Year Ended (in thousands) 2021 $ 16,132 2022 10,426 2023 7,594 2024 7,149 2025 6,126 $ 47,427

Property, Plant and Equipment,

Property, Plant and Equipment, net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, net	Property, Plant and Equipment, net December 31, 2020 December 31, 2019 (in thousands) Cost Land $ 3,724 $ 3,724 Building 84,195 85,050 Computer equipment and software 438,742 434,707 Office furniture and fixtures 90,477 88,401 Laboratory equipment 44,101 40,611 Leasehold improvements 30,588 31,144 Motor vehicles 158 199 Foreign exchange movement 11,728 (1,371) 703,713 682,465 Less accumulated depreciation and asset write offs (522,473) (518,383) Foreign exchange movement (6,897) 1,005 Property, plant and equipment (net) $ 174,343 $ 165,087

Other Liabilities

Other Liabilities	12 Months Ended
	Dec. 31, 2020
Other Liabilities Disclosure [Abstract]
Other Liabilities	Other Liabilities December 31, 2020 December 31, 2019 (in thousands) General trade and overhead liabilities* $ 188,638 $ 174,131 Personnel related liabilities 161,363 157,146 Operating lease liability (note 22) 24,334 28,320 Facility related liabilities 9,441 11,138 Other liabilities 8,726 6,531 Restructuring charges (note 14) 7,219 1,232 Short term government grants (note 11) 48 45 $ 399,769 $ 378,543 *includes amounts due to third parties in respect o f accrued reimbursable expenses of $138.2 million at December 31, 2020 and $142.6 million at December 31, 2019.

Non-Current Other Liabilities

Non-Current Other Liabilities	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Non-Current Other Liabilities	Non-Current Other Liabilities December 31, 2020 December 31, 2019 (in thousands) Other non-current liabilities $ 15,971 $ 11,459 Defined benefit pension obligations, net (note 9) 10,395 6,053 $ 26,366 $ 17,512

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefits	Employee Benefits Certain Company employees are eligible to participate in a defined contribution plan (the "Plan"). Participants in the Plan may elect to defer a portion of their pre-tax earnings into a pension plan, which is run by an independent party. The Company matches participant's contributions typically at 8% of the participant's annual compensation. Contributions to the plan are recorded as an expense in the selling, general and administrative line in the Consolidated Statement of Operations. Contributions for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 were $28,036,000, $26,483,000 and $25,241,000 respectively. The Company's United States operations maintain a retirement plan (the "U.S. Plan") that qualifies as a deferred salary arrangement under Section 401(k) of the Internal Revenue Code. Participants in the U.S. Plan may elect to defer a portion of their pre-tax earnings, up to the Internal Revenue Service annual contribution limit. The Company matches participant's contributions up to 3% and matches 50% of participant's contributions thereafter to a maximum Company contribution of 4.5% of the participant's annual compensation. Contributions to this U.S. Plan are recorded, in the year contributed, as an expense in the Consolidated Statement of Operations. Contributions for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 were $17,048,000, $16,469,000 and $15,532,000 respectively. ICON Development Solutions Limited pension plan One of the Company's subsidiaries, ICON Development Solutions Limited, operates a defined benefit pension plan in the United Kingdom for its employees. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2020, December 31, 2019 and December 31, 2018, consist of units held in independently administered funds. The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60 'Defined Benefit Plans – Other Postretirement' . The plan has been closed to new entrants with effect from July 1, 2003. Funded status December 31, 2020 December 31, 2019 (in thousands) Projected benefit obligation $ (43,988) $ (37,036) Fair value of plan assets 34,612 32,016 Funded status $ (9,376) $ (5,020) Non-current other liabilities (note 8) $ (9,376) $ (5,020) Change in benefit obligation December 31, 2020 December 31, 2019 (in thousands) Benefit obligation at beginning of year $ 37,036 $ 30,045 Service cost 100 107 Interest cost 746 867 Plan participants' contributions 22 24 Benefits paid (724) (177) Actuarial loss 5,294 4,756 Foreign currency exchange rate changes 1,514 1,414 Benefit obligation at end of year $ 43,988 $ 37,036 Change in plan assets December 31, 2020 December 31, 2019 (in thousands) Fair value of plan assets at beginning of year $ 32,016 $ 27,297 Actual return on plan assets 2,092 3,504 Employer contributions 109 148 Plan participants' contributions 22 24 Benefits paid (724) (177) Foreign currency exchange rate changes 1,097 1,220 Fair value of plan assets at end of year $ 34,612 $ 32,016 The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company. The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit cost: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Service cost $ 100 $ 107 $ 124 Interest cost 746 867 895 Expected return on plan assets (1,214) (574) (624) Amortization of net loss 160 67 248 Net periodic benefit cost $ (208) $ 467 $ 643 The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2020, December 31, 2019 and December 31, 2018: December 31, 2020 December 31, 2019 December 31, 2018 Discount rate 2.1 % 2.9 % 2.5 % Rate of compensation increase 3.3 % 3.7 % 3.7 % Expected rate of return on plan assets 4.0 % 2.1 % 2.0 % Other comprehensive income December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Actuarial loss/(gain) - benefit obligation $ 5,294 $ 4,756 $ (3,844) Actuarial (gain)/loss – plan assets (878) (2,930) 1,208 Actuarial loss recognized in net periodic benefit cost (160) (67) (248) Total $ 4,256 $ 1,759 $ (2,884) The estimated net loss and prior service cost for the defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year are $0.6 million and $Nil respectively. Benefit Obligation The following assumptions were used in determining the benefit obligation at December 31, 2020 and December 31, 2019: December 31, 2020 December 31, 2019 Discount rate 1.5 % 2.1 % Rate of compensation increase 3.4 % 3.3 % A single discount rate is used which, when used to discount the projected benefit cashflows underlying a pension scheme with a 26 year duration, gives the same result as a full AA corporate bond yield curve. Actuarial losses on the benefit obligation during 2020 resulted from changes in the assumptions compared to those adopted at December 2019. Changes in the assumptions reflect the changes in market conditions from December 2019 to December 2020 and the actuarial loss is primarily due to the change in the discount rate and a reduction in the inflation risk premium. Plan Assets The assets of the scheme are held on an investment platform with Mobius which invests in a number of investment funds with Legal & General, Stone Harbor, Ninety One and Barings. The overall investment strategy is that approximately 19% of investments are in senior secured loans, 20% in corporate bonds, high yield bonds and multi-asset credit fund and 21% in world equities respectively. There is no self-investment in employer related assets. The expected long-term rate of return on assets at December 31, 2020 of 3.4% was calculated as the value of the fund after application of a market value reduction factor. The expected long term rates of return on different asset classes are as follows: Asset Category Expected long-term return per annum Corporate Bonds (including 50% high yield bonds) 2.8 % Equities 5.2 % Secured Loans and Multi Asset Credit 3.0 % The long-term expected rate of return on cash is determined by reference to traditional corporate bond rates at the latest Balance Sheet date. The long-term expected returns on traditional corporate and government bonds are determined by reference to corporate bond yields and gilt yields respectively at the Balance Sheet date. The long-term expected returns on equities are based on the rate of return on government bonds with an allowance for out-performance. The long-term expected return on high yield bonds, secured loans and multi asset credit is based on the return on traditional corporate bonds with an allowance for out-performance. The underlying asset split of the fund is shown below. Asset Category December 31, 2020 December 31, 2019 Corporate Bonds (including 50% high yield bonds) 40 % 40 % Equities 21 % 21 % Secured Loans and Multi Asset Credit 39 % 39 % 100 % 100 % Applying the above expected long term rates of return to the asset distribution at December 31, 2020, gives rise to an expected overall rate of return of scheme assets of approximately 3.4% per annum. Plan Asset Fair Value Measurements Quoted Prices in Active Markets for Identical Assets December 31, 2020 December 31, 2019 (in thousands) Cash $ 11 $ 17 Fixed Income Securities L&G Life GPBF All World Equity Index Fund 7,460 6,753 L&G Life DC Active Corporate Bond 6,797 6,412 Stone Harbor High Yield Bond Fund 6,861 6,292 Ninety One Global Total Return Credit 3,472 — Investec Global Total Return Credit Fund — 3,166 Stone Harbor Multi Asset Credit Portfolio 3,389 3,151 Barings European Loan Fund Buy & Hold 6,622 6,225 $ 34,612 $ 32,016 Cash Flows The Company expects to contribute $0.1 million to the pension fund in the year ending December 31, 2021. The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2021 $ 342 2022 258 2023 343 2024 421 2025 458 Years 2026 - 2030 3,700 The expected cash flows are estimated figures based on the members expected to retire over the next 10 years assuming no early retirements, withdrawals or commutation of pension for cash. At the present time it is not clear whether annuities will be purchased when members reach retirement or whether pensions will be paid each month out of scheme assets. The cash flows above have been estimated on the assumption that pensions will be paid monthly out of scheme assets. If annuities are purchased, then the expected benefit payments will be significantly different from those shown above. Aptiv Solutions pension plan On May 7, 2014 the Company acquired 100% of the common stock of Aptiv Solutions ("Aptiv"). The acquisition of Aptiv was accounted for as a business combination in accordance with ASC 805 'Business Combinations' . The Company has a defined benefit plan covering its employees in Switzerland as mandated by the Swiss government. Benefits are based on the employee's years of service and compensation. Benefits are paid directly by the Company when they become due, in conformity with the funding requirements of applicable government regulations. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2020 and December 31, 2019 consist of units held in independently administered funds. The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60 'Defined Benefit Plans – Other Postretirement' . Funded status December 31, 2020 December 31, 2019 (in thousands) Projected benefit obligation $ (8,620) $ (7,047) Fair value of plan assets 7,601 6,014 Funded status $ (1,019) $ (1,033) Non-current other liabilities (note 8) $ (1,019) $ (1,033) Change in benefit obligation December 31, 2020 December 31, 2019 (in thousands) Benefit obligation at beginning of year $ 7,047 $ 5,279 Service cost 139 115 Interest cost 21 40 Plan participants' contributions 81 67 Prior service cost (23) — Transferred balances/(benefits paid) 245 (72) Actuarial loss 406 1,479 Foreign currency exchange rate changes 704 139 Benefit obligation at end of year $ 8,620 $ 7,047 Change in plan assets December 31, December 31, 2020 2019 (in thousands) Fair value of plan assets at beginning of year $ 6,014 $ 4,707 Expected return on plan assets 21 35 Actual return on plan assets 519 1,114 Scheme contributions 105 89 Plan participants' contributions 81 67 Transferred balances/ (benefits paid) 245 (72) Foreign currency exchange rate changes 616 74 Fair value of plan assets at end of year $ 7,601 $ 6,014 The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company. The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit cost: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Service cost $ 139 $ 115 $ 138 Interest cost 21 40 47 Expected return on plan assets (21) (35) (41) Amortization of net gain (6) (93) (69) Amortization of prior service credit (12) (9) (8) Settlement — — (93) Net periodic benefit cost/(credit) $ 121 $ 18 $ (26) The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2020, December 31, 2019 and December 31, 2018: December 31, 2020 December 31, 2019 December 31, 2018 Discount rate 0.30 % 0.80 % 0.80 % Rate of compensation increase 2.00 % 2.00 % 2.00 % Expected rate of return on plan assets 0.34 % 0.80 % 0.80 % Other comprehensive income December 31, 2020 December 31, 2019 December 31, 2018 Actuarial loss/(gain) - benefit obligation $ 406 $ 1,479 $ (372) Actuarial (gain)/loss – plan assets (519) (1,114) 240 Prior service credit recognized in net periodic benefit cost — — 93 Actuarial gain recognized in net periodic benefit cost 6 93 69 Amortization of net prior service credit 12 9 8 Net prior service cost occurring during the year (23) — (9) Total $ (118) $ 467 $ 29 The estimated net gain and prior service credit for the defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year are $7,000 and $12,000 respectively. Benefit Obligation The following assumptions were used in determining the benefit obligation at December 31, 2020 and December 31, 2019: December 31, 2020 December 31, 2019 Discount rate 0.15 % 0.30 % Rate of compensation increase 2.00 % 2.00 % The discount rate is determined by reference to Swiss corporate bond yields at the Balance Sheet date. Actuarial losses on the benefit obligation during 2020 resulted from actual experience with plan members during 2020 being different from that assumed at December 2019 and due to changes in the assumptions compared to those adopted at December 2019. Changes in the assumptions reflect the changes in market conditions from December 2019 to December 2020 and the actuarial loss is primarily due to the change in the discount rate. Plan Assets The pension plan is an insured arrangement with Swiss Life. The assets are an insurance contract whose value depends on the amount saved by employees and the interest granted by Swiss Life. The value of assets does not depend on the performance of any underlying assets. There is no self-investment in employer related assets. Cash Flows The Company expects to contribute $0.1 million to its pension fund in the year ending December 31, 2021. The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2021 $ 348 2022 342 2023 334 2024 326 2025 377 Years 2025 - 2029 1,587 The expected cash flows are estimated figures based on the members already receiving benefits plus an additional amount for people expected to retire, leave the Company, become disabled or die in the next 10 years. The amounts assume no early retirements and allow for expected experience in line with Swiss population statistics. At the present time it is not clear whether annuities will be purchased when members reach retirement or whether pensions will be paid each month out of scheme assets. The cash flows above have been estimated on the assumption that employees will take 50% of their benefits as a lump sum and the remainder will be paid monthly out of scheme assets as a pension. If employees choose a different option, the actual future benefit payments will be significantly different from those shown above.

Equity Incentive Schemes and St

Equity Incentive Schemes and Stock Compensation Charges	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Equity Incentive Schemes and Stock Compensation Charges	Equity Incentive Schemes and Stock Compensation Charges Share Options On July 21, 2008 the Company adopted the Employee Share Option Plan 2008 (the "2008 Employee Plan") pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any employee, or any Director holding a salaried office or employment with the Company or a Subsidiary for the purchase of ordinary shares. On the same date, the Company also adopted the Consultants Share Option Plan 2008 (the "2008 Consultants Plan"), pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any consultant, adviser or non-executive Director retained by the Company or any Subsidiary for the purchase of ordinary shares. On February 14, 2017 both the 2008 Employee Plan and the 2008 Consultants Plan (together the "2008 Option Plans") were amended and restated in order to increase the number of options that can be issued under the 2008 Consultants Plan from 0.4 million to 1.0 million and to extend the date for options to be granted under the 2008 Option Plans. An aggregate of 6.0 million ordinary shares have been reserved under the 2008 Employee Plan, as reduced by any shares issued or to be issued pursuant to options granted under the 2008 Consultants Plan, under which a limit of 1.0 million shares applies. Further, the maximum number of ordinary shares with respect to which options may be granted under the 2008 Employee Option Plan, during any calendar year to any employee shall be 0.4 million ordinary shares. There is no individual limit under the 2008 Consultants Plan. No options may be granted under the 2008 Option Plans after February 14, 2027. Each option granted under the 2008 Option Plans will be an employee stock option, or NSO, as described in Section 422 or 423 of the Internal Revenue Code. Each grant of an option under the 2008 Options Plans will be evidenced by a Stock Option Agreement between the optionee and the Company. The exercise price will be specified in each Stock Option Agreement, however option prices will not be less than 100% of the fair market value of an ordinary share on the date the option is granted. On January 17, 2003 the Company adopted the Share Option Plan 2003 (the "2003 Share Option Plan") pursuant to which the Compensation and Organization Committee of the Board could grant options to officers and other employees of the Company or its subsidiaries for the purchase of ordinary shares. An aggregate of 6.0 million ordinary shares were reserved under the 2003 Share Option Plan; and in no event could the number of ordinary shares issued pursuant to options awarded under this plan exceed 10% of the outstanding shares, as defined in the 2003 Share Option Plan, at the time of the grant, unless the Board expressly determined otherwise. Further, the maximum number of ordinary shares with respect to which options could be granted under the 2003 Share Option Plan during any calendar year to any employee was 0.4 million ordinary shares. The 2003 Share Option Plan expired on January 17, 2013. No new options may be granted under this plan. Share option awards are granted with an exercise price equal to the market price of the Company's shares at date of grant. Prior to 2018, share options typically vest over a period of five years from date of grant and expire eight years from date of grant. Share options granted to non-executive directors during 2018 vest over 12 months and expire eight years from the date of grant. The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2020, December 31, 2019 and December 31, 2018: Options Granted Weighted Average Exercise Price Weighted Average Grant Date Fair Value Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 167,557 $ 118.90 $ 36.84 Exercised (408,699) $ 41.12 $ 13.55 Canceled (9,505) $ 32.35 $ 11.39 Outstanding at December 31, 2018 920,746 $ 74.32 $ 22.39 Granted 97,112 $ 140.13 $ 43.43 Exercised (329,870) $ 65.54 $ 19.78 Canceled/expired (31,881) $ 88.12 $ 26.77 Outstanding at December 31, 2019 656,107 $ 87.80 $ 26.60 Granted 107,737 $ 159.83 $ 42.43 Exercised (193,417) $ 68.19 $ 20.91 Canceled/expired (16,681) $ 92.21 $ 27.93 Outstanding at December 31, 2020 553,746 $ 108.53 $ 31.63 Vested and exercisable at December 31, 2020 224,845 $ 83.13 $ 24.90 The weighted average remaining contractual life of options outstanding and options exercisable at December 31, 2020, was 4.86 years and 3.60 years respectively (2019: 4.76 years and 3.64 years respectively). 157,797 options are expected to vest during the year ended December 31, 2021 (186,479 options were expected to vest during the year ended December 31, 2020). The intrinsic value of options exercised during the year ended December 31, 2020 amounted to $21.3 million. The intrinsic value of options outstanding and options exercisable at December 31, 2020 amounted to $47.9 million and $25.1 million respectively. Intrinsic value is calculated based on the market value versus strike price of the Company's shares at the date of exercise and at the year end. Non-vested shares outstanding as at December 31, 2020 are as follows: Options Weighted Average Exercise Price Weighted Average Fair Value Non-vested outstanding at December 31, 2019 358,030 $ 103.68 $ 31.28 Granted 107,737 159.83 42.43 Vested (124,605) 92.25 27.51 Forfeited (12,261) 117.41 34.50 Non-vested outstanding at December 31, 2020 328,901 $ 125.89 $ 36.24 Outstanding and exercisable share options: The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2020: Options Outstanding Options Exercisable Range Exercise Number of Weighted Weighted Average Exercise Price Number of Weighted Average Exercise Price $ 32.37 2,500 0.33 2,500 $ 37.90 920 0.92 920 $ 40.83 21,360 1.39 21,360 $ 47.03 479 1.16 479 $ 48.67 1,027 1.20 1,027 $ 65.60 29,381 3.38 19,528 $ 66.47 2,621 2.38 2,621 $ 68.39 24,508 2.18 24,508 $ 71.95 73,193 3.17 59,284 $ 83.47 58,159 4.17 10,671 $ 90.03 45,154 4.38 27,698 $ 115.11 68,016 5.17 8,866 $ 125.74 39,664 5.38 36,885 $ 137.47 6,045 6.37 895 $ 140.38 75,061 6.17 7,603 $ 159.33 98,128 7.17 — $ 166.51 7,530 7.47 — $32.37-$166.51 553,746 4.86 $ 108.53 224,845 $ 83.13 Options outstanding include both vested and unvested options as at December 31, 2020. Options exercisable represent options which have vested at December 31, 2020. From the date of grant, substantially all options vest over a five Fair value of Stock Options Assumptions The weighted average fair value of options granted during the years ended December 31, 2020, December 31, 2019 and December 31, 2018 was calculated using the Black-Scholes option pricing model. The weighted average fair values and assumptions were as follows: Year Ended December 31, 2020 December 31, 2019 December 31, 2018 Weighted average fair value $ 42.43 $ 43.43 $ 36.84 Assumptions: Expected volatility 30 % 30 % 30 % Dividend yield — % — % — % Risk-free interest rate 0.57 % 2.55 % 2.76 % Expected life 5.0 years 5.0 years 5.0 years Expected volatility is based on the historical volatility of our common stock over a period equal to the expected term of the options; the expected life represents the weighted average period of time that options granted are expected to be outstanding given consideration to vesting schedules and our historical experience of past vesting and termination patterns. The risk-free rate is based on the U.S. government zero-coupon bonds yield curve in effect at time of the grant for periods corresponding with the expected life of the option. Restricted Share Units and Performance Share Units On April 23, 2013 the Company adopted the 2013 Employees Restricted Share Unit and Performance Share Unit Plan (the "2013 RSU Plan") pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may select any employee, or any Director holding a salaried office or employment with the Company, or a Subsidiary to receive an award under the plan. On May 11, 2015 the 2013 RSU Plan was amended and restated in order to increase the number of shares that can be issued under the RSU Plan by 2.5 million shares. Accordingly, an aggregate of 4.1 million ordinary shares have been reserved for issuance under the 2013 RSU Plan. The shares are awarded at par value and vest over a service period. Awards under the 2013 RSU Plan may be settled in cash or shares at the option of the Company. 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 during 2019 and 2020 vest over twelve months. The Company has awarded RSUs and PSUs to certain key individuals of the Group. The following table summarizes RSU and PSU activity for the year ended December 31, 2020: PSU Outstanding PSU PSU RSU Outstanding RSU RSU Outstanding at December 31, 2019 175,989 $ 110.79 1.04 389,900 $ 119.07 1.43 Granted 57,184 $ 165.30 137,572 $ 168.03 Shares vested (63,516) $ 83.92 (144,747) $ 98.47 Forfeited (10,016) $ 134.27 (41,301) $ 133.63 Outstanding at December 31, 2020 159,641 $ 137.64 1.14 341,424 $ 145.77 1.41 The fair value of RSUs vested for the year ended December 31, 2020 totaled $14.3 million (2019: $16.5 million.The share price range for the year was $83.47 - $156.21 (2019: $65.60 - $147.55). The fair value of PSUs vested for the year ended December 31, 2020 totaled $5.3 million (2019: $8.5 million). The share price range for the year was $83.47 - $90.03 (2019: $65.60 - $71.95). The PSUs vest based on service and specified EPS targets over the period 2018 – 2020, 2019 – 2021 and 2020 – 2022. Depending on the actual amount of EPS from 2018 to 2022, up to an additional 71,939 PSUs may also be granted. Non-cash stock compensation expense Income from operations for the year ended December 31, 2020 is stated after charging $26.3 million in respect of non-cash stock compensation expense. Non-cash stock compensation expense has been allocated as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Direct costs $ 8,557 $ 14,777 $ 17,408 Selling, general and administrative 17,714 12,042 14,186 $ 26,271 $ 26,819 $ 31,594 Total non-cash stock compensation expense not yet recognized at December 31, 2020 amounted to $45.9 million. The weighted average period over which this is expected to be recognized is 2.12 years. The income tax expense for the year ended December 31, 2020 reflects a net income tax benefit of $6.9 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2020 was $2.5 million. The income tax expense for the year ended December 31, 2019 reflects a net income tax benefit of $8.2 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2019 was $1.9 million. The income tax expense for the year ended December 31, 2018 reflects a net income tax benefit of $12.7 million in connection with stock compensation (including excess tax benefits) and the total tax benefit realized in connection with stock options exercised during 2018 was $3.6 million.

Government Grants

Government Grants	12 Months Ended
	Dec. 31, 2020
Deferred Revenue Disclosure [Abstract]
Government Grants	Government Grants December 31, 2020 December 31, 2019 (in thousands) Received $ 3,539 $ 3,539 Less accumulated amortization (2,881) (2,836) Foreign exchange translation adjustment 228 155 Total government grants 886 858 Less current portion (48) (45) Non-current government grants $ 838 $ 813 Grants received may be refundable in full if certain events occur. Such events, as set out in the related grant agreements, include sale of the related asset, liquidation of the Company or failure to comply with other conditions of the grant agreements. No loss contingency has been recognized as the likelihood of such events arising has been assessed as remote. Government grants amortized to the profit and loss account amounted to $45,000 for the year ended December 31, 2020 (December 31, 2019: $44,000). As at December 31, 2020 the Company had $6.0 million (December 31, 2019: $3.1 million) in restricted retained earnings, pursuant to the terms of grant agreements.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Share Capital	Share Capital Holders of ordinary shares will be entitled to receive such dividends as may be recommended by the Board of Directors of the Company and approved by the shareholders and/or such interim dividends as the Board of Directors of the Company may decide. On liquidation or a winding up of the Company, the par value of the ordinary shares will be repaid out of the assets available for distribution among the holders of the ordinary shares of the Company. Holders of ordinary shares have no conversion or redemption rights. On a show of hands, every holder of an ordinary share present in person or proxy at a general meeting of shareholders shall have one vote, for each ordinary share held with no individual having more than one vote. During the year ended December 31, 2020, 193,417 options were exercised by employees at an average exercise price of $68.19 per share for total proceeds of $13.2 million. During the year ended December 31, 2020, 144,172 ordinary shares were issued in respect of certain RSUs and 63,516 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2019, 329,870 options were exercised by employees at an average exercise price of $65.54 per share for total proceeds of $21.6 million. During the year ended December 31, 2019, 237,119 ordinary shares were issued in respect of certain RSUs and 118,611 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2018, 408,699 options were exercised by employees at an average exercise price of $41.12 per share for total proceeds of $16.8 million. During the year ended December 31, 2018, 273,742 ordinary shares were issued in respect of certain RSUs and 215,826 ordinary shares were issued in respect of PSUs previously awarded by the Company. (a) Share Repurchase Program On July 31, 2015 the Company commenced a buyback program of up to $400 million under which the Company could acquire its outstanding ordinary shares (by way of redemption), in accordance with Irish law, the United States securities laws and the Company's constitutional documents through open market share acquisitions. A total of 5,316,062 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $400 million. The share buyback program was completed in December 31, 2015, with a total of 6,198,481 ordinary shares redeemed during the year ended December 31, 2015 for total consideration of $457.9 million (including the program for $400 million). A resolution was passed at the Company’s Annual General Meeting (“AGM”) on July 22, 2016, which authorized the Directors to purchase (buyback) up to 10% of the outstanding shares in the Company. This authorization was renewed at the Company's AGM on each of July 25, 2017, July 24, 2018, July 23, 2019 and 21 July 2020. On October 3, 2016, the Company commenced a share buyback program of up to $400 million. The share buyback program was completed during the year ended December 31, 2018 with a total of 4,026,576 ordinary shares redeemed for a total consideration of $372.1 million. On January 8, 2019, the Company commenced a further share buyback program of up to 1.0 million ordinary shares which was completed during the year ended December 31, 2019. These shares were redeemed by the Company for a total consideration of $141.6 million. On October 22, 2019, the Company commenced a further share buyback program. At December 31, 2019, 35,100 ordinary shares were redeemed by the Company for a total consideration of $5.3 million. During the year ended December 31, 2020 1,235,218 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $175.0 million. All ordinary shares that were redeemed under the buyback program were canceled in accordance with the Constitution of the Company and the nominal value of these shares transferred to other undenominated capital as required under Irish Company law. Under the repurchase program, a broker purchased the Company's shares from time to time on the open market or in privately negotiated transactions in accordance with agreed terms and limitations. The program was designed to allow share repurchases during periods when the Company would ordinarily not be permitted to do so because it may be in possession of material non-public or price-sensitive information or due to applicable insider trading laws or self-imposed trading blackout periods. The Company's instructions to the broker were irrevocable and the trading decisions in respect of the repurchase program were made independently of and uninfluenced by the Company. The Company confirms that on entering the share repurchase plans it had no material non-public, price-sensitive or inside information regarding the Company or its securities. Furthermore, the Company will not enter into additional plans whilst in possession of such information. The timing and actual number of shares acquired by way of the redemption will be dependent on market conditions, legal and regulatory requirements and the other terms and limitations contained in the program. In addition, acquisitions under the program may be suspended or discontinued in certain circumstances in accordance with the agreed terms. Therefore, there can be no assurance as to the timing or number of shares that may be acquired under the program.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company's United States and Irish based subsidiaries file income tax returns in the United States and Ireland respectively. Other foreign subsidiaries are taxed separately under the laws of their respective countries. The components of income before income tax expense are as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 280,310 $ 323,726 $ 243,988 United States 41,950 21,073 27,499 Other 58,945 82,190 93,127 $ 381,205 $ 426,989 $ 364,614 The components of income tax expense are as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Income tax expense: Current tax expense: Ireland $ 28,963 $ 35,955 $ 28,042 United States 3,022 5,073 2,885 Other 14,963 11,642 9,379 Total current tax expense 46,948 52,670 40,306 Deferred tax expense/ (benefit): Ireland 1,654 2,833 1,054 United States 4,577 (3,502) 875 Other (5,304) (868) (277) 927 (1,537) 1,652 Income tax expense allocated to continuing operations 47,875 51,133 41,958 Income tax expense was allocated to the following components of other comprehensive income: Currency impact on long term funding 68 25 119 Fair value of cash flow hedge — — (148) Total $ 47,943 $ 51,158 $ 41,929 Ireland's statutory income tax rate is 12.5%. The Company's consolidated reported income tax expense differed from the amount that would result from applying the Irish statutory rate as set forth below: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Taxes at Irish statutory rate of 12.5% (2019:12.5%; 2018:12.5%) $ 47,651 $ 53,374 $ 45,577 Foreign and other income taxed at higher rates 7,943 7,356 7,649 Research & development tax incentives (1,243) (893) (1,243) Movement in valuation allowance 3,581 (10) 5,667 Effects of change in tax rates 108 359 (147) Decrease in unrecognized tax benefits (1,672) (1,273) (5,423) Impact of stock compensation (5,150) (7,383) (8,301) Other (3,343) (397) (1,821) $ 47,875 $ 51,133 $ 41,958 The tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below: December 31, 2020 December 31, 2019 (in thousands) Deferred tax liabilities: Property, plant and equipment $ 1,359 $ 1,102 Right-of-use-assets 9,402 11,838 Goodwill 31,629 27,590 Intangible assets 13,398 11,805 Other 1,009 6,284 56,797 58,619 Deferred tax assets: Operating loss and tax credits carryforwards 42,794 37,865 Property, plant and equipment 6,040 5,257 Lease liabilities 9,394 11,754 Accrued expenses and payments on account 24,368 26,380 Stock compensation 3,672 5,009 Deferred compensation 3,184 2,744 Unearned revenue 2,257 3,933 Other 155 604 Total deferred tax assets 91,864 93,546 Valuation allowance for deferred tax assets (32,768) (27,721) Deferred tax assets recognized 59,096 65,825 Overall net deferred tax asset $ 2,299 $ 7,206 At December 31, 2020 Ireland subsidiaries had tax credit carryforwards for income tax purposes that may be carried forward indefinitely, available for offset against future tax liabilities, if any, of $11.7 million. At December 31, 2020 U.S. subsidiaries had U.S. federal and state net operating loss ("NOL") carryforwards of approximately $16.6 million and $51.1 million, respectively. These NOLs are available for offset against future taxable income and the expiry dates are shown in the table below. Of the $16.6 million U.S. federal NOLs, approximately $6.4 million is available for offset against future U.S. federal taxable income in 2021. The subsidiaries' ability to use the remaining U.S. federal and state NOL carryforwards is limited on an annual basis due to change of ownership in 2014, 2017, and 2019, as defined by Section 382 of the Internal Revenue Code of 1986, as amended. Of the U.S. federal NOLs, $15.2 million are limited by Section 382 as follows: $10.0 million for the years 2021 - 2022, $4.8 million in 2023 - 2027, $0.4 million for the years 2028 – 2036. A s at December 31, 2020, U.S subsidiaries also had disallowed interest carryforwards of $29.1 million that can be carried forward indefinitely. These carryforwards are available for offset against future taxable income in the event that the U.S subsidiaries have excess capacity for interest deductions in future years. At December 31, 2020 other than those in the U.S. and Ireland, we had operating loss carryforwards for income tax purposes that may be carried forward indefinitely, available to offset against future taxable income, if any, of approximately $42.1 million. At December 31, 2020 those subsidiaries also had additional operating loss carryforwards of $15.1 million which are due to expire between 2021 and 2027 and operating loss carryforwards of $18.3 million which are due to expire between 2028 and 2037. In addition, at December 31, 2020 those subsidiaries had tax credit carryforwards for income tax purposes that may be carried forward indefinitely, available to offset against future tax liabilities, if any, of $5.5 million. The expected expiry dates of these US losses are as follows: Federal State (in thousands) 2022-2035 $ 13,295 $ 51,092 2037 1,921 — Indefinite 1,363 49 $ 16,579 $ 51,141 In addition, we also have general business tax credit carryforwards of approximately $0.8 million that are available to reduce future U.S. federal and state income taxes. The general business tax credits are non-refundable and are due to expire between the years 2026-2038. The valuation allowance at December 31, 2020 was approximately $32.8 million. The valuation allowance for deferred tax assets as of December 31, 2019 and December 31, 2018 was $27.7 million and $27.3 million respectively. The net change in the total valuation allowance was an increase of $5.1 million during 2020 and an increase of $0.4 million during 2019. Of the total increase of $5.1 million in 2020, $3.6 million was recognized within income tax expense and $1.5 million was recognized in Other Comprehensive Income. Of the total increase of $0.4 million in 2019, $Nil resulted in a current year income tax expense, and $0.4 million was recognized in Other Comprehensive Income. The valuation allowances at December 31, 2020 and December 31, 2019 were primarily related to operating loss and tax credit carry forwards that, in the judgment of management, are not more likely than not to be realized. In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers the scheduled reversal of deferred tax liabilities and projected future taxable income in making this assessment. In respect of deferred tax assets not subject to a valuation allowance, management considers that it is more likely than not that these deferred tax assets will be realized on the basis that there will be sufficient reversals of deferred tax liabilities and taxable income in future periods. The Company has recognized a deferred tax liability of $0.9 million (2019: $5.4 million) for investments in foreign subsidiaries where the Company does not consider the earnings to be indefinitely reinvested. For the deferred tax liability not recognized in respect of temporary differences related to investments in foreign subsidiaries which are considered to be indefinitely reinvested, it is not practicable to calculate the exact unrecognized deferred tax liability, however it is not expected to be material as Ireland allows a tax credit in respect of distributions from foreign subsidiaries at the statutory tax rate in the jurisdiction of the subsidiary so that no material tax liability would be expected to arise in Ireland in the event these earnings were ever remitted. In addition, withholding taxes applicable to remittances from foreign subsidiaries would not be expected to be material given Ireland’s tax treaty network and the EU parent subsidiary directive. A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Unrecognized tax benefits at start of year $ 20,156 $ 21,433 $ 23,720 Increase related to prior year tax positions 401 — 2,084 Decrease related to prior year tax positions (1,271) — (2,915) Increase related to current year tax positions 2,931 1,588 3,065 Settlements (369) (347) (182) Lapse of statute of limitations (2,770) (2,518) (4,339) Unrecognized tax benefits at end of year $ 19,078 $ 20,156 $ 21,433 The relevant statute of limitations for unrecognized tax benefits totaling $3.2 million could potentially expire during 2021. Included in the balance of total unrecognized tax benefits at December 31, 2020 were potential benefits of $19.1 million, which if recognized, would affect the effective rate on income tax from continuing operations. The balance of total unrecognized tax benefits at December 31, 2019 and December 31, 2018 included potential benefits which, if recognized, would affect the effective rate of income tax from continuing operations of $20.2 million and $21.4 million respectively. Interest and penalties recognized during the year ended December 31, 2020 amounted to a net release of $0.6 million (2019: Nil, 2018: net expense of $1.3 million) and are included within the income tax expense. Total accrued interest and penalties as of December 31, 2020 and December 31, 2019 were $0.5 million and $1.1 million respectively and are included in closing income taxes payable at those dates. Our major tax jurisdictions are the United States and Ireland. We may potentially be subjected to tax audits in both our major jurisdictions. In the United States, tax periods open to audit include the years ended December 31, 2017, December 31, 2018, December 31, 2019 and December 31, 2020. In Ireland, tax periods open to audit include the years ended December 31, 2016, December 31, 2017, December 31, 2018, December 31, 2019 and December 31, 2020. During such audits, local tax authorities may challenge the positions taken by us in our tax returns.

Restructuring charges

Restructuring charges	12 Months Ended
	Dec. 31, 2020
Restructuring and Related Activities [Abstract]
Restructuring charges	Restructuring charges Restructuring recognized during the year ended December 31, 2020 comprise: Year Ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Restructuring charges $ 18,089 $ — $ 12,490 Net charge $ 18,089 $ — $ 12,490 Restructuring charges A restructuring charge of $18.1 million was recognized during the year ended December 31, 2020 under a restructuring plan adopted following a review of operations. The restructuring plan reflected resource rationalization across the business to improve resource utilization, resulting in a charge of $11.4 million and office consolidation resulting in a charge for onerous lease obligation of $6.7 million, including the recognition of an impairment of right of use assets of $5.4 million (see note 22 - Operating Leases ) and provision for other related costs of $1.3 million. Workforce reductions Onerous leases Total (in thousands) Initial restructuring charge recorded $ 11,391 $ 6,698 $ 18,089 Net charge $ 11,391 $ 6,698 $ 18,089 Workforce reductions liability Onerous lease liability Total (in thousands) Initial restructuring provision $ 11,391 $ 6,698 $ 18,089 Utilization (6,987) (1,309) $ (8,296) Foreign exchange movement — 325 $ 325 Provision at December 31, 2020 $ 4,404 $ 5,714 $ 10,118 Future minimum lease payments (including related costs), associated with the 2020 restructuring plan, under the non-cancelable onerous leases as of December 31, 2020 were as follows: Minimum rental payments (in thousands) 2021 $ 2,602 2022 1,896 2023 999 2024 139 2025 139 Thereafter 402 Total future minimum lease payments (including related costs) 6,177 Lease imputed interest (463) Total $ 5,714 Prior Period Restructuring Charges A restructuring charge of $12.5 million was recognized during the year ended December 31, 2018, under a restructuring plan adopted following a review of operations. The restructuring plan reflected resource rationalization across the business to improve resource utilizations, resulting in a charge of $9.7 million and office consolidation resulting in the recognition of an onerous lease obligation of $2.8 million. No additional charge was recorded during the twelve months ended December 31, 2020. Workforce Onerous Total (in thousands) Total provision recognized $ 9,684 $ 2,806 $ 12,490 Utilized (5,399) (672) $ (6,071) Provision at December 31, 2018 $ 4,285 $ 2,134 $ 6,419 Utilized (3,554) (1,228) (4,782) Provision at December 31, 2019 $ 731 $ 906 $ 1,637 Utilized (731) (276) (1,007) Provision at December 31, 2020 $ — $ 630 $ 630 At December 31, 2020 $7.2 million is included within other liabilities and $3.5 million within non-current other liabilities.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Litigation On April 20, 2020, an individual named Chrystal Miller (“Miller”) who previously worked for a subsidiary of the Company, ICON Clinical Research LLC (“ICON Clinical”), filed a putative class action lawsuit, Chrystal Miller v. ICON plc et al., in the Superior Court of California, County of San Mateo. Her lawsuit was brought against the Company, ICON Clinical, DOCS Global, Inc., a subsidiary of the Company, and an individual former employee of ICON Clinical (collectively, “the ICON defendants”). The complaint alleges that the ICON defendants violated the California Labor Code and the California Business & Professions Code by failing to pay overtime wages, provide meal and rest periods, provide accurate, itemized wage statements, and timely pay all final wages to Clinical Research Associates employed in California since April 20, 2016. The suit seeks monetary damages, interest, injunctive relief, and attorneys’ fees and costs. On June 22, 2020, the case was removed to the U.S. District Court, Northern District of California. On November 4, 2020, the federal court remanded the case to San Mateo Superior Court. On December 23, 2020, ICON Clinical filed a cross-complaint against Miller alleging causes of action for fraud, negligent misrepresentation, conversion, breach of contract, and accounting. The ICON defendants deny the allegations that they have acted unlawfully and are vigorously defending the lawsuit. Operating Leases The Company has several non-cancelable operating leases, primarily for facilities, that expire over the next twelve years. These leases generally contain renewal options and require the Company to pay all executory costs such as maintenance and insurance. See note 22 - Operating leases for rental expense pursuant to ASC 842 for the years ended December 31, 2020 and December 31, 2019 and future minimum rental commitments as of December 31, 2020.

Disaggregation of Revenue

Disaggregation of Revenue	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	Disaggregation of Revenue Revenue disaggregated by customer profile is as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Top client $ 337,904 $ 350,287 $ 352,335 Clients 2-5 754,906 704,963 671,723 Clients 6-10 350,865 347,832 385,741 Clients 11-25 501,643 529,713 461,351 Other 851,970 873,044 724,627 Total $2,797,288 $2,805,839 $2,595,777 Accounts receivable and unbilled revenue are as follows: December 31, 2020 December 31, 2019 (in thousands) Billed services (accounts receivable) $ 722,420 $ 535,088 Unbilled services (unbilled revenue) 428,684 422,769 Accounts receivable and unbilled revenue 1,151,104 957,857 Allowance for credit losses (note 18) (7,149) — Provision for doubtful debts (note 18) — (7,380) Accounts receivable and unbilled revenue, net $ 1,143,955 $ 950,477 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2020 December 31, 2019 $ Change % Change Unbilled services (unbilled revenue) $ 428,684 $ 422,769 $ 5,915 1.4 % Unearned revenue (payments on account) (660,883) (366,988) (293,895) 80.1 % Net balance $ (232,199) $ 55,781 $ (287,980) 516.3 % Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected. These assets are recorded as unbilled revenue and therefore contract assets rather than accounts 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 of set-off between unbilled services (contract assets) and unearned revenue (contract liabilities), each are separately presented gross on the Consolidated Balance Sheet. The Company is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support a clinical trial. The progress towards completion for clinical service contracts is measured based on total project costs (including reimbursable costs). Amounts owed to investigators and others in respect of reimbursable expenses at December 31, 2020 and December 31, 2019 were $138.2 million and $142.6 million (see note 7 - Other liabilities ). Unbilled services as at December 31, 2020 increased by $5.9 million as compared to December 31, 2019. Unearned revenue increased by $293.9 million over the same period resulting in a decrease of $288.0 million in the net balance of unbilled services and unearned revenue or payments on account between December 31, 2019 and December 31, 2020. 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 contract when a cost-based input method of revenue recognition is applied and revenue recognized exceeds the amount billed to the customer. The credit loss expense recognized on the Group's receivables and unbilled services was $2.9 million and $0.4 million for the twelve months ended December 31, 2020 and 2019, respectively.

Accounts receivable, unbilled r

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities)	Disaggregation of Revenue Revenue disaggregated by customer profile is as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Top client $ 337,904 $ 350,287 $ 352,335 Clients 2-5 754,906 704,963 671,723 Clients 6-10 350,865 347,832 385,741 Clients 11-25 501,643 529,713 461,351 Other 851,970 873,044 724,627 Total $2,797,288 $2,805,839 $2,595,777 Accounts receivable and unbilled revenue are as follows: December 31, 2020 December 31, 2019 (in thousands) Billed services (accounts receivable) $ 722,420 $ 535,088 Unbilled services (unbilled revenue) 428,684 422,769 Accounts receivable and unbilled revenue 1,151,104 957,857 Allowance for credit losses (note 18) (7,149) — Provision for doubtful debts (note 18) — (7,380) Accounts receivable and unbilled revenue, net $ 1,143,955 $ 950,477 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2020 December 31, 2019 $ Change % Change Unbilled services (unbilled revenue) $ 428,684 $ 422,769 $ 5,915 1.4 % Unearned revenue (payments on account) (660,883) (366,988) (293,895) 80.1 % Net balance $ (232,199) $ 55,781 $ (287,980) 516.3 % Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected. These assets are recorded as unbilled revenue and therefore contract assets rather than accounts 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 of set-off between unbilled services (contract assets) and unearned revenue (contract liabilities), each are separately presented gross on the Consolidated Balance Sheet. The Company is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support a clinical trial. The progress towards completion for clinical service contracts is measured based on total project costs (including reimbursable costs). Amounts owed to investigators and others in respect of reimbursable expenses at December 31, 2020 and December 31, 2019 were $138.2 million and $142.6 million (see note 7 - Other liabilities ). Unbilled services as at December 31, 2020 increased by $5.9 million as compared to December 31, 2019. Unearned revenue increased by $293.9 million over the same period resulting in a decrease of $288.0 million in the net balance of unbilled services and unearned revenue or payments on account between December 31, 2019 and December 31, 2020. 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 contract when a cost-based input method of revenue recognition is applied and revenue recognized exceeds the amount billed to the customer. The credit loss expense recognized on the Group's receivables and unbilled services was $2.9 million and $0.4 million for the twelve months ended December 31, 2020 and 2019, respectively.

Provision for Credit Losses

Provision for Credit Losses	12 Months Ended
	Dec. 31, 2020
Accounts Receivable, after Allowance for Credit Loss [Abstract]
Provision for Credit Losses	Provision for Credit Losses The Company does business with most major international pharmaceutical companies. Provision for credit losses at December 31, 2020 and provision for doubtful debts at December 31, 2019 comprises: December 31, 2020 December 31, 2019 (in thousands) Opening provision $ 7,380 $ 8,889 Amounts used during the year (2,561) — Amounts provided during the year 2,692 1,691 Amounts released during the year (510) (3,226) Foreign exchange 148 26 Closing provision $ 7,149 $ 7,380

Business Segment and Geographic

Business Segment and Geographical Information	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Business Segment and Geographical Information	Business Segment and Geographical Information The Company is a clinical research organization ("CRO"), providing outsourced development services on a global basis to the pharmaceutical, biotechnology and medical device industries. 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 most major therapeutic areas on a global basis and has the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full-service" solution. The Company has expanded predominately 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 business segment, which is the provision of outsourced development services on a global basis to the pharmaceutical, biotechnology and medical devices industries. Revenues are allocated to individual entities based on where the work is performed in accordance with the Company's global transfer pricing model. Revenues and income from operations in Ireland are a function of our global contracting model and the Group’s transfer pricing model. Given ICON Clinical Research Limited's ("ICON Ireland") role in the development and management of the Group, it's ownership of key intellectual property, customer relationships, its key role in the mitigation of risks faced by the Group, plus the responsibility for maintaining the Group's global network, ICON Ireland acts as the Group entrepreneur and enters into the majority of the Group's customer contracts. As such, ICON Ireland remunerates most of the other operating entities ("cost plus service providers") in the ICON Group on the basis of a guaranteed cost plus mark-up for the services they perform in each of their local territories. The cost plus mark-up for each ICON entity is established to ensure that each of ICON Ireland and the ICON entities in the various geographical areas that are involved in the conduct of services for customers, earn an appropriate arms-length return having regard to the assets owned, risks borne and functions performed by each entity from these intercompany transactions. The cost plus mark-up policy is reviewed annually to ensure that it is market appropriate. Under this method, the residual operating profits (or losses) of the Group, once the cost plus service providers have been paid their respective intercompany service fee, are retained by ICON Ireland. The geographic split of revenue disclosed for each region outside Ireland is the cost plus revenue attributable to these entities. The revenues disclosed as relating to Ireland are the net revenues after deducting the cost plus revenues attributable to the activities performed outside Ireland. The Company's areas of operation outside of Ireland include the United States, United Kingdom, Austria, Belgium, Bulgaria, Czech Republic, France, Germany, Hungary, Italy, Latvia, Poland, Portugal, Romania, Russia, Serbia, Spain, Sweden, The Netherlands, Turkey, Ukraine, Canada, Argentina, Brazil, Chile, Colombia, Mexico, Peru, China (including Hong Kong), India, Israel, Japan, Singapore, South Korea, The Philippines, Taiwan, Thailand, Australia, New Zealand and South Africa. There have been no changes to the basis of segmentation or the measurement basis for the segment results since the prior year. Reportable segment information at December 31, 2020 and December 31, 2019 and for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 is as follows: a) The distribution of revenue by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 1,181,292 $ 1,252,834 $ 1,066,200 Rest of Europe 416,884 388,916 379,883 U.S. 925,563 892,497 894,978 Other 273,549 271,592 254,716 Total $ 2,797,288 $ 2,805,839 $ 2,595,777 b) The distribution of income from operations, including restructuring, by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 276,478 $ 314,287 $ 257,089 Rest of Europe 35,765 37,997 36,280 U.S. 58,018 60,272 58,561 Other 21,239 20,850 21,427 Total $ 391,500 $ 433,406 $ 373,357 c) The distribution of income from operations, excluding restructuring, by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 295,360 $ 314,287 $ 269,196 Rest of Europe 35,402 37,997 36,904 U.S. 57,680 60,272 58,340 Other 21,147 20,850 21,407 Total $ 409,589 $ 433,406 $ 385,847 d) The distribution of long-lived assets (including operating right-of-use assets (year ended December 31, 2020 and 2019)), net, by geographical area was as follows: December 31, 2020 December 31, 2019 (in thousands) Ireland $ 118,361 $ 110,522 Rest of Europe 36,723 41,970 U.S. 65,152 72,578 Other 38,668 44,994 Total $ 258,904 $ 270,064 e) The distribution of depreciation, amortization and reduction in carrying value of operating right-of-use assets (year ended December 31, 2020 and 2019 only) by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 33,516 $ 30,635 $ 34,721 Rest of Europe 18,569 14,370 5,331 U.S. 36,060 33,922 21,605 Other 11,872 12,995 4,259 Total $ 100,017 $ 91,922 $ 65,916 f) The distribution of total assets by geographical area was as follows: Year Ended December 31, 2020 December 31, 2019 (in thousands) Ireland $ 1,675,980 $ 1,323,181 Rest of Europe 671,218 660,797 U.S. 909,202 755,271 Other 179,206 168,263 Total $ 3,435,606 $ 2,907,512 g) The distribution of capital expenditures (including operating right-of-use assets (year ended December 31, 2020 and 2019 only)) by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 29,309 $ 32,526 $ 30,942 Rest of Europe 9,298 13,971 2,590 U.S. 14,347 18,638 9,311 Other 10,201 15,010 5,554 Total $ 63,155 $ 80,145 $ 48,397 h) The following table sets forth the clients which represented 10% or more of the Company's revenue in each of the periods set out below. Year ended December 31, 2020 December 31, 2019 December 31, 2018 Client A 12 % 12 % 14 % Client B <10% 10 % <10% i) The distribution of interest income by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 1,620 $ 3,674 $ 2,620 Rest of Europe 570 2,242 1,750 U.S. 22 111 17 Other 512 832 372 Total $ 2,724 $ 6,859 $ 4,759 j) The distribution of the income tax charge by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 30,616 $ 38,788 $ 29,096 Rest of Europe 4,609 4,234 (434) U.S. 7,600 1,571 3,761 Other 5,050 6,540 9,535 Total $ 47,875 $ 51,133 $ 41,958

Supplemental Disclosure of Cash

Supplemental Disclosure of Cash Flow Information	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Supplemental Disclosure of Cash Flow Information	Supplemental Disclosure of Cash Flow Information Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Cash paid for interest $ 13,062 $ 13,059 $ 13,060 Cash paid for income taxes (net of refunds) $ 27,604 $ 29,836 $ 18,558

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income Year ended December 31, 2020 December 31, 2019 (in thousands) Currency translation adjustments $ (15,894) $ (64,023) Currency impact on long term funding (9,329) (7,726) Actuarial loss on defined benefit pension plan (note 9) (9,364) (5,226) Unrealized capital gain – investments (note 3) — 231 Realized gain on interest rate hedge 4,658 4,658 Amortization of gain on interest rate hedge (4,658) (3,733) Loss on interest rate hedge (905) — Amortization of loss on interest rate hedge 15 — Total $ (35,477) $ (75,819)

Operating Leases

Operating Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Operating Leases	Operating leases Lease costs recorded under operating leases were as follows: Year ended December 31, 2020 December 31, 2019 (in thousands) Operating lease costs $ 31,088 $ 34,759 Income from sub-leases (940) (1,761) Net operating lease costs $ 30,148 $ 32,998 Of the total cost of $30.1 million incurred in the year ended December 31, 2020, $27.6 million is recorded within selling, general and administration costs and $2.5 million is recorded within direct costs. Of the total cost of $33.0 million incurred in the year ended December 31, 2019, $30.5 million is recorded within selling, general and administration costs and $2.5 million is recorded within direct costs. During the years ended December 31, 2020 and December 31, 2019, the Group did not incur any costs related to variable lease payments. The adoption of ASC 842 resulted in the recognition of operating right-of-use assets and lease liabilities of $106.5 million at January 1, 2019. Right-of-use assets obtained, in exchange for lease obligations, net of early termination options now reasonably certain to be exercised, during the years ended December 31, 2020 and December 31, 2019 totaled $12.1 million and $29.5 million, respectively. During the year ended December 31, 2020, office consolidations resulted in the recognition of an onerous lease obligation. The right-of-use assets related to these offices have been impaired to the extent they are considered onerous and a loss of $5.4 million was recorded (see note 14 - Restructuring charges ). No impairment losses were recognized during the year ended December 31, 2019. The weighted average remaining lease term and weighted-average discount rate at December 31, 2020 were 4.45 years and 2.53%, respectively. The weighted average remaining lease term and weighted-average discount rate at December 31, 2019 were 5.19 years and 2.91%, respectively. Future minimum lease payments under non-cancelable leases as of December 31, 2020 were as follows: Minimum rental payments (in thousands) 2021 $ 26,164 2022 21,510 2023 16,108 2024 10,165 2025 6,046 Thereafter 10,490 Total future minimum lease payments 90,483 Lease imputed interest (5,348) Total $ 85,135 Operating lease liabilities are presented as current and non-current. Operating lease liabilities of $24.3 million and $28.3 million have been included in other liabilities

Debt - Senior Notes

Debt - Senior Notes	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt - Senior Notes	Debt – Senior Notes As at December 31, 2020 and December 31, 2019, the Company had an outstanding principal amount of indebtedness of $350.0 million under a $350.0 million Note Purchase and Guarantee Agreement. On December 15, 2015, ICON Investments Five Unlimited Company issued Senior Notes, '2015 Senior Notes' for aggregate gross proceeds of $350.0 million in a private placement. The 2015 Senior Notes matured on December 15, 2020 and this debt was repaid in full. On December 8, 2020, ICON Investments Five Unlimited Company issued new senior notes, '2020 Senior Notes' for aggregate gross proceeds of $350.0 million in a private placement which was guaranteed by ICON plc and ICON Global Treasury Unlimited Company, a subsidiary of the Company. The 2020 Senior Notes were issued in two tranches; Series A Notes of $275.0 million which will mature on December 8, 2023 and Series B Notes of $75.0 million which will mature on December 8, 2025. The interest rate in respect of the 2015 Senior Notes was fixed at 3.64% for the five year term. The interest cost was recognized in interest expense in the period since drawdown. In October 2015, the Company entered into an interest rate hedge in respect of the planned issuance of the 2015 Senior Notes in December 2015. The interest rate hedge matured in November 2015 when the interest rate on the 2015 Senior Notes was fixed. The interest rate hedge was effective in accordance with ASC 815 'Derivatives and Hedging' . The cash proceeds ($4.6 million), representing the realized gain on the interest rate hedge, were received on maturity in November 2015 and are recorded within Other Comprehensive Income. The realized gain was amortized to the Consolidated Statements of Operations over the term of the 2015 Senior Notes (net against interest payable). The interest rate in respect of the 2020 Senior Notes is fixed at 2.32% and 2.43% for Series A Notes and Series B Notes respectively. The interest is payable semi-annually on the 2020 Senior Notes on each June 8 and December 8; this will commence on June 8, 2021 and the interest cost in the period since drawdown was recognized in interest expense. The Company entered into an interest rate hedge in respect of the planned issuance of the 2020 Senior Notes in June 2020. The interest rate hedge matured in July 2020 when the interest rates on the 2020 Senior Notes were fixed. The interest rate hedge was effective in accordance with ASC 815 'Derivatives and Hedging' . There was a cash outflow ($0.9 million) on maturity in July 2020, representing the realized loss on the interest rate hedge. This was recorded within Other Comprehensive Income and the realized loss is amortized to the Consolidated Statement of Operations over the term of the 2020 Senior Notes. The 2020 Senior Notes agreement also includes certai n financial covenants that require compliance with a consolidated leverage ratio and a maximum amount of priority debt, each of which are defined in the Note Purchase and Guarantee Agreement. The Company was in compliance with these covenants at December 31, 2020. The Senior Notes have not been registered under the Securities Act of 1933, as amended, and may not be offered or sold in the United States absent registration or an applicable exemption from registration requirements. We hereby agree to furnish to the SEC, upon its request, a copy of any instrument defining the rights of holders of long-term debt of the Company or of its subsidiaries for which consolidated or unconsolidated financial statements are required to be filed.

Impact of New Accounting Pronou

Impact of New Accounting Pronouncements	12 Months Ended
	Dec. 31, 2020
Accounting Changes and Error Corrections [Abstract]
Impact of New Accounting Pronouncements	Impact of New Accounting Pronouncements Impact of new accounting pronouncements adopted during fiscal year ended December 31, 2020 (or previously) Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13 'Measurement of Credit Losses on Financial Instruments (Topic 326)' , which significantly changes the way entities recognize impairment of many financial assets by requiring immediate recognition of estimated credit losses expected to occur over their remaining life. The update provides guidance on the measurement of credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The amendment replaces the previous incurred loss impairment approach with a methodology to reflect expected credit losses and requires consideration of a broader range of reasonable and supportable information to explain credit loss estimates. The updated guidance was applied on a modified retrospective approach and was effective for the Group for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The adoption of ASU 2016-13 did not have a significant impact on the financial statements. Fair Value Measurement In August 2018, the FASB issued ASU 2018-13 'Fair Value Measurement - Disclosure Framework (Topic 820)' . The updated guidance requires entities to disclose changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Amendments in this guidance also require disclosure of transfers into and out of Level 3 of the fair value hierarchy, purchases and issues of Level 3 assets and liabilities, and clarify that the measurement uncertainty disclosure is about the uncertainty in measurement as of the reporting date. The guidance removed requirements to disclose the amounts and reasons for transfers between Level 1 and Level 2, policy for timing between of transfers between levels, and the valuation processes for Level 3 fair value measurements. The updated guidance is effective for the Group for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The adoption of ASU 2018-13 did not have a significant impact on the financial statements. Other Pronouncements We have reviewed the FASB issued Accounting Standards Update (ASU) accounting pronouncements and interpretations thereof that have effective dates during the periods reported. The following relevant ASUs have also been adopted in 2020: ASU 2020-04 'Reference Rate Reform (Topic 848) - Facilitation of the Effects of Reference Rate Reform on Financial Reporting' provides optional expedients and exceptions for applying US GAAP to contracts, hedging relationships, and other transactions affected by the reference rate reform. The amendments apply only to contracts and transactions that reference LIBOR or another reference rate expected to be discontinued as part of the reform. This ASU was issued on March 12, 2020 and applies only to contracts or transactions entered into or evaluated before December 31, 2022. This ASU was effective and adopted upon issuance. The adoption of ASU 2020-04 did not have a significant impact on the financial statements. In August 2018, the FASB issued ASU 2018-15 'Intangibles - Goodwill and Other - Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (Topic 350)' . The amendments in the update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The ASU is effective for the Group for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years and was applied prospectively to all relevant implementation costs incurred after the adoption date. The adoption of ASU 2018-15 did not have a significant impact on the financial statements. In November 2019, the FASB issued ASU 2019-08 'Codification Improvements—Share-Based Consideration Payable to a Customer' . The amendments in this update require that an entity measure and classify share-based payment awards granted to a customer by applying the guidance in ASC 718. The amount recorded as a reduction of the transaction price is required to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with ASC 718. The grant date is the date at which a grantor (supplier) and a grantee (customer) reach a mutual understanding of the key terms and conditions of a share-based payment award. The classification and subsequent measurement of the award are subject to the guidance in ASC 718 unless the share-based payment award is subsequently modified and the grantee is no longer a customer. The amendments in this update are effective for the Group for fiscal years beginning after December 15, 2019. The adoption of ASU 2019-08 did not have a significant impact on the financial statements. In August 2018, the FASB issued ASU 2018-14 'Compensation - Retirement Benefits - Defined Benefit Plans (Topic 715)' . The updated guidance requires additional disclosures of weighted-average interest credit rates for cash balance plans and an explanation of the reasons for significant gains and losses related to changes in the benefit obligation. Amendments in the guidance also clarify the requirement to disclose the projected benefit obligation (PBO) and fair value of plan assets for plans with PBOs in excess of plan assets. The same disclosure is needed for the accumulated benefit obligation (ABO) and fair value of plan assets for plans with ABOs in excess of plan assets. The guidance removes certain previous disclosure requirements no longer considered cost beneficial. The amendments are effective for fiscal years ending after December 15, 2020, with early adoption permitted. The adoption of ASU 2018-14 did not to have a significant impact on the financial statements. In January 2017, the FASB issued ASU 2017-04 'Intangibles - Goodwill and Other: Simplifying the test for goodwill impairment' which requires an entity to no longer perform a hypothetical purchase price allocation to measure goodwill impairment. Instead, impairment will be measured using the difference between the carrying amount and the fair value of the reporting unit. The ASU is effective for public businesses, that are SEC filers, for annual and interim periods in fiscal years beginning after December 15, 2019, with early adoption permitted. The company early adopted this ASU from January 1, 2018. The adoption of this ASU did not have a significant impact on the financial statements. Impact of new accounting pronouncements which will be adopted during fiscal year ended December 31, 2021 In August 2020, the FASB issues ASU 2020-06 ‘Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity’ which removes the separation models in ASC 470 ‘Debt’ for convertible debt with cash conversion features and convertible instruments with beneficial conversion features. The ASU also removes from ASC 815 ‘Derivatives and Hedge Accounting’ certain conditions for equity classification for contracts on an entity’s own equity. This ASU can be adopted using either the retrospective approach or the modified retrospective approach and is effective for the Group for fiscal years ending after December 15, 2021. The guidance may be early adopted for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. This ASU is not expected to have a significant impact on the financial statements. In December 2019, the FASB issued ASU 2019-12 'Simplifying the Accounting for Income Taxes (Topic 740)' . The amendments in this update simplify the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of GAAP for other areas of Topic 740 by clarifying and amending existing guidance. The Company will adopt the amendments in this ASU on a prospective basis, except where the required method of adoption is retrospective or modified retrospective. ASU 2019-12 is effective for the Company for fiscal years commencing on or after December 15, 2020. This ASU is not expected to have a significant impact on the financial statements. In January 2020, the FASB issued ASU 2020-01, 'Investments-Equity Securities (Topic 321), Investments-Equity Method and Joint Ventures (Topic 323), and Derivatives and Hedging (Topic 815) '. ASU 2020-01 states any equity security transitioning from the alternative method of accounting under Topic 321 to the equity method, or vice versa, due to an observable transaction will be remeasured immediately before the transition. In addition, the ASU clarifies the accounting for certain non-derivative forward contracts or purchased call options to acquire equity securities stating such instruments will be measured using the fair value principles of Topic 321 before settlement or exercise. The ASU is effective for fiscal years beginning after December 15, 2020, and will be applied on a prospective basis. Early adoption is permitted. The adoption of this ASU is not expected to have a significant impact of the financial statements. Impact of other new accounting pronouncements No other new accounting pronouncement issued or effective has had, or is expected to have, a significant impact on the Company’s consolidated financial statements.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Parties	Related Parties Subsidiaries of the Company earned revenue of $321,000 (December 31, 2019: $95,000) from DS Biopharma Limited (formerly Dignity Sciences Limited) during the year. Dr. John Climax is Chief Executive Officer and a Director and shareholder of DS Biopharma Limited. $41,000 was recorded as due from DS Biopharma Limited at December 31, 2020 (December 31, 2019: $36,000). Dr Ronan Lambe resigned as a Director of ICON plc on July 24, 2018. The contract terms were agreed on an arm’s length basis. On July 24, 2020, a subsidiary of the Company, ICON Clinical Research Limited, entered into an agreement to jointly establish a new company, Oncacare, with a third party. The Company has invested $4.9 million to obtain a 49% interest in the voting share capital of Oncacare, of which, $2.5 million has been paid with the remainder to be paid in the next twelve months. This balance remains outstanding at the year ended December 31, 2020. The majority investor has the right to sell the 51% majority voting share capital exclusively to the Company in an eighteen month period, commencing January 1, 2023 and ICON also has the right to acquire the 51% majority voting share capital from August 1, 2025.

Net Income Per Ordinary Share

Net Income Per Ordinary Share	12 Months Ended
	Dec. 31, 2020
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. Basic and diluted net income per ordinary share attributable to the Group includes the adjustment to reflect the accretion of the noncontrolling interest in MeDiNova to its redemption value. The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows: December 31, 2020 December 31, 2019 December 31, 2018 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 52,859,911 53,859,537 54,118,764 Effect of dilutive share options outstanding 423,674 473,924 671,899 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 53,283,585 54,333,461 54,790,663 The reconciliation between net income attributable to the Group per the Statement of Operating Income and the net income used to calculate net income per ordinary share attributable to the Group is as follows: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Net income attributable to the Group $ 332,331 373,986 $ 322,656 Noncontrolling interest adjustment to redemption amount (4,522) (5,048) — Net income attributable to the Group (including NCI redemption adjustment) 327,809 368,938 322,656 December 31, 2020 December 31, 2019 December 31, 2018 Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment): Basic $ 6.20 $ 6.85 $ 5.96 Diluted $ 6.15 $ 6.79 $ 5.89

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 24, 2021, ICON plc announced it has entered into a Merger Agreement to acquire PRA Health Sciences, Inc. (‘PRA’) in a cash and stock transaction valued at approximately $12 billion. The per share merger consideration consists of $80 in cash and 0.4125 shares of ICON stock. The boards of directors of both companies have approved the acquisition. Subject to receipt of regulatory clearances and approval by shareholders of both companies, the acquisition is expected to close in the third quarter of 2021, and upon completion, ICON shareholders will own approximately 66% of the combined company and PRA shareholders will own approximately 34% of the combined company. In conjunction with the execution of the Merger Agreement, ICON entered into a commitment letter with Citigroup Global Markets Inc. (“Citi”) pursuant to which Citi has committed to provide to ICON a senior secured revolving credit facility in an aggregate principal amount of up to $300.0 million and a senior secured bridge loan facility in an aggregate principal amount of up to $6,060.0 million to, among other things, finance ICON’s obligations in respect of the transactions contemplated by the Merger Agreement. The effectiveness of such credit facilities is subject to the occurrence of customary closing conditions, including the consummation of the Merger.

Impact of Change in Accounting

Impact of Change in Accounting Policies	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Impact of Change in Accounting Policies	Impact of change in accounting policies Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13 'Measurement of Credit Losses on Financial Instruments (Topic 326)' , which significantly changes the way entities recognize impairment of many financial assets by requiring immediate recognition of estimated credit losses expected to occur over their remaining life. The update provides guidance on the measurement of credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The amendment replaces the previous incurred loss impairment approach with a methodology to reflect expected credit losses and requires consideration of a broader range of reasonable and supportable information to explain credit loss estimates. The updated guidance was applied on a modified retrospective approach and was effective for the Group for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company applied the requirements of the new standard from January 1, 2020 and no adjustment was required to opening retained earnings as of that date. The Company's estimate of expected credit losses considers historical credit loss information that is adjusted, where necessary, for current conditions and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. The Company's receivables and unbilled services are predominantly due from large and mid-tier pharmaceutical and biotechnology companies that share similar risk characteristics. The Company monitors their portfolio of receivables and unbilled services for any deterioration in current or expected credit quality (for example, expected delinquency level), and adjusts the allowance for credit losses as required. Changes in the allowance for credit losses are recorded as a provision for (or reversal of) credit loss expense in the Consolidated Statement of Operations. Losses are charged against the allowance when management believes the uncollectibility of a previously provisioned amount is confirmed. Leases The new leasing standard (ASU No. 2016-02 ' Leases ') was issued in February 2016. ASC 842 ' Leases ' (ASC 842) supersedes the requirements in ASC 840 ' Leases ' (ASC 840) and requires that lessees recognize rights and obligations from virtually all leases (other than leases that meet the definition of a short-term lease) on their Balance Sheets as right-of-use assets with corresponding lease liabilities. The ASU also provides additional guidance on how to classify leases and how to determine the lease term for accounting purposes. In July 2018, the FASB issued ASU No. 2018-11 ' Leases (Topic 842): Targeted Improvements ', which provides the option to adopt the standard retrospectively for each prior period presented, as initially set out in ASU No. 2016-02, or as of the adoption date with a cumulative-effect adjustment to the opening balance of retained earnings. ASC 842 became effective for ICON plc with effect from January 1, 2019. ICON adopted the new standard as of January 1, 2019 under the cumulative effect adjustment approach. Under this transition method, the new standard is applied from January 1, 2019 without restatement of comparative period amounts. Results for the year ended December 31, 2018 are therefore presented under the previous leasing accounting principles, ASC 840. Operating lease liabilities and right-of-use assets have been recorded on the Consolidated Balance Sheets as at January 1, 2019 of $106.5 million. There is no impact of adopting ASC 842 on opening retained earnings at January 1, 2019.

Significant accounting polici_2

Significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Consolidation	Basis of consolidation    The consolidated financial statements include the financial statements of the Company and all of its subsidiaries. All significant intercompany profits, transactions and account balances have been eliminated. The results of subsidiary undertakings acquired in the period are included in the Consolidated Statement of Operations from the date of acquisition.
Use of Estimates	Use of estimates    The preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reported period. Actual results could differ from those estimates. The principal management estimates and judgments used in preparing the financial statements relate to revenue recognition and taxation.
Revenue Recognition	Revenue recognition The Company primarily 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, and laboratory 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). 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. We have concluded that 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 project. 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 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 third party costs) at each reporting period as a percentage of forecasted total project costs. 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 We have concluded that 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 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. 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 amounts corresponds to the value of the Company's performance and the transfer of value to the customer. 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.
Direct Costs	Third party costs (Reimbursable expenses) Reimbursable expenses comprise investigator payments and certain other costs which are reimbursed by clients under terms specific to each contract to the investigators. See note 2 (e) Direct costs below. (e) Direct costs Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs. Reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time i.e. the full-service obligation is in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred. Investigator payment costs are recorded and reported reflecting investigator activity over the life of the contract. Investigator payments are made based on predetermined contractual arrangements. Payments may differ from the recording and reporting of the expense which is based on activity.
Advertising Costs	Advertising costs    All costs associated with advertising and promotion are expensed as incurred.
Foreign Currencies and Translation of Subsidiaries	Foreign currencies and translation of subsidiaries    ICON plc's financial statements are prepared in United States dollars. Transactions in currencies other than the functional currency of the individual entities within the ICON Group are recorded at the rate ruling at the date of the transaction. Monetary assets and liabilities denominated in currencies other than the functional currency of the individual entities within the ICON Group are translated into the functional currency of that entity at exchange rates prevailing at the Balance Sheet date. Adjustments resulting from these translations are charged or credited to income.The financial statements of subsidiaries with other functional currencies are translated at period end rates for the Consolidated Balance Sheets and average rates for the Consolidated Statements of Operations. Translation gains and losses arising are reported as a movement on accumulated other comprehensive income. Foreign currency transaction gains and losses are reported in other comprehensive income rather than through income where the foreign currency transaction is 'long-term investment' in nature i.e. settlement is not planned or anticipated in the foreseeable future.
Disclosure of Fair Value of Financial Instruments	Disclosure of fair value of financial instruments Cash, cash equivalents, other receivables, available for sale investments, accounts receivable, accounts payable, investigator payments and income taxes payable have carrying amounts that approximate fair value due to the short term maturities of these instruments. Other liabilities' carrying amounts approximate fair value based on net present value of estimated future cash flows. Debt is measured at historical cost. Financial instruments are measured in the Consolidated Balance Sheets at amortized cost or fair value using a fair value hierarchy of valuation inputs. For financial instruments measured at amortized cost, the fair value of these financial instruments closely approximates their fair value. The fair value hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. These levels are: Level 1: Level 2: Inputs are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. The Group's Senior notes (private placement debt) are carried at $350.0 million (prior to related financing costs). The carrying value at December 31, 2020, closely approximates fair value as the Group's Senior notes were entered into close to the end of the reporting period. The Company classifies its investments in short term debt or equity investments as available for sale, as it does not actively trade such securities nor does it intend to hold them to maturity. The fair value of short term investments are represented by level 1 fair value measurements – quoted prices in active markets for identical assets. The unrealized movements in fair value are recognized in equity until disposal or sale, at which time, those unrealized movements from prior periods are recognized in the Consolidated Statement of Operations. Losses other than temporary, which reduce the carrying amount below cost are recognized in Consolidated Statement of Operations.
Business Combinations	Business combinations The cost of a business combination is measured as the aggregate of the fair values at the date of exchange of assets given, liabilities incurred or assumed and equity instruments issued in exchange for control. Where a business combination agreement provides for an adjustment to the cost of the acquisition which is contingent upon future events, the amount of the estimated adjustment is recognized at the acquisition date at the fair value of the contingent consideration. Any changes to this estimate outside the measurement period will depend on the classification of the contingent consideration. If the contingent consideration is classified as equity it shall not be re-measured and the settlement shall be accounted for within equity. If the contingent consideration is classified as a liability any adjustments will be accounted for through the Consolidated Statement of Operations or Other Comprehensive Income depending on whether the liability is considered a financial instrument. The assets, liabilities and contingent liabilities of businesses acquired are measured at their fair values at the date of acquisition. In the case of a business combination which is completed in stages, the fair values of the identifiable assets, liabilities and contingent liabilities are determined at the date of each exchange transaction. When the initial accounting for a business combination is determined provisionally, any subsequent adjustments to the provisional values allocated to the identifiable assets, liabilities and contingent liabilities are made within twelve months of the acquisition date and presented as adjustments to goodwill in the reporting period in which the adjustments are determined.
Goodwill and Impairment	Goodwill and Impairment Goodwill represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill primarily comprises acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. Goodwill is stated net of any provision for impairment. The Company tests goodwill annually for any impairments or whenever events occur which may indicate impairment. The Company applied the provisions of ASU 2017-04 'Intangibles - Goodwill and Other: Simplifying the test for goodwill impairment'
Intangible Assets	Intangible assets    Intangible assets are amortized on a straight line basis over their estimated useful life.
Cash and Cash Equivalents	Cash and cash equivalents    Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value.
Investment in Debt, Equity and Other	Investments in debt, equity and other Available for sale investments The Company classifies short-term investments as available for sale in accordance with the terms of ASC 320 'Investments - Debt and Equity Securities' . Realized gains and losses are determined using specific identification. The investments are reported at fair value, with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of the investments are represented by accrued interest and unrealized gains/losses. Long term investments The Company classifies its interests in funds having considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2020 meet the definition of equity securities without readily determinable fair values. Effective from January 1, 2018, the Company concluded that the interests held at December 31, 2019 and December 31, 2020 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. Any increases or decreases in fair value are recognized in net income in the period. These are therefore measured at Level 3 of the fair value hierarchy. Equity method investments The Company’s investments that are not consolidated are accounted for under the equity method if the Company exercises significant influence that is considered to be greater than minor. These investments are classified as equity method investments on the accompanying Consolidated Balance Sheet. The Company records its pro rata share of the earnings/losses of these investments in Share of equity method investments in the Consolidated Statement of Operations. The Company reviews these for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable.
Accounts receivable, net and unbilled revenue	Accounts receivable, net and unbilled revenue Accounts receivable and unbilled revenue are recorded at fair value less an estimate of the credit losses expected to be incurred on the Company's accounts receivable portfolio. The Company's estimate of expected credit losses considers historical credit loss information that is adjusted, where necessary, for current conditions and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. The Company's receivables and unbilled services are predominantly due from large and mid-tier pharmaceutical and biotechnology companies that share similar risk characteristics. The Company monitors their portfolio of receivables and unbilled services for any deterioration in current or expected credit quality (for example, expected delinquency level), and adjusts the allowance for credit losses as required. Changes in the allowance for credit losses are recorded as a provision for (or reversal of) credit loss expense in the Consolidated Statement of Operations. Losses are charged against the allowance when management believes the uncollectibility of a previously provisioned amount is confirmed. Accounts receivable factoring Where the Company enters into an agreement to sell certain portfolios of its accounts receivable balances, the sale is accounted for in accordance with ASC Topic 860 'Transfers and Servicing'
Inventory	Inventory    Inventory is valued at the lower of cost and net realizable value and after provisions for obsolescence. The cost of inventories comprises the purchase price and attributable costs, less trade discounts.
Property, Plant and Equipment	Property, plant and equipment Property, plant and equipment is stated at cost less accumulated depreciation. Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5 Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter.
Leases	Leases The Company adopted ASC 842 'Leases' ( ASC 842), with a date of initial application of January 1, 2019. The lease accounting policy applied in preparation of the results for the year ended December 31, 2019 and December 31, 2020 therefore reflect application of ASC 842. The Company adopted the standard using the cumulative-effect adjustment approach. Under this transition method, the Company applied the ASC 842 as at the date of initial application (i.e. January 1, 2019), without restatement of comparative period amounts. The cumulative effect of applying the standard is recorded as an adjustment to the opening consolidated balance sheet as at the date of initial application (see note 24 - Impact of new accounting pronouncements for further details). The comparative information for the year ended December 31, 2018 has not been adjusted and therefore continues to be reported under ASC 840 'Leases' (ASC 840) . ASC 842 requires lessees to recognize the rights and obligations resulting from virtually all leases on the Consolidated Balance Sheet as right-of-use (ROU) assets with corresponding lease liabilities. The most significant impact of application of the standard for the Company related to the recognition of right-of-use assets and lease liabilities on the Consolidated Balance Sheet for operating leases for certain property, vehicles and equipment. Prior to application of ASC 842, costs in respect of operating leases were charged to the Consolidated Statements of Operations on a straight-line basis over the lease term. Pursuant to certain practical expedients available as part of adopting ASC 842, ICON did not reassess whether existing or expired supplier contracts are or contain leases, the classification of existing or expired leases, or whether unamortized initial direct costs meet the new definition of initial direct costs under ASC 842. Additionally, the Company elected to use hindsight in determining the lease term and in assessing impairment of ROU assets, if any. The Company determines if an arrangement is a lease at inception. Finance leases, if any, are depreciated on the same basis as property, plant and equipment. At December 31, 2020 and December 31, 2019, the Company did not account for any leases as finance leases. Operating leases are included in operating right-of-use assets, other liabilities and non-current operating lease liabilities on our Consolidated Balance Sheet with the lease charge recognized on a straight-line basis over the lease term. ROU assets and lease liabilities are recognized based on the present value of future minimum lease payments over the lease term at commencement date or date of transition. Our lease terms may also include options to extend or terminate. The Company actively reviews options to extend or terminate leases and adjusts the ROU asset and lease liability when it is reasonably certain the option will be exercised. The ROU asset is adjusted for any prepayments made at the date of commencement and any initial direct costs incurred. As most of the Company's leases do not provide an implicit rate, the discount rate used is based on the rate of traded corporate bonds available at the commencement date adjusted for country risk, liquidity and lease term. Leasehold improvements are amortized over the shorter of the depreciable lives of the corresponding fixed assets or the lease term including any applicable renewals. Certain property leases include variable lease payments resulting from periodic rent increases based on an index which cannot be reasonably estimated at the lease commencement date. These costs are expensed as incurred on the Consolidated Statements of Operations. The Company accounts for lease and non-lease components separately with lease components flowing through the Consolidated Balance Sheet and non-lease components expensed directly to the Consolidated Statements of Operations. In some cases, the Company enters into sublease agreements and becomes both a lessee and a lessor for the same underlying asset. Although subleases are accounted for separately from the lease they relate to, subleases are accounted for in the same way as other leases.
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 amounts of existing assets and liabilities and their respective tax bases and for operating loss and tax credit carry-forwards. 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 income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance to the amount 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 GILTI (“global intangible low-taxed income”) as a period item in the period it arises and has therefore not provided for deferred taxes in respect of this item.
Government Grants	Government grants Government grants received relating to capital expenditures are shown by deducting the grant from the asset's carrying amount and crediting them to income on a basis consistent with the depreciation policy of the relevant assets. Grants relating to categories of operating expenditures are shown as deferred income and credited to income in the period in which the expenditure to which they relate is charged. Under the grant agreements amounts received may become repayable in full should certain circumstances specified within the grant agreements occur, including downsizing by the Company, disposing of the related assets, ceasing to carry on its business or the appointment of a receiver over any of its assets. The Company has not recognized any loss contingency having assessed as remote the likelihood of these events arising.
Research and Development Credits	Research and development credits    Research and development credits are available to the Company under the tax laws in certain jurisdictions, based on qualifying research and development spend as defined under those tax laws. Research and development credits are generally recognized as a reduction of income tax expense. However, certain tax jurisdictions provide refundable credits that are not wholly dependent on the Company's ongoing income tax status or income tax position. In these circumstances the benefit of these credits is not recorded as a reduction to income tax expense, but rather as a reduction of operating expenditure.
Pension Costs	Pension costs The Company contributes to defined contribution plans covering all eligible employees. The Company contributes to these plans based upon various fixed percentages of employee compensation and such contributions are expensed as incurred. The Company operates, through two subsidiaries, a defined benefit plan for certain of its United Kingdom and Swiss employees. The Company accounts for the costs of these plans in accordance with ASC 715-30 'Defined Benefit Plans – Pension' . These plans are presented in accordance with the requirements of ASC 715-60 'Defined Benefit Plans – Other Postretirement' .
Redeemable Non-Controlling Interests and Equity	Redeemable noncontrolling interests and equity The Company acquired a majority ownership interest in MeDiNova during the year ended December 31, 2019. Included in the purchase agreement were put and call option arrangements with the noncontrolling interest holders that required (put option) or enabled (call option) the Company to purchase the remaining minority ownership at a future date. The option was accounted for as temporary equity, which is presented separately as redeemable noncontrolling interest on the Consolidated Balance Sheet. This classification reflects the assessment that the instruments are contingently redeemable in accordance with ASC 480-10-S99 'Distinguishing Liabilities from Equity'. On March 9, 2020, ICON exercised its option to call the remaining shares and took 100% ownership of MeDiNova.
Net Income per Ordinary Share	Net income per ordinary share Basic net income per ordinary share attributable to the Company has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares. There is no difference in net income used for basic and diluted net income per ordinary share. Basic and diluted net income per ordinary share attributable to the Company includes the adjustment to reflect the accretion of the noncontrolling interest in MeDiNova to its redemption value (see note 26 - Net income per ordinary share ).
Share-Based Compensation	Share-based compensation The Company accounts for its share options, Restricted Share Units ("RSUs") and Performance Share Units ("PSUs") in accordance with the provisions of ASC 718 'Compensation – Stock Compensation'. Share-based compensation expense for equity-settled awards made to employees and directors is measured and recognized based on estimated grant date fair values. These equity-settled awards include employee share options, RSUs and PSUs. Share-based compensation expense for share options awarded to employees and directors is estimated at the grant date based on each option's fair value as calculated using the Black-Scholes option-pricing model. Share-based compensation for RSUs and PSUs awarded to employees and directors is calculated based on the market value of the Company's shares on the date of award of the RSUs and PSUs. The value of awards expected to vest is recognized as an expense over the requisite service periods. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates. Estimating the grant date fair value of share options as of the grant date using an option-pricing model, such as the Black-Scholes model, is affected by the Company's share price as well as assumptions regarding a number of complex variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards.
Impairment of Long-Lived Assets	Impairment of long-lived assets    Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured at the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount of the asset or fair value less selling costs.
Derivative Financial Instruments	Derivative financial instruments We enter into transactions in the normal course of business using various financial instruments in order to hedge against exposure to fluctuating exchange and interest rates. We use derivative financial instruments to reduce exposure to fluctuations in interest rates. A derivative is a financial instrument or other contract whose value changes in response to some underlying variable, which has an initial net investment smaller than would be required for other instruments that have a similar response to the variable and that will be settled at a future date. We do not enter into derivative financial instruments for trading or speculative purposes. We did not hold any interest rate swap contracts or forward currency contracts at December 31, 2020 or December 31, 2019. We use derivative financial instruments to reduce exposure to fluctuations in foreign exchange rates. During the years ended December 31, 2018, December 31, 2019 and December 31, 2020 we entered into forward currency contracts in respect of identified exposure arising from euro payments. All contracts expired during the year in which the contract was entered into. Our accounting policies for derivative financial instruments are based on whether they meet the criteria for designation as cash flow or fair value hedges. A designated hedge of the exposure to variability in the future cash flows of an asset or a liability, or of a forecast transaction, is referred to as a cash flow hedge. A designated hedge of the exposure to changes in fair value of an asset or a liability is referred to as a fair value hedge. The criterion for designating a derivative as a hedge includes the assessment of the instrument's effectiveness in risk reduction, matching of the derivative instrument to its underlying transaction and the probability that the underlying transaction will occur. For derivatives with cash flow hedge accounting designation, we report the gain or loss from the effective portion of the hedge as a component of Other Comprehensive Income and reclassify it into earnings in the same period or periods in which the hedged transaction affects earnings and within the same Consolidated Statement of Operations line item as the impact of the hedged transaction. For derivatives with fair value hedge accounting designation, we recognize gains or losses from the change in fair value of these derivatives, as well as the offsetting change in the fair value of the underlying hedged item, in earnings. Fair value gains and losses arising on derivative financial instruments not qualifying for hedge accounting are reported in our Consolidated Statement of Operations.
Financing Costs and Gain on Interest Rate Hedge	Financing costs and gain on interest rate hedge The interest rate in respect of the 2015 Senior Notes was fixed at 3.64% for the 5 year term of the agreement. The associated interest costs were recognized in interest expense in the period from drawdown in December 2015 until its maturity in December 2020. The interest rate in respect of the 2020 Senior Notes is fixed at 2.32% for Series A Notes which will mature on December 8, 2023 and 2.43% for Series B Notes which will mature on December 8, 2025. The associated interest cost is recognized in the period since drawdown in December 2020. Cash proceeds ($4.6 million) received in November 2015 in respect of the realized hedge gain are amortized to the Consolidated Statement of Operations, net against interest payable, over the period of the 2015 Senior Notes. There was a cash outflow ($0.9 million) in July 2020, representing a realized loss on the interest hedge on the 2020 Senior Notes. This was recorded within Other Comprehensive Income and the realized loss is amortized to the Consolidated Statement of Operations over the term of the 2020 Senior Notes. Deferred financing costs (including issue costs relating to the Senior Notes) are reported at cost less accumulated amortization and the related amortization expense is included in interest expense, in our Consolidated Statement of Operations.
Reclassifications	Reclassifications    Certain amounts in the consolidated financial statements have been reclassified where necessary to conform to the current year presentation.

Significant accounting polici_3

Significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Adjustments Resulting From Foreign Currency Translations	Amounts charged or credited to the Consolidated Statements of Operations for the years ended December 31, 2020, December 31, 2019 and December 31, 2018 were as follows: Year ended 2020 2019 2018 (in thousands) Amounts charged/(credited) $ 5,979 $ 590 $ (3,876)
Estimated Useful Lives of Assets	Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Cost and Fair Value of Investments	December 31, 2020 December 31, 2019 (in thousands) Available for sale investments at start of year $ 49,628 $ 59,910 Purchases — 9,603 Sales and maturities (47,902) (21,686) Interest on short term investments — 1,065 Realized gain on sale of short term investments 234 55 Unrealized capital (loss)/gain – investments (231) 681 $ 1,729 $ 49,628
Available For Sale Short Term Investments by Major Security Type	The following table represents our available for sale short term investments by major security type as of December 31, 2020: Maturity by period Cost Fair Value Less than 1 1 to 5 (in millions) Term deposits $ 1.73 $ 1.73 $ 0.50 $ 1.23 Total ($ in millions) $ 1.73 $ 1.73 $ 0.50 $ 1.23
Equity Method Investments	The following table represents our equity method investments at December 31, 2020: Ownership Percentage Carrying Value Carrying Value At date of investment December 31, 2020 December 31, 2020 July 24, 2020 (in thousands) Oncacare Limited 49 % $ 4,534 $ 4,900

Goodwill (Tables)

Goodwill (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	December 31, 2020 December 31, 2019 (in thousands) Opening goodwill $ 883,170 $ 756,260 Current year acquisitions 27,191 126,932 Prior period acquisition (note 4 (b),(c)) 123 — Foreign exchange movement 25,773 (22) Closing goodwill $ 936,257 $ 883,170
Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed	The following table summarizes the Company’s fair values of the assets acquired and liabilities assumed: January 22, 2020 (in thousands) Cash & cash equivalents $ 10,170 Property, plant and equipment 45 Operating right of use assets 539 Goodwill * 27,191 Customer relationships 11,725 Order backlog 2,883 Accounts receivable 3,033 Prepayments and other current assets 158 Accounts payable (368) Unearned revenue (989) Other liabilities (2,202) Current lease liabilities (219) Non-current lease liabilities (320) Non-current deferred tax liability (4,090) Net assets acquired $ 47,556 Cash outflows $ 46,992 Working capital adjustment paid 564 Contingent consideration ** — Total consideration $ 47,556 * Goodwill represents the acquisition of an established workforce that specializes in medical device development and market access. None of the goodwill recognized is expected to be deductible for income tax purposes. ** The fair value of the contingent consideration was estimated at the date of acquisition. Depending on performance of the company, the total consideration could have increased by a maximum of $6.7 million in contingent consideration. At the acquisition date and at December 31, 2020, the fair value of this contingent consideration payable to MedPass is $Nil. September 24, 2019 (in thousands) Cash & cash equivalents $ 3,292 Property, plant and equipment 564 Operating right of use assets 820 Goodwill * 22,865 Customer relationships 8,159 Order backlog 2,163 Accounts receivable 4,544 Unbilled revenue 186 Prepayments and other current assets 181 Other receivables 6 Accounts payable (799) Unearned revenue (2,411) Other liabilities (933) Current lease liabilities (289) Non-current lease liabilities (531) Net assets acquired $ 37,817 Cash outflows 34,976 Working capital adjustment paid 341 Contingent consideration 2,500 Total consideration $ 37,817 * Goodwill represents the acquisition of an established workforce and the capability to provide at-home trial services and site support solutions. The full amount of the goodwill recognized is expected to be deductible for income tax purposes. May 23, 2019 (in thousands) Cash & cash equivalents $ 7,719 Property, plant and equipment 670 Operating right of use assets 1,558 Goodwill * 81,760 Customer relationships 3,887 Order backlog 171 Patient database 2,542 Accounts receivable 3,488 Unbilled revenue 4,272 Other receivables 819 Prepayments and other current assets 406 Accounts payable (5,484) Unearned revenue (5,796) Other liabilities (6,860) Current lease liabilities (430) Non-current lease liabilities (1,128) Non-current deferred tax liability (1,345) Net assets acquired $ 86,249 Cash outflows $ 54,123 Working capital adjustment received (466) Redeemable noncontrolling interest ** 32,592 Total consideration $ 86,249 *Goodwill represents the acquisition of an established workforce and access to a broad site network in Europe and Africa. None of the goodwill recognized is expected to be deductible for income tax purposes. **The fair value of the redeemable noncontrolling interest on May 23, 2019 was $32.6 million which was estimated by applying an income based approach. The valuation approach used was based on the future earnings of the company times an appropriate earnings multiple. On March 9, 2020 ICON exercised its option to call the outstanding shares in the noncontrolling interest to take 100% ownership of MeDiNova. Effective from this date, the noncontrolling interest was derecognized and a liability of $44.7 million was recognized, representing the assessment of the redemption value of the noncontrolling interest. This liability was settled on July 17, 2020 for $43.9 million. January, 25 2019 (in thousands) Cash & cash equivalent $ 686 Property, plant and equipment 1,697 Operating right of use assets 2,866 Goodwill * 22,430 Customer relationships * 10,708 Order backlog * 2,787 Accounts receivable 3,100 Unbilled revenue 2,421 Other receivables 43 Prepayments and other current assets 908 Deferred tax asset 1,568 Accounts payable (1,280) Unearned revenue (540) Other liabilities (1,232) Current lease liabilities (699) Non-current lease liabilities (2,167) Non-current other liabilities (1,123) Net assets acquired $ 42,173 Cash outflows $ 42,349 Working capital adjustment (176) Total consideration $ 42,173
Summary of Proforma Effect in Net Revenue Net Income and Earnings Per Share	The proforma effect of the MedPass acquisition if completed on January 1, 2019 would have resulted in revenue, net income and earnings per share for the fiscal years ended December 31, 2020 and December 31, 2019 as follows: Year Ended 2020 2019 (in thousands) Revenue $ 2,798,180 $ 2,820,796 Net income $ 332,521 $ 377,485 (in ones) Basic earnings per share $ 6.21 $ 6.91 Diluted earnings per share $ 6.16 $ 6.85 Year Ended 2019 2018 (in thousands) Revenue $ 2,818,280 $ 2,609,233 Net income $ 376,636 $ 324,671 (in ones) Basic earnings per share $ 6.90 $ 6.00 Diluted earnings per share $ 6.84 $ 5.93 Year Ended 2019 2018 (in thousands) Revenue $ 2,807,788 $ 2,599,091 Net income $ 375,979 $ 323,759 (in ones) Basic earnings per share $ 6.89 $ 5.98 Diluted earnings per share $ 6.83 $ 5.91 Year Ended 2019 2018 (in thousands) Revenue $ 2,806,908 $ 2,612,346 Net income $ 373,930 $ 321,698 (in ones) Basic earnings per share $ 6.85 $ 5.94 Diluted earnings per share $ 6.79 $ 5.87

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	December 31, 2020 December 31, 2019 Cost (in thousands) Customer relationships $ 144,101 $ 132,192 Technology asset 11,169 11,169 Order backlog 39,224 36,318 Trade names/ brands 2,766 2,766 Volunteer list 1,325 1,325 Non-compete arrangements 489 489 Patient database 2,542 2,542 Foreign exchange movement 209 (5,728) Total cost 201,825 181,073 Accumulated amortization (135,430) (116,196) Foreign exchange movement 65 3,017 Net book value $ 66,460 $ 67,894
Schedule of Future Intangible Asset Amortization Expense	Future intangible asset amortization expense for the years ended December 31, 2021 to December 31, 2025 is as follows: Year Ended (in thousands) 2021 $ 16,132 2022 10,426 2023 7,594 2024 7,149 2025 6,126 $ 47,427

Property, Plant and Equipment_2

Property, Plant and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property Plant and Equipment, Net	December 31, 2020 December 31, 2019 (in thousands) Cost Land $ 3,724 $ 3,724 Building 84,195 85,050 Computer equipment and software 438,742 434,707 Office furniture and fixtures 90,477 88,401 Laboratory equipment 44,101 40,611 Leasehold improvements 30,588 31,144 Motor vehicles 158 199 Foreign exchange movement 11,728 (1,371) 703,713 682,465 Less accumulated depreciation and asset write offs (522,473) (518,383) Foreign exchange movement (6,897) 1,005 Property, plant and equipment (net) $ 174,343 $ 165,087

Other Liabilities (Tables)

Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Other Liabilities Disclosure [Abstract]
Schedule of Other Liabilities	December 31, 2020 December 31, 2019 (in thousands) General trade and overhead liabilities* $ 188,638 $ 174,131 Personnel related liabilities 161,363 157,146 Operating lease liability (note 22) 24,334 28,320 Facility related liabilities 9,441 11,138 Other liabilities 8,726 6,531 Restructuring charges (note 14) 7,219 1,232 Short term government grants (note 11) 48 45 $ 399,769 $ 378,543 *includes amounts due to third parties in respect o f accrued reimbursable expenses of $138.2 million at December 31, 2020 and $142.6 million at December 31, 2019.

Non-Current Other Liabilities (

Non-Current Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Non-Current Other Liabilities	December 31, 2020 December 31, 2019 (in thousands) Other non-current liabilities $ 15,971 $ 11,459 Defined benefit pension obligations, net (note 9) 10,395 6,053 $ 26,366 $ 17,512

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan	Funded status December 31, 2020 December 31, 2019 (in thousands) Projected benefit obligation $ (43,988) $ (37,036) Fair value of plan assets 34,612 32,016 Funded status $ (9,376) $ (5,020) Non-current other liabilities (note 8) $ (9,376) $ (5,020) Change in benefit obligation December 31, 2020 December 31, 2019 (in thousands) Benefit obligation at beginning of year $ 37,036 $ 30,045 Service cost 100 107 Interest cost 746 867 Plan participants' contributions 22 24 Benefits paid (724) (177) Actuarial loss 5,294 4,756 Foreign currency exchange rate changes 1,514 1,414 Benefit obligation at end of year $ 43,988 $ 37,036 Change in plan assets December 31, 2020 December 31, 2019 (in thousands) Fair value of plan assets at beginning of year $ 32,016 $ 27,297 Actual return on plan assets 2,092 3,504 Employer contributions 109 148 Plan participants' contributions 22 24 Benefits paid (724) (177) Foreign currency exchange rate changes 1,097 1,220 Fair value of plan assets at end of year $ 34,612 $ 32,016 Funded status December 31, 2020 December 31, 2019 (in thousands) Projected benefit obligation $ (8,620) $ (7,047) Fair value of plan assets 7,601 6,014 Funded status $ (1,019) $ (1,033) Non-current other liabilities (note 8) $ (1,019) $ (1,033) Change in benefit obligation December 31, 2020 December 31, 2019 (in thousands) Benefit obligation at beginning of year $ 7,047 $ 5,279 Service cost 139 115 Interest cost 21 40 Plan participants' contributions 81 67 Prior service cost (23) — Transferred balances/(benefits paid) 245 (72) Actuarial loss 406 1,479 Foreign currency exchange rate changes 704 139 Benefit obligation at end of year $ 8,620 $ 7,047 Change in plan assets December 31, December 31, 2020 2019 (in thousands) Fair value of plan assets at beginning of year $ 6,014 $ 4,707 Expected return on plan assets 21 35 Actual return on plan assets 519 1,114 Scheme contributions 105 89 Plan participants' contributions 81 67 Transferred balances/ (benefits paid) 245 (72) Foreign currency exchange rate changes 616 74 Fair value of plan assets at end of year $ 7,601 $ 6,014
Schedule of Components of Net Periodic Benefit Cost	The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit cost: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Service cost $ 100 $ 107 $ 124 Interest cost 746 867 895 Expected return on plan assets (1,214) (574) (624) Amortization of net loss 160 67 248 Net periodic benefit cost $ (208) $ 467 $ 643 December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Service cost $ 139 $ 115 $ 138 Interest cost 21 40 47 Expected return on plan assets (21) (35) (41) Amortization of net gain (6) (93) (69) Amortization of prior service credit (12) (9) (8) Settlement — — (93) Net periodic benefit cost/(credit) $ 121 $ 18 $ (26)
Summary of Assumptions Used in Calculating Pension Benefit Obligations	The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2020, December 31, 2019 and December 31, 2018: December 31, 2020 December 31, 2019 December 31, 2018 Discount rate 2.1 % 2.9 % 2.5 % Rate of compensation increase 3.3 % 3.7 % 3.7 % Expected rate of return on plan assets 4.0 % 2.1 % 2.0 % December 31, 2020 December 31, 2019 Discount rate 1.5 % 2.1 % Rate of compensation increase 3.4 % 3.3 % December 31, 2020 December 31, 2019 December 31, 2018 Discount rate 0.30 % 0.80 % 0.80 % Rate of compensation increase 2.00 % 2.00 % 2.00 % Expected rate of return on plan assets 0.34 % 0.80 % 0.80 % December 31, 2020 December 31, 2019 Discount rate 0.15 % 0.30 % Rate of compensation increase 2.00 % 2.00 %
Schedule of Amounts Recognized in Other Comprehensive Income (Loss)	Other comprehensive income December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Actuarial loss/(gain) - benefit obligation $ 5,294 $ 4,756 $ (3,844) Actuarial (gain)/loss – plan assets (878) (2,930) 1,208 Actuarial loss recognized in net periodic benefit cost (160) (67) (248) Total $ 4,256 $ 1,759 $ (2,884) Other comprehensive income December 31, 2020 December 31, 2019 December 31, 2018 Actuarial loss/(gain) - benefit obligation $ 406 $ 1,479 $ (372) Actuarial (gain)/loss – plan assets (519) (1,114) 240 Prior service credit recognized in net periodic benefit cost — — 93 Actuarial gain recognized in net periodic benefit cost 6 93 69 Amortization of net prior service credit 12 9 8 Net prior service cost occurring during the year (23) — (9) Total $ (118) $ 467 $ 29
Summary of Expected Long Term Rates of Return on Different Asset Classes	The expected long term rates of return on different asset classes are as follows: Asset Category Expected long-term return per annum Corporate Bonds (including 50% high yield bonds) 2.8 % Equities 5.2 % Secured Loans and Multi Asset Credit 3.0 %
Schedule of Underlying Asset Split of Fund	The underlying asset split of the fund is shown below. Asset Category December 31, 2020 December 31, 2019 Corporate Bonds (including 50% high yield bonds) 40 % 40 % Equities 21 % 21 % Secured Loans and Multi Asset Credit 39 % 39 % 100 % 100 %
Schedule of Plan Asset Fair Value Measurements	Quoted Prices in Active Markets for Identical Assets December 31, 2020 December 31, 2019 (in thousands) Cash $ 11 $ 17 Fixed Income Securities L&G Life GPBF All World Equity Index Fund 7,460 6,753 L&G Life DC Active Corporate Bond 6,797 6,412 Stone Harbor High Yield Bond Fund 6,861 6,292 Ninety One Global Total Return Credit 3,472 — Investec Global Total Return Credit Fund — 3,166 Stone Harbor Multi Asset Credit Portfolio 3,389 3,151 Barings European Loan Fund Buy & Hold 6,622 6,225 $ 34,612 $ 32,016
Schedule of Annual Benefit Payments which Reflect Expected Future Service	The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2021 $ 342 2022 258 2023 343 2024 421 2025 458 Years 2026 - 2030 3,700 (in thousands) 2021 $ 348 2022 342 2023 334 2024 326 2025 377 Years 2025 - 2029 1,587

Equity Incentive Schemes and _2

Equity Incentive Schemes and Stock Compensation Charges (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Transactions for Company's Share Option Plans	The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2020, December 31, 2019 and December 31, 2018: Options Granted Weighted Average Exercise Price Weighted Average Grant Date Fair Value Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 167,557 $ 118.90 $ 36.84 Exercised (408,699) $ 41.12 $ 13.55 Canceled (9,505) $ 32.35 $ 11.39 Outstanding at December 31, 2018 920,746 $ 74.32 $ 22.39 Granted 97,112 $ 140.13 $ 43.43 Exercised (329,870) $ 65.54 $ 19.78 Canceled/expired (31,881) $ 88.12 $ 26.77 Outstanding at December 31, 2019 656,107 $ 87.80 $ 26.60 Granted 107,737 $ 159.83 $ 42.43 Exercised (193,417) $ 68.19 $ 20.91 Canceled/expired (16,681) $ 92.21 $ 27.93 Outstanding at December 31, 2020 553,746 $ 108.53 $ 31.63 Vested and exercisable at December 31, 2020 224,845 $ 83.13 $ 24.90
Schedule of Non Vested Shares Outstanding	Non-vested shares outstanding as at December 31, 2020 are as follows: Options Weighted Average Exercise Price Weighted Average Fair Value Non-vested outstanding at December 31, 2019 358,030 $ 103.68 $ 31.28 Granted 107,737 159.83 42.43 Vested (124,605) 92.25 27.51 Forfeited (12,261) 117.41 34.50 Non-vested outstanding at December 31, 2020 328,901 $ 125.89 $ 36.24
Summary of Information Concerning Outstanding and Exercisable Share Options	The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2020: Options Outstanding Options Exercisable Range Exercise Number of Weighted Weighted Average Exercise Price Number of Weighted Average Exercise Price $ 32.37 2,500 0.33 2,500 $ 37.90 920 0.92 920 $ 40.83 21,360 1.39 21,360 $ 47.03 479 1.16 479 $ 48.67 1,027 1.20 1,027 $ 65.60 29,381 3.38 19,528 $ 66.47 2,621 2.38 2,621 $ 68.39 24,508 2.18 24,508 $ 71.95 73,193 3.17 59,284 $ 83.47 58,159 4.17 10,671 $ 90.03 45,154 4.38 27,698 $ 115.11 68,016 5.17 8,866 $ 125.74 39,664 5.38 36,885 $ 137.47 6,045 6.37 895 $ 140.38 75,061 6.17 7,603 $ 159.33 98,128 7.17 — $ 166.51 7,530 7.47 — $32.37-$166.51 553,746 4.86 $ 108.53 224,845 $ 83.13
Summary of Weighted Average Fair Values and Assumptions Used	The weighted average fair values and assumptions were as follows: Year Ended December 31, 2020 December 31, 2019 December 31, 2018 Weighted average fair value $ 42.43 $ 43.43 $ 36.84 Assumptions: Expected volatility 30 % 30 % 30 % Dividend yield — % — % — % Risk-free interest rate 0.57 % 2.55 % 2.76 % Expected life 5.0 years 5.0 years 5.0 years
Summary of RSU and PSU Activity	The following table summarizes RSU and PSU activity for the year ended December 31, 2020: PSU Outstanding PSU PSU RSU Outstanding RSU RSU Outstanding at December 31, 2019 175,989 $ 110.79 1.04 389,900 $ 119.07 1.43 Granted 57,184 $ 165.30 137,572 $ 168.03 Shares vested (63,516) $ 83.92 (144,747) $ 98.47 Forfeited (10,016) $ 134.27 (41,301) $ 133.63 Outstanding at December 31, 2020 159,641 $ 137.64 1.14 341,424 $ 145.77 1.41
Schedule of Non-cash Stock Compensation Expense	Non-cash stock compensation expense has been allocated as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Direct costs $ 8,557 $ 14,777 $ 17,408 Selling, general and administrative 17,714 12,042 14,186 $ 26,271 $ 26,819 $ 31,594

Government Grants (Tables)

Government Grants (Tables)	12 Months Ended
	Dec. 31, 2020
Deferred Revenue Disclosure [Abstract]
Government Grants	December 31, 2020 December 31, 2019 (in thousands) Received $ 3,539 $ 3,539 Less accumulated amortization (2,881) (2,836) Foreign exchange translation adjustment 228 155 Total government grants 886 858 Less current portion (48) (45) Non-current government grants $ 838 $ 813

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Summary of Components of Income Before Provision for Income Taxes	The components of income before income tax expense are as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 280,310 $ 323,726 $ 243,988 United States 41,950 21,073 27,499 Other 58,945 82,190 93,127 $ 381,205 $ 426,989 $ 364,614
Summary of Components of Provision for Income Taxes	The components of income tax expense are as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Income tax expense: Current tax expense: Ireland $ 28,963 $ 35,955 $ 28,042 United States 3,022 5,073 2,885 Other 14,963 11,642 9,379 Total current tax expense 46,948 52,670 40,306 Deferred tax expense/ (benefit): Ireland 1,654 2,833 1,054 United States 4,577 (3,502) 875 Other (5,304) (868) (277) 927 (1,537) 1,652 Income tax expense allocated to continuing operations 47,875 51,133 41,958 Income tax expense was allocated to the following components of other comprehensive income: Currency impact on long term funding 68 25 119 Fair value of cash flow hedge — — (148) Total $ 47,943 $ 51,158 $ 41,929
Schedule of Reconciliation of Consolidated Reported Provision for Income Taxes and Statutory Rate	The Company's consolidated reported income tax expense differed from the amount that would result from applying the Irish statutory rate as set forth below: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Taxes at Irish statutory rate of 12.5% (2019:12.5%; 2018:12.5%) $ 47,651 $ 53,374 $ 45,577 Foreign and other income taxed at higher rates 7,943 7,356 7,649 Research & development tax incentives (1,243) (893) (1,243) Movement in valuation allowance 3,581 (10) 5,667 Effects of change in tax rates 108 359 (147) Decrease in unrecognized tax benefits (1,672) (1,273) (5,423) Impact of stock compensation (5,150) (7,383) (8,301) Other (3,343) (397) (1,821) $ 47,875 $ 51,133 $ 41,958
Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities	The tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below: December 31, 2020 December 31, 2019 (in thousands) Deferred tax liabilities: Property, plant and equipment $ 1,359 $ 1,102 Right-of-use-assets 9,402 11,838 Goodwill 31,629 27,590 Intangible assets 13,398 11,805 Other 1,009 6,284 56,797 58,619 Deferred tax assets: Operating loss and tax credits carryforwards 42,794 37,865 Property, plant and equipment 6,040 5,257 Lease liabilities 9,394 11,754 Accrued expenses and payments on account 24,368 26,380 Stock compensation 3,672 5,009 Deferred compensation 3,184 2,744 Unearned revenue 2,257 3,933 Other 155 604 Total deferred tax assets 91,864 93,546 Valuation allowance for deferred tax assets (32,768) (27,721) Deferred tax assets recognized 59,096 65,825 Overall net deferred tax asset $ 2,299 $ 7,206
Schedule of Expected Expiry Dates of NOL's	The expected expiry dates of these US losses are as follows: Federal State (in thousands) 2022-2035 $ 13,295 $ 51,092 2037 1,921 — Indefinite 1,363 49 $ 16,579 $ 51,141
Schedule of Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Unrecognized tax benefits at start of year $ 20,156 $ 21,433 $ 23,720 Increase related to prior year tax positions 401 — 2,084 Decrease related to prior year tax positions (1,271) — (2,915) Increase related to current year tax positions 2,931 1,588 3,065 Settlements (369) (347) (182) Lapse of statute of limitations (2,770) (2,518) (4,339) Unrecognized tax benefits at end of year $ 19,078 $ 20,156 $ 21,433

Restructuring charges (Tables)

Restructuring charges (Tables)	12 Months Ended
	Dec. 31, 2020
Restructuring and Related Activities [Abstract]
Schedule of Restructuring Charges	Restructuring recognized during the year ended December 31, 2020 comprise: Year Ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Restructuring charges $ 18,089 $ — $ 12,490 Net charge $ 18,089 $ — $ 12,490
Schedule of Restructuring Reserve by Type of Cost	Workforce reductions Onerous leases Total (in thousands) Initial restructuring charge recorded $ 11,391 $ 6,698 $ 18,089 Net charge $ 11,391 $ 6,698 $ 18,089 Workforce reductions liability Onerous lease liability Total (in thousands) Initial restructuring provision $ 11,391 $ 6,698 $ 18,089 Utilization (6,987) (1,309) $ (8,296) Foreign exchange movement — 325 $ 325 Provision at December 31, 2020 $ 4,404 $ 5,714 $ 10,118 Workforce Onerous Total (in thousands) Total provision recognized $ 9,684 $ 2,806 $ 12,490 Utilized (5,399) (672) $ (6,071) Provision at December 31, 2018 $ 4,285 $ 2,134 $ 6,419 Utilized (3,554) (1,228) (4,782) Provision at December 31, 2019 $ 731 $ 906 $ 1,637 Utilized (731) (276) (1,007) Provision at December 31, 2020 $ — $ 630 $ 630
Schedule of Operating Lease Maturity	Future minimum lease payments (including related costs), associated with the 2020 restructuring plan, under the non-cancelable onerous leases as of December 31, 2020 were as follows: Minimum rental payments (in thousands) 2021 $ 2,602 2022 1,896 2023 999 2024 139 2025 139 Thereafter 402 Total future minimum lease payments (including related costs) 6,177 Lease imputed interest (463) Total $ 5,714 Minimum rental payments (in thousands) 2021 $ 26,164 2022 21,510 2023 16,108 2024 10,165 2025 6,046 Thereafter 10,490 Total future minimum lease payments 90,483 Lease imputed interest (5,348) Total $ 85,135

Disaggregation of Revenue (Tabl

Disaggregation of Revenue (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Revenue disaggregated by customer profile is as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Top client $ 337,904 $ 350,287 $ 352,335 Clients 2-5 754,906 704,963 671,723 Clients 6-10 350,865 347,832 385,741 Clients 11-25 501,643 529,713 461,351 Other 851,970 873,044 724,627 Total $2,797,288 $2,805,839 $2,595,777

Accounts receivable, unbilled_2

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Contracts with Customers, Asset and Liabilities	Accounts receivable and unbilled revenue are as follows: December 31, 2020 December 31, 2019 (in thousands) Billed services (accounts receivable) $ 722,420 $ 535,088 Unbilled services (unbilled revenue) 428,684 422,769 Accounts receivable and unbilled revenue 1,151,104 957,857 Allowance for credit losses (note 18) (7,149) — Provision for doubtful debts (note 18) — (7,380) Accounts receivable and unbilled revenue, net $ 1,143,955 $ 950,477 Unbilled services and unearned revenue or payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2020 December 31, 2019 $ Change % Change Unbilled services (unbilled revenue) $ 428,684 $ 422,769 $ 5,915 1.4 % Unearned revenue (payments on account) (660,883) (366,988) (293,895) 80.1 % Net balance $ (232,199) $ 55,781 $ (287,980) 516.3 %

Provision for Credit Losses (Ta

Provision for Credit Losses (Tables)	12 Months Ended
	Dec. 31, 2020
Accounts Receivable, after Allowance for Credit Loss [Abstract]
Schedule of Provision for Credit Losses	The Company does business with most major international pharmaceutical companies. Provision for credit losses at December 31, 2020 and provision for doubtful debts at December 31, 2019 comprises: December 31, 2020 December 31, 2019 (in thousands) Opening provision $ 7,380 $ 8,889 Amounts used during the year (2,561) — Amounts provided during the year 2,692 1,691 Amounts released during the year (510) (3,226) Foreign exchange 148 26 Closing provision $ 7,149 $ 7,380

Business Segment and Geograph_2

Business Segment and Geographical Information (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Schedule of Distribution of Net Revenue by Geographical Area	The distribution of revenue by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 1,181,292 $ 1,252,834 $ 1,066,200 Rest of Europe 416,884 388,916 379,883 U.S. 925,563 892,497 894,978 Other 273,549 271,592 254,716 Total $ 2,797,288 $ 2,805,839 $ 2,595,777
Schedule of Distribution of Income from Operations by Geographical Area	The distribution of income from operations, including restructuring, by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 276,478 $ 314,287 $ 257,089 Rest of Europe 35,765 37,997 36,280 U.S. 58,018 60,272 58,561 Other 21,239 20,850 21,427 Total $ 391,500 $ 433,406 $ 373,357 c) The distribution of income from operations, excluding restructuring, by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 295,360 $ 314,287 $ 269,196 Rest of Europe 35,402 37,997 36,904 U.S. 57,680 60,272 58,340 Other 21,147 20,850 21,407 Total $ 409,589 $ 433,406 $ 385,847
Schedule of Distribution of Long-lived Assets, Net, by Geographical Area	The distribution of long-lived assets (including operating right-of-use assets (year ended December 31, 2020 and 2019)), net, by geographical area was as follows: December 31, 2020 December 31, 2019 (in thousands) Ireland $ 118,361 $ 110,522 Rest of Europe 36,723 41,970 U.S. 65,152 72,578 Other 38,668 44,994 Total $ 258,904 $ 270,064
Schedule of Distribution of Depreciation and Amortization by Geographical Area	The distribution of depreciation, amortization and reduction in carrying value of operating right-of-use assets (year ended December 31, 2020 and 2019 only) by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 33,516 $ 30,635 $ 34,721 Rest of Europe 18,569 14,370 5,331 U.S. 36,060 33,922 21,605 Other 11,872 12,995 4,259 Total $ 100,017 $ 91,922 $ 65,916
Schedule of Distribution of Total Assets by Geographical Area	The distribution of total assets by geographical area was as follows: Year Ended December 31, 2020 December 31, 2019 (in thousands) Ireland $ 1,675,980 $ 1,323,181 Rest of Europe 671,218 660,797 U.S. 909,202 755,271 Other 179,206 168,263 Total $ 3,435,606 $ 2,907,512
Schedule of Distribution of Capital Expenditures by Geographical Area	The distribution of capital expenditures (including operating right-of-use assets (year ended December 31, 2020 and 2019 only)) by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 29,309 $ 32,526 $ 30,942 Rest of Europe 9,298 13,971 2,590 U.S. 14,347 18,638 9,311 Other 10,201 15,010 5,554 Total $ 63,155 $ 80,145 $ 48,397
Schedule of Company's Net Revenue for Major Clients	The following table sets forth the clients which represented 10% or more of the Company's revenue in each of the periods set out below. Year ended December 31, 2020 December 31, 2019 December 31, 2018 Client A 12 % 12 % 14 % Client B <10% 10 % <10%
Schedule of Distribution of Interest Income by Geographical Area	The distribution of interest income by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 1,620 $ 3,674 $ 2,620 Rest of Europe 570 2,242 1,750 U.S. 22 111 17 Other 512 832 372 Total $ 2,724 $ 6,859 $ 4,759
Schedule of Distribution of Tax Charge by Geographical Area	The distribution of the income tax charge by geographical area was as follows: Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Ireland $ 30,616 $ 38,788 $ 29,096 Rest of Europe 4,609 4,234 (434) U.S. 7,600 1,571 3,761 Other 5,050 6,540 9,535 Total $ 47,875 $ 51,133 $ 41,958

Supplemental Disclosure of Ca_2

Supplemental Disclosure of Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Schedule of Supplemental Disclosure of Cash Flow Information	Year ended December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Cash paid for interest $ 13,062 $ 13,059 $ 13,060 Cash paid for income taxes (net of refunds) $ 27,604 $ 29,836 $ 18,558

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Accumulated Other Comprehensive Income	Year ended December 31, 2020 December 31, 2019 (in thousands) Currency translation adjustments $ (15,894) $ (64,023) Currency impact on long term funding (9,329) (7,726) Actuarial loss on defined benefit pension plan (note 9) (9,364) (5,226) Unrealized capital gain – investments (note 3) — 231 Realized gain on interest rate hedge 4,658 4,658 Amortization of gain on interest rate hedge (4,658) (3,733) Loss on interest rate hedge (905) — Amortization of loss on interest rate hedge 15 — Total $ (35,477) $ (75,819)

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lease Cost	Lease costs recorded under operating leases were as follows: Year ended December 31, 2020 December 31, 2019 (in thousands) Operating lease costs $ 31,088 $ 34,759 Income from sub-leases (940) (1,761) Net operating lease costs $ 30,148 $ 32,998
Schedule of Operating Lease Maturity	Future minimum lease payments (including related costs), associated with the 2020 restructuring plan, under the non-cancelable onerous leases as of December 31, 2020 were as follows: Minimum rental payments (in thousands) 2021 $ 2,602 2022 1,896 2023 999 2024 139 2025 139 Thereafter 402 Total future minimum lease payments (including related costs) 6,177 Lease imputed interest (463) Total $ 5,714 Minimum rental payments (in thousands) 2021 $ 26,164 2022 21,510 2023 16,108 2024 10,165 2025 6,046 Thereafter 10,490 Total future minimum lease payments 90,483 Lease imputed interest (5,348) Total $ 85,135

Net Income Per Ordinary Share (

Net Income Per Ordinary Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share	The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows: December 31, 2020 December 31, 2019 December 31, 2018 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 52,859,911 53,859,537 54,118,764 Effect of dilutive share options outstanding 423,674 473,924 671,899 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 53,283,585 54,333,461 54,790,663
Reconciliation of Net Income Attributable to the Group Per the Statement of Operating Income and Net Income Used For Net Income Per Ordinary Share	The reconciliation between net income attributable to the Group per the Statement of Operating Income and the net income used to calculate net income per ordinary share attributable to the Group is as follows: December 31, 2020 December 31, 2019 December 31, 2018 (in thousands) Net income attributable to the Group $ 332,331 373,986 $ 322,656 Noncontrolling interest adjustment to redemption amount (4,522) (5,048) — Net income attributable to the Group (including NCI redemption adjustment) 327,809 368,938 322,656 December 31, 2020 December 31, 2019 December 31, 2018 Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment): Basic $ 6.20 $ 6.85 $ 5.96 Diluted $ 6.15 $ 6.79 $ 5.89

Description of business (Detail

Description of business (Details)	12 Months Ended
	Dec. 31, 2020EmployeeLocationCountry
Product Information [Line Items]
Number of employees | Employee	15,730
Number of locations in which company operates | Location	93
Number of countries in which company operates | Country	41
Geographic Concentration Risk | United States | Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	33.10%
Geographic Concentration Risk | Europe | Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	57.10%
Geographic Concentration Risk | Rest of World | Net Revenue
Product Information [Line Items]
Percentage of company revenue (in percent)	9.80%

Significant accounting polici_4

Significant accounting policies - Narrative (Details)	1 Months Ended		12 Months Ended
	Jul. 31, 2020USD ($)	Nov. 30, 2015USD ($)	Dec. 31, 2020USD ($)subsidiary	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Mar. 09, 2020
Debt Instrument [Line Items]
Advertising and promotion costs			$ 7,020,328	$ 8,088,451	$ 6,516,637
Short-term debt			0	349,640,000
Goodwill impairment charge			0	0	0
Carrying value of inventory			$ 4,800,000	3,200,000
Number of subsidiaries | subsidiary			2
Debt instrument, term			5 years
Cash outflow	$ 900,000		$ 905,000	0	$ 0
Senior Notes
Debt Instrument [Line Items]
Short-term debt			$ 350,000,000
Debt instrument, interest rate (in percent)			3.64%
Cash proceeds from interest rate hedge		$ 4,600,000
Senior Notes
Debt Instrument [Line Items]
Short-term debt			$ 350,000,000	$ 350,000,000
Series A note | Senior Notes
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)			2.32%
Series B note | Senior Notes
Debt Instrument [Line Items]
Debt instrument, interest rate (in percent)			2.43%
MeDiNova
Debt Instrument [Line Items]
Ownership percentage						100.00%

Significant accounting polici_5

Significant accounting policies - Adjustments Resulting from Foreign Currency Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Foreign Currency [Abstract]
Amounts charged/(credited)	$ 5,979	$ 590	$ (3,876)

Significant accounting polici_6

Significant accounting policies - Estimated Useful Lives of Assets (Details)	12 Months Ended
	Dec. 31, 2020
Building
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years
Office furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Motor vehicles
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years

Investments - Available For Sal

Investments - Available For Sale Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation Of Debt Securities, Available-For-Sale [Roll Forward]
Available for sale investments at start of year	$ 49,628	$ 59,910
Purchases	0	9,603	$ 80,956
Sales and maturities	(47,902)	(21,686)	(99,865)
Interest on short term investments	0	1,065	1,329
Realized gain on sale of short term investments	234	55	56
Unrealized capital (loss)/gain - investments	(231)	681	(155)
Available for sale investments at end of year	$ 1,729	$ 49,628	$ 59,910

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended			24 Months Ended	36 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2020	Jul. 24, 2020
Investments, Debt and Equity Securities [Abstract]
Short term investments average maturity period	3 years 4 months 24 days	7 months 24 days
Purchase of investments in equity - long term	$ 3,212	$ 3,890	$ 6,163	$ 10,100	$ 13,300
Increase (decrease) in fair value of investments in equity	2,500	(800)
Carrying value of long-term investments in equity	15,800	10,100		10,100	15,800
Commitment to acquire future long-term investments	21,900				21,900
Equity method investments	$ 4,534	0		$ 0	$ 4,534	$ 4,900
Ownership percentage	49.00%				49.00%
Majority investor, ownership percentage	51.00%				51.00%
Period majority voting share capital can be sold to the company	18 months
Payments to acquire equity method investments	$ 2,450	0	0
Loss on equity method investments	$ 366	$ 0	$ 0

Investments - Available For S_2

Investments - Available For Sale Short Term Investments by Major Security Type (Details) $ in Thousands	Dec. 31, 2020USD ($)
Debt Securities, Available-for-sale [Line Items]
Cost Total	$ 1,730
Fair Value Total	1,730
Less than 1 year	500
1 to 5 years	1,230
Term deposits
Debt Securities, Available-for-sale [Line Items]
Cost Total	1,730
Fair Value Total	1,730
Less than 1 year	500
1 to 5 years	$ 1,230

Investments - Equity Method Inv

Investments - Equity Method Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jul. 24, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Ownership percentage	49.00%
Equity method investments	$ 4,534	$ 4,900	$ 0

Goodwill - Schedule of Goodwill

Goodwill - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Opening goodwill	$ 883,170	$ 756,260
Current year acquisitions	27,191	126,932
Prior period acquisition (note 4 (b),(c))	123	0
Foreign exchange movement	25,773	(22)
Closing goodwill	$ 936,257	$ 883,170

Goodwill - Narrative (Details)

Goodwill - Narrative (Details) - USD ($)	3 Months Ended	7 Months Ended	11 Months Ended			12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2020	Mar. 09, 2020	Jan. 22, 2020	Sep. 25, 2019	Sep. 24, 2019	May 23, 2019	Jan. 25, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 883,170,000	$ 883,170,000	$ 936,257,000	$ 883,170,000	$ 883,170,000	$ 936,257,000							$ 756,260,000
Medpass Group Limited
Business Acquisition [Line Items]
Percentage of share capital acquired (in percent)								100.00%
Accounts receivable adjustment						200,000
Unearned revenue adjustment						800,000
Business combination, fair value decrease to operating right-to-use assets						800,000
Other liabilities adjustment						(800,000)
Business combination, fair value decrease to operating current lease liabilities						100,000
Business combination, fair value decrease to operating non-current lease liabilities						700,000
Business combination, fair value decrease to operating non-current deferred tax liability						600,000
Revenue of acquiree since acquisition date			13,200,000
Net income of acquiree since acquisition date			2,500,000
Goodwill								$ 27,191,000
Valuation of contingent consideration at acquisition			0			0		$ 0
Symphony
Business Acquisition [Line Items]
Percentage of share capital acquired (in percent)										100.00%
Accounts receivable adjustment						600,000
Unearned revenue adjustment						1,000,000
Other liabilities adjustment						(100,000)
Revenue of acquiree since acquisition date	4,500,000
Net income of acquiree since acquisition date	1,000,000
Goodwill									$ 22,865,000
Unbilled revenue adjustment						100,000
Accounts payable adjustment						300,000
Valuation of contingent consideration at acquisition	$ 2,500,000	2,500,000		2,500,000	2,500,000				$ 2,500,000
Settlement of contingent consideration						500,000
Symphony | Minimum
Business Acquisition [Line Items]
Valuation of contingent consideration at acquisition			0			0
Symphony | Maximum
Business Acquisition [Line Items]
Valuation of contingent consideration at acquisition			$ 2,500,000			2,500,000
MeDiNova
Business Acquisition [Line Items]
Percentage of share capital acquired (in percent)							100.00%				60.00%
Business combination, fair value decrease to operating right-to-use assets						300,000
Other liabilities adjustment						1,600,000
Business combination, fair value decrease to operating current lease liabilities						100,000
Business combination, fair value decrease to operating non-current lease liabilities						300,000
Business combination, fair value decrease to operating non-current deferred tax liability						$ 2,200,000
Revenue of acquiree since acquisition date		6,500,000
Net income of acquiree since acquisition date		$ 4,700,000
Goodwill											$ 81,760,000
MMD
Business Acquisition [Line Items]
Percentage of share capital acquired (in percent)												100.00%
Unearned revenue adjustment				(900,000)
Other liabilities adjustment				100,000
Revenue of acquiree since acquisition date					17,900,000
Net income of acquiree since acquisition date					$ 1,100,000
Goodwill												$ 22,430,000
Unbilled revenue adjustment				(2,100,000)
Accounts payable adjustment				$ (600,000)

Goodwill - Summary of Estimates

Goodwill - Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($)	Jul. 17, 2020	Jan. 22, 2020	Sep. 24, 2019	May 23, 2019	Jan. 25, 2019	Jul. 17, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 09, 2020	Sep. 25, 2019	Jul. 27, 2017
Business Acquisition [Line Items]
Goodwill							$ 883,170,000	$ 936,257,000	$ 883,170,000	$ 756,260,000
Cash outflows								47,931,000	131,272,000	$ 1,645,000
Derecognition of noncontrolling interest, and establishing a liability						$ 44,700,000
Medpass Group Limited
Business Acquisition [Line Items]
Cash & cash equivalents		$ 10,170,000
Property, plant and equipment		45,000
Operating right of use assets		539,000
Goodwill		27,191,000
Accounts receivable		3,033,000
Prepayments and other current assets		158,000
Accounts payable		(368,000)
Unearned revenue		(989,000)
Other liabilities		(2,202,000)
Current lease liabilities		(219,000)
Non-current lease liabilities		(320,000)
Non-current deferred tax liability		(4,090,000)
Net assets acquired		47,556,000
Cash outflows		46,992,000
Working capital adjustment paid		564,000
Contingent consideration		0						0
Total consideration		47,556,000
Goodwill, expected tax deductible amount		0
Contingent consideration, range of outcomes, maximum value		6,700,000
Other liabilities adjustment								800,000
Unearned revenue adjustment								(800,000)
Symphony
Business Acquisition [Line Items]
Cash & cash equivalents												$ 3,292,000
Property, plant and equipment												564,000
Operating right of use assets												820,000
Goodwill												22,865,000
Accounts receivable												4,544,000
Unbilled revenue												186,000
Prepayments and other current assets												181,000
Other receivables												6,000
Accounts payable												(799,000)
Unearned revenue												(2,411,000)
Other liabilities												(933,000)
Current lease liabilities												(289,000)
Non-current lease liabilities												(531,000)
Net assets acquired												37,817,000
Cash outflows			$ 34,976,000
Working capital adjustment paid			341,000
Contingent consideration							2,500,000		$ 2,500,000			2,500,000
Total consideration			37,817,000
Unbilled revenue adjustment								100,000
Accounts payable adjustment								300,000
Other liabilities adjustment								100,000
Unearned revenue adjustment								(1,000,000)
MeDiNova
Business Acquisition [Line Items]
Cash & cash equivalents				$ 7,719,000
Property, plant and equipment				670,000
Operating right of use assets				1,558,000
Goodwill				81,760,000
Accounts receivable				3,488,000
Unbilled revenue				4,272,000
Prepayments and other current assets				406,000
Other receivables				819,000
Accounts payable				(5,484,000)
Unearned revenue				(5,796,000)
Other liabilities				(6,860,000)
Current lease liabilities				(430,000)
Non-current lease liabilities				(1,128,000)
Non-current deferred tax liability				(1,345,000)
Net assets acquired				86,249,000
Cash outflows				54,123,000
Working capital adjustment paid				466,000
Redeemable noncontrolling interest				32,592,000
Total consideration				86,249,000
Business combination acquisition percentage (in percent)											100.00%
Payments for previous acquisition	$ 43,900,000
Other liabilities adjustment								$ (1,600,000)
MMD
Business Acquisition [Line Items]
Cash & cash equivalents					$ 686,000
Property, plant and equipment					1,697,000
Operating right of use assets					2,866,000
Goodwill					22,430,000
Accounts receivable					3,100,000
Unbilled revenue					2,421,000
Prepayments and other current assets					908,000
Other receivables					43,000
Deferred tax asset					1,568,000
Accounts payable					(1,280,000)
Unearned revenue					(540,000)
Other liabilities					(1,232,000)
Current lease liabilities					(699,000)
Non-current lease liabilities					(2,167,000)
Non-current other liabilities					(1,123,000)
Net assets acquired					42,173,000
Cash outflows					42,349,000
Working capital adjustment paid					176,000
Total consideration					42,173,000
Unbilled revenue adjustment							(2,100,000)
Deferred tax asset adjustment							3,700,000
Accounts payable adjustment							(600,000)
Other liabilities adjustment							(100,000)
Property, plant and equipment adjustment							100,000
Unearned revenue adjustment							$ 900,000
Customer relationships | Medpass Group Limited
Business Acquisition [Line Items]
Intangible assets		11,725,000
Customer relationships | Symphony
Business Acquisition [Line Items]
Intangible assets			8,200,000									8,159,000
Customer relationships | MeDiNova
Business Acquisition [Line Items]
Intangible assets				3,887,000
Customer relationships | MMD
Business Acquisition [Line Items]
Intangible assets					10,708,000
Customer relationships | Mapi Group
Business Acquisition [Line Items]
Intangible assets													$ 18,400,000
Order backlog | Medpass Group Limited
Business Acquisition [Line Items]
Intangible assets		$ 2,883,000
Order backlog | Symphony
Business Acquisition [Line Items]
Intangible assets			$ 2,200,000									$ 2,163,000
Order backlog | MeDiNova
Business Acquisition [Line Items]
Intangible assets				171,000
Order backlog | MMD
Business Acquisition [Line Items]
Intangible assets					$ 2,787,000
Order backlog | Mapi Group
Business Acquisition [Line Items]
Intangible assets													$ 13,000,000
Patient database | MeDiNova
Business Acquisition [Line Items]
Intangible assets				$ 2,542,000

Goodwill - Summary of Proforma

Goodwill - Summary of Proforma Effect in Net Revenue, Net Income and Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Medpass Group Limited
Business Acquisition [Line Items]
Revenue	$ 2,798,180	$ 2,820,796
Net income	$ 332,521	$ 377,485
Basic earnings per share (USD per share)	$ 6.21	$ 6.91
Diluted earnings per share (USD per share)	$ 6.16	$ 6.85
Symphony
Business Acquisition [Line Items]
Revenue		$ 2,818,280	$ 2,609,233
Net income		$ 376,636	$ 324,671
Basic earnings per share (USD per share)		$ 6.90	$ 6
Diluted earnings per share (USD per share)		$ 6.84	$ 5.93
MeDiNova
Business Acquisition [Line Items]
Revenue		$ 2,807,788	$ 2,599,091
Net income		$ 375,979	$ 323,759
Basic earnings per share (USD per share)		$ 6.89	$ 5.98
Diluted earnings per share (USD per share)		$ 6.83	$ 5.91
MMD
Business Acquisition [Line Items]
Revenue		$ 2,806,908	$ 2,612,346
Net income		$ 373,930	$ 321,698
Basic earnings per share (USD per share)		$ 6.85	$ 5.94
Diluted earnings per share (USD per share)		$ 6.79	$ 5.87

Intangible Assets - Summary of

Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	$ 201,825	$ 181,073
Foreign exchange movement	209	(5,728)
Accumulated amortization	(135,430)	(116,196)
Foreign exchange movement	65	3,017
Net book value	66,460	67,894
Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	144,101	132,192
Technology asset
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	11,169	11,169
Order backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	39,224	36,318
Trade names/ brands
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	2,766	2,766
Volunteer list
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	1,325	1,325
Non-compete arrangements
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	489	489
Patient database
Acquired Finite-Lived Intangible Assets [Line Items]
Total cost	$ 2,542	$ 2,542

Intangible Assets - Narrative (

Intangible Assets - Narrative (Details) $ in Thousands

Jan. 22, 2020USD ($)

Sep. 24, 2019USD ($)

May 23, 2019USD ($)

Jan. 25, 2019USD ($)

Jul. 27, 2017USD ($)

Sep. 15, 2016USD ($)

Dec. 04, 2015USD ($)Site

Feb. 27, 2015USD ($)

May 07, 2014USD ($)

Feb. 28, 2012USD ($)

Feb. 15, 2012USD ($)

Jul. 14, 2011USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Dec. 31, 2020USD ($)

Mar. 09, 2020

Sep. 25, 2019USD ($)

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 19,234

$ 15,947

$ 15,351

Medpass Group Limited

Indefinite-lived Intangible Assets [Line Items]

Percentage of share capital acquired (in percent)

100.00%

Amortization of intangibles

$ 1,800

Medpass Group Limited | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 11,725

Amortization period

13 years

Medpass Group Limited | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 2,883

Amortization period

3 years

Symphony

Indefinite-lived Intangible Assets [Line Items]

Percentage of share capital acquired (in percent)

100.00%

Amortization of intangibles

$ 2,200

Symphony | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 8,200

$ 8,159

Amortization period

12 years

Symphony | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 2,200

$ 2,163

Amortization period

2 years

MeDiNova

Indefinite-lived Intangible Assets [Line Items]

Percentage of share capital acquired (in percent)

60.00%

100.00%

Amortization of intangibles

$ 1,300

MeDiNova | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 3,887

Amortization period

12 years

MeDiNova | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 171

Amortization period

1 year

MeDiNova | Patient database

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 2,542

Amortization period

7 years

MMD

Indefinite-lived Intangible Assets [Line Items]

Percentage of share capital acquired (in percent)

100.00%

Amortization of intangibles

$ 4,000

MMD | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 10,708

Amortization period

16 years

MMD | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 2,787

Amortization period

2 years

Mapi Group

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 13,300

Mapi Group | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 18,400

Amortization period

8 years

Mapi Group | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 13,000

Amortization period

8 years 6 months

ICON Government & Public Health Solutions

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 5,600

ICON Government & Public Health Solutions | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 4,000

Amortization period

7 years

ICON Government & Public Health Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 1,700

Amortization period

2 years

ICON Government & Public Health Solutions | Brand

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 1,400

Amortization period

5 years

PMG

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 8,000

Number of clinical research sites acquired | Site

52

PMG | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 6,900

Amortization period

7 years

PMG | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 3,000

Amortization period

2 years

MediMedia Pharma Solutions

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 21,500

MediMedia Pharma Solutions | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 22,800

Amortization period

7 years

MediMedia Pharma Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 2,500

Amortization period

1 year

Aptiv Solutions

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 28,300

Aptiv Solutions | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 21,400

Amortization period

7 years

Aptiv Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 7,900

Amortization period

3 years

PriceSpective

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 9,800

PriceSpective | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 10,200

Amortization period

10 years

PriceSpective | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 400

Amortization period

9 months 18 days

PriceSpective | Non-compete arrangements

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 400

Amortization period

3 years

BeijingWits Medical

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 2,000

BeijingWits Medical | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 1,800

Amortization period

10 years

BeijingWits Medical | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 400

Amortization period

4 years

BeijingWits Medical | Non-compete arrangements

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 100

Amortization period

5 years

Firecrest Clinical Limited

Indefinite-lived Intangible Assets [Line Items]

Amortization of intangibles

$ 16,500

Firecrest Clinical Limited | Customer relationships

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 5,200

Amortization period

7 years 6 months

Firecrest Clinical Limited | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 1,200

Amortization period

1 year 2 months 12 days

Firecrest Clinical Limited | Technology asset

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 11,200

Amortization period

7 years 6 months

Firecrest Clinical Limited | Trade names/ brands

Indefinite-lived Intangible Assets [Line Items]

Value of intangible assets acquired

$ 1,400

Amortization period

4 years 6 months

Intangible Assets - Future Inta

Intangible Assets - Future Intangible Asset Amortization Expense (Details) $ in Thousands	Dec. 31, 2020USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2021	$ 16,132
2022	10,426
2023	7,594
2024	7,149
2025	6,126
Finite lived intangible assets, amortization expense, net, total	$ 47,427

Property, Plant and Equipment_3

Property, Plant and Equipment, net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Land	$ 3,724	$ 3,724
Building	84,195	85,050
Computer equipment and software	438,742	434,707
Office furniture and fixtures	90,477	88,401
Laboratory equipment	44,101	40,611
Leasehold improvements	30,588	31,144
Motor vehicles	158	199
Foreign exchange movement on property, plant and equipment	11,728	(1,371)
Property, plant and equipment, gross	703,713	682,465
Less accumulated depreciation and asset write offs	(522,473)	(518,383)
Foreign exchange movement in acccumulated depreciation and asset write offs	(6,897)	1,005
Property, plant and equipment (net)	$ 174,343	$ 165,087

Other Liabilities (Details)

Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other Liabilities Disclosure [Abstract]
General trade and overhead liabilities	$ 188,638	$ 174,131
Personnel related liabilities	161,363	157,146
Operating lease liability (note 22)	24,334	28,320
Facility related liabilities	9,441	11,138
Other liabilities	8,726	6,531
Restructuring charges (note 14)	7,219	1,232
Short term government grants (note 11)	48	45
Other liabilities	399,769	378,543
Amounts due to third parties for reimbursable expenses	$ 138,200	$ 142,600

Non-Current Other Liabilities_2

Non-Current Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Other non-current liabilities	$ 15,971	$ 11,459
Defined benefit pension obligations, net (note 9)	10,395	6,053
Non-current other liabilities	$ 26,366	$ 17,512

Employee Benefits - Narrative (

Employee Benefits - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 07, 2014
Defined Benefit Plan Disclosure [Line Items]
Contributions percentage of annual compensation (in percent)	8.00%
Defined contribution plan, cost	$ 28,036,000	$ 26,483,000	$ 25,241,000
Expected long-term return per annum	3.40%
Assumption for cash flow estimate, percentage of lump sum (in percent)	50.00%
Corporate Bonds (including 50% high yield bonds)
Defined Benefit Plan Disclosure [Line Items]
Percentage of investments (in percent)	20.00%
Expected long-term return per annum	2.80%
Equities
Defined Benefit Plan Disclosure [Line Items]
Percentage of investments (in percent)	21.00%
Expected long-term return per annum	5.20%
Secured Loans and Multi Asset Credit
Defined Benefit Plan Disclosure [Line Items]
Percentage of investments (in percent)	19.00%
Expected long-term return per annum	3.00%
United States
Defined Benefit Plan Disclosure [Line Items]
Contributions percentage of annual compensation (in percent)	3.00%
Defined contribution plan, cost	$ 17,048,000	$ 16,469,000	$ 15,532,000
Participant's contributions percentage matches (in percent)	50.00%
Participant's contributions percentage of annual compensation (in percent)	4.50%
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Net loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ 600,000
Net prior service cost for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ 0
Pension plan period	26 years
Expected long-term return per annum	4.00%	2.10%	2.00%
Contribution to pension fund in the year ending December 31, 2021	$ 100,000
Retirement period used as a basis to estimate expected cash flows	10 years
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Net loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ (7,000)
Net prior service cost for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ (12,000)
Expected long-term return per annum	0.34%	0.80%	0.80%
Contribution to pension fund in the year ending December 31, 2021	$ 100,000
Retirement period used as a basis to estimate expected cash flows	10 years
Switzerland | Aptiv Solutions
Defined Benefit Plan Disclosure [Line Items]
Percentage of share capital acquired (in percent)				100.00%

Employee Benefits - Funded Stat

Employee Benefits - Funded Status (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ (43,988)	$ (37,036)
Fair value of plan assets	34,612	32,016	$ 27,297
Funded status	(9,376)	(5,020)
Non-current other liabilities (note 8)	(9,376)	(5,020)
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	(8,620)	(7,047)
Fair value of plan assets	7,601	6,014	$ 4,707
Funded status	(1,019)	(1,033)
Non-current other liabilities (note 8)	$ (1,019)	$ (1,033)

Employee Benefits - Change in B

Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United Kingdom
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	$ 37,036	$ 30,045
Service cost	100	107	$ 124
Interest cost	746	867	895
Plan participants' contributions	22	24
Benefits paid	724	177
Actuarial loss	5,294	4,756
Foreign currency exchange rate changes	1,514	1,414
Benefit obligation at end of year	43,988	37,036	30,045
Switzerland
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	7,047	5,279
Service cost	139	115	138
Interest cost	21	40	47
Plan participants' contributions	81	67
Prior service cost	(23)	0
Benefits paid	245	72
Actuarial loss	406	1,479
Foreign currency exchange rate changes	704	139
Benefit obligation at end of year	$ 8,620	$ 7,047	$ 5,279

Employee Benefits - Change in P

Employee Benefits - Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United Kingdom
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 32,016	$ 27,297
Expected return on plan assets	1,214	574	$ 624
Actual return on plan assets	2,092	3,504
Employer contributions	109	148
Plan participants' contributions	22	24
Benefits paid	(724)	(177)
Foreign currency exchange rate changes	1,097	1,220
Fair value of plan assets at end of year	34,612	32,016	27,297
Switzerland
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	6,014	4,707
Expected return on plan assets	21	35	41
Actual return on plan assets	519	1,114
Scheme contributions	105	89
Plan participants' contributions	81	67
Transferred balances/ (benefits paid)	245	(72)
Foreign currency exchange rate changes	616	74
Fair value of plan assets at end of year	$ 7,601	$ 6,014	$ 4,707

Employee Benefits - Components

Employee Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 100	$ 107	$ 124
Interest cost	746	867	895
Expected return on plan assets	(1,214)	(574)	(624)
Amortization of net loss	160	67	248
Net periodic benefit cost	(208)	467	643
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Service cost	139	115	138
Interest cost	21	40	47
Expected return on plan assets	(21)	(35)	(41)
Amortization of net loss	(6)	(93)	(69)
Amortization of prior service credit	(12)	(9)	(8)
Settlement	0	0	(93)
Net periodic benefit cost	$ 121	$ 18	$ (26)

Employee Benefits - Net Periodi

Employee Benefits - Net Periodic Pension Benefit Cost Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Expected rate of return on plan assets	3.40%
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.10%	2.90%	2.50%
Rate of compensation increase	3.30%	3.70%	3.70%
Expected rate of return on plan assets	4.00%	2.10%	2.00%
Other comprehensive income
Actuarial loss/(gain) - benefit obligation	$ 5,294	$ 4,756	$ (3,844)
Actuarial (gain)/loss – plan assets	(878)	(2,930)	1,208
Actuarial loss recognized in net periodic benefit cost	(160)	(67)	(248)
Total	$ 4,256	$ 1,759	$ (2,884)
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Discount rate	0.30%	0.80%	0.80%
Rate of compensation increase	2.00%	2.00%	2.00%
Expected rate of return on plan assets	0.34%	0.80%	0.80%
Other comprehensive income
Actuarial loss/(gain) - benefit obligation	$ 406	$ 1,479	$ (372)
Actuarial (gain)/loss – plan assets	(519)	(1,114)	240
Prior service credit recognized in net periodic benefit cost	0	0	93
Actuarial loss recognized in net periodic benefit cost	6	93	69
Amortization of net prior service credit	12	9	8
Net prior service cost occurring during the year	(23)	0	(9)
Total	$ (118)	$ 467	$ 29

Employee Benefits - Assumptions

Employee Benefits - Assumptions Used in Determining Benefit Obligation (Details)	Dec. 31, 2020	Dec. 31, 2019
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Discount rate	1.50%	2.10%
Rate of compensation increase	3.40%	3.30%
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Discount rate	0.15%	0.30%
Rate of compensation increase	2.00%	2.00%

Employee Benefits - Expected Ra

Employee Benefits - Expected Rate of Return and Actual Plan Asset Allocation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
High yield percentage of corporate bond investment	50.00%
Expected long-term return per annum	3.40%
Actual plan asset allocations	100.00%	100.00%
Corporate Bonds (including 50% high yield bonds)
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	2.80%
Actual plan asset allocations	40.00%	40.00%
Equities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	5.20%
Actual plan asset allocations	21.00%	21.00%
Secured Loans and Multi Asset Credit
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	3.00%
Actual plan asset allocations	39.00%	39.00%
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	4.00%	2.10%	2.00%

Employee Benefits - Plan Asset

Employee Benefits - Plan Asset Fair Value Measurements (Details) - Fair Value, Inputs, Level 1 - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 34,612	$ 32,016
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	17
L&G Life GPBF All World Equity Index Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,460	6,753
L&G Life DC Active Corporate Bond
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,797	6,412
Stone Harbor High Yield Bond Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,861	6,292
Ninety One Global Total Return Credit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,472	0
Investec Global Total Return Credit Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3,166
Stone Harbor Multi Asset Credit Portfolio
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,389	3,151
Barings European Loan Fund Buy & Hold
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,622	$ 6,225

Employee Benefits - Annual Bene

Employee Benefits - Annual Benefit Payments (Details) $ in Thousands	Dec. 31, 2020USD ($)
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
2021	$ 342
2022	258
2023	343
2024	421
2025	458
Years 2026 - 2030	3,700
Switzerland
Defined Benefit Plan Disclosure [Line Items]
2021	348
2022	342
2023	334
2024	326
2025	377
Years 2026 - 2030	$ 1,587

Equity Incentive Schemes and _3

Equity Incentive Schemes and Stock Compensation Charges - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 14, 2017	May 11, 2015	Jan. 17, 2003	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2021	May 16, 2019	Feb. 13, 2017
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Weighted average contractual term of options outstanding				4 years 10 months 9 days	4 years 9 months 3 days
Exercisable - weighted average remaining contractual life				3 years 7 months 6 days	3 years 7 months 20 days
Options expected to vest (in shares)				186,479			186,479
Intrinsic value of option exercised				$ 21,300
Intrinsic value of option outstanding				47,900			$ 47,900
Intrinsic value of option exercisable				25,100			25,100
Total compensation costs				26,271	$ 26,819	$ 31,594
Non-cash stock compensation expense not yet recognized				$ 45,900			$ 45,900
Unrecognized stock-based compensation expense, weighted average period				2 years 1 month 13 days
Income tax benefit related to stock compensation				$ 6,900	8,200	12,700
Cash tax benefit related to stock options exercised				$ 2,500	1,900	$ 3,600
Forecast
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Options expected to vest (in shares)									157,797
Consultants Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)	1,000,000										400,000
Limit of shares issued or to be issued pursuant to options granted (in shares)	1,000,000
Employee Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)	6,000,000
Maximum number of shares per employee (in shares)	400,000
Option Plans 2008 | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Percentage of option price for fair value of ordinary share (in percent)	100.00%
Employee Stock Plan, 2003 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)			6,000,000
Maximum number of shares per employee (in shares)			400,000
Maximum number of award as percentage of shares outstanding (in percent)			10.00%
Employee Stock Option
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period				5 years
Employee Stock Option | Vested and Unvested Options
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Options vesting percentage				20.00%
Employee Stock Option | Award date, prior to 2018
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period								5 years
Shares expiration period								8 years
Employee Stock Option | Award date, 2018 and thereafter
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period						12 months
Shares expiration period						8 years
Restricted Stock Units 2013
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)		4,100,000
Additional number of ordinary shares which have been reserved for issuance (in shares)		2,500,000
Consultants Restricted Stock Units 2019
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)										250,000
Consultants Restricted Stock Units 2019 | Non-executive director
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period							12 months
Restricted Stock Units (RSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested				$ 14,300	$ 16,500
Restricted Stock Units (RSUs) | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)				$ 83.47	$ 65.60		$ 83.47
Restricted Stock Units (RSUs) | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)				$ 156.21	$ 147.55		156.21
Performance Share Unit (PSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested				$ 5,300	$ 8,500
Performance Share Unit (PSUs) | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)				$ 83.47	$ 65.60		83.47
Performance Share Unit (PSUs) | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Share price of equity instruments other than options, vested in period (in USD per share)				$ 90.03	$ 71.95		$ 90.03
PSUs Based on Service and EPS Targets | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units to be granted (in shares)				71,939			71,939

Equity Incentive Schemes and _4

Equity Incentive Schemes and Stock Compensation Charges - Summary of Stock Option Activity (Details) - Stock Option And Award Plans - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options Granted Under Plans
Beginning balance (in shares)	656,107	920,746	1,171,393
Granted (in shares)	107,737	97,112	167,557
Exercised (in shares)	(193,417)	(329,870)	(408,699)
Canceled/expired (in shares)	(16,681)	(31,881)	(9,505)
Ending balance (in shares)	553,746	656,107	920,746
Vested and exercisable at end of period (in shares)	224,845
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 87.80	$ 74.32	$ 56.02
Granted (USD per share)	159.83	140.13	118.90
Exercised (USD per share)	68.19	65.54	41.12
Canceled/expired (USD per share)	92.21	88.12	32.35
Ending balance (USD per share)	108.53	87.80	74.32
Vested and exercisable at end of period (USD per share)	83.13
Weighted Average Grant Date Fair Value
Beginning balance (USD per share)	26.60	22.39	17.15
Granted (USD per share)	42.43	43.43	36.84
Exercised (USD per share)	20.91	19.78	13.55
Canceled/expired (USD per share)	27.93	26.77	11.39
Ending balance (USD per share)	31.63	$ 26.60	$ 22.39
Vested and exercisable at end of period (USD per share)	$ 24.90

Equity Incentive Schemes and _5

Equity Incentive Schemes and Stock Compensation Charges - Summary of Movement in Non-Vested Share Options (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options Outstanding Number of Shares
Beginning Balance (in shares)	358,030
Granted (in shares)	107,737
Vested (in shares)	(124,605)
Forfeited (in shares)	(12,261)
Ending Balance (in shares)	328,901	358,030
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 103.68
Granted (USD per share)	159.83
Vested (USD per share)	92.25
Forfeited (USD per share)	117.41
Ending balance (USD per share)	125.89	$ 103.68
Weighted Average Fair Value
Beginning Balance (USD per share)	31.28
Granted (USD per share)	42.43	43.43	$ 36.84
Vested (USD per share)	27.51
Forfeited (USD per share)	34.50
Ending Balance (USD per share)	$ 36.24	$ 31.28

Equity Incentive Schemes and _6

Equity Incentive Schemes and Stock Compensation Charges - Outstanding and Exercisable Share Options (Details)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 166.51
Minimum Range Exercise Price (USD per share) | $ / shares	$ 32.37
Number of Shares (in shares)	553,746
Weighted Average Remaining Contractual Life	4 years 10 months 9 days
Weighted Average Exercise Price (USD per share) | $ / shares	$ 108.53
Number of Shares (in shares)	224,845
Weighted Average Exercise Price (USD per share) | $ / shares	$ 83.13
Range 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 32.37
Number of Shares (in shares)	2,500
Weighted Average Remaining Contractual Life	3 months 29 days
Number of Shares (in shares)	2,500
Range 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 37.90
Number of Shares (in shares)	920
Weighted Average Remaining Contractual Life	11 months 1 day
Number of Shares (in shares)	920
Range 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 40.83
Number of Shares (in shares)	21,360
Weighted Average Remaining Contractual Life	1 year 4 months 20 days
Number of Shares (in shares)	21,360
Range 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 47.03
Number of Shares (in shares)	479
Weighted Average Remaining Contractual Life	1 year 1 month 28 days
Number of Shares (in shares)	479
Range 5
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 48.67
Number of Shares (in shares)	1,027
Weighted Average Remaining Contractual Life	1 year 2 months 12 days
Number of Shares (in shares)	1,027
Range 6
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 65.60
Number of Shares (in shares)	29,381
Weighted Average Remaining Contractual Life	3 years 4 months 17 days
Number of Shares (in shares)	19,528
Range 7
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 66.47
Number of Shares (in shares)	2,621
Weighted Average Remaining Contractual Life	2 years 4 months 17 days
Number of Shares (in shares)	2,621
Range 8
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 68.39
Number of Shares (in shares)	24,508
Weighted Average Remaining Contractual Life	2 years 2 months 4 days
Number of Shares (in shares)	24,508
Range 9
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 71.95
Number of Shares (in shares)	73,193
Weighted Average Remaining Contractual Life	3 years 2 months 1 day
Number of Shares (in shares)	59,284
Range 10
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 83.47
Number of Shares (in shares)	58,159
Weighted Average Remaining Contractual Life	4 years 2 months 1 day
Number of Shares (in shares)	10,671
Range 11
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 90.03
Number of Shares (in shares)	45,154
Weighted Average Remaining Contractual Life	4 years 4 months 17 days
Number of Shares (in shares)	27,698
Range 12
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 115.11
Number of Shares (in shares)	68,016
Weighted Average Remaining Contractual Life	5 years 2 months 1 day
Number of Shares (in shares)	8,866
Range 13
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 125.74
Number of Shares (in shares)	39,664
Weighted Average Remaining Contractual Life	5 years 4 months 17 days
Number of Shares (in shares)	36,885
Range 14
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 137.47
Number of Shares (in shares)	6,045
Weighted Average Remaining Contractual Life	6 years 4 months 13 days
Number of Shares (in shares)	895
Range 15
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 140.38
Number of Shares (in shares)	75,061
Weighted Average Remaining Contractual Life	6 years 2 months 1 day
Number of Shares (in shares)	7,603
Range 16
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 159.33
Number of Shares (in shares)	98,128
Weighted Average Remaining Contractual Life	7 years 2 months 1 day
Number of Shares (in shares)	0
Range 17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (USD per share) | $ / shares	$ 166.51
Number of Shares (in shares)	7,530
Weighted Average Remaining Contractual Life	7 years 5 months 19 days
Number of Shares (in shares)	0

Equity Incentive Schemes and _7

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Weighted Average Fair Values and Assumptions Used (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value (USD per share)	$ 42.43	$ 43.43	$ 36.84
Assumptions:
Expected volatility (in percent)	30.00%	30.00%	30.00%
Dividend yield (in percent)	0.00%	0.00%	0.00%
Risk free interest rate (in percent)	0.57%	2.55%	2.76%
Expected life	5 years	5 years	5 years

Equity Incentive Schemes and _8

Equity Incentive Schemes and Stock Compensation Charges - Summary of RSU and PSU Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Performance Share Unit (PSUs)
Outstanding number of shares [Roll Forward]
Outstanding at beginning of period (in shares)	175,989
Granted (in shares)	57,184
Shares vested (in shares)	(63,516)
Forfeited (in shares)	(10,016)
Outstanding at ending of period (in shares)	159,641	175,989
Weighted Average Fair Value
Outstanding at beginning of period (USD per share)	$ 110.79
Granted (USD per share)	165.30
Shares vested (USD per share)	83.92
Forfeited (USD per share)	134.27
Outstanding at end of period (USD per share)	$ 137.64	$ 110.79
Weighted Average Remaining Contractual Life
Outstanding at end of period	1 year 1 month 20 days	1 year 14 days
Restricted Stock Units (RSUs)
Outstanding number of shares [Roll Forward]
Outstanding at beginning of period (in shares)	389,900
Granted (in shares)	137,572
Shares vested (in shares)	(144,747)
Forfeited (in shares)	(41,301)
Outstanding at ending of period (in shares)	341,424	389,900
Weighted Average Fair Value
Outstanding at beginning of period (USD per share)	$ 119.07
Granted (USD per share)	168.03
Shares vested (USD per share)	98.47
Forfeited (USD per share)	133.63
Outstanding at end of period (USD per share)	$ 145.77	$ 119.07
Weighted Average Remaining Contractual Life
Outstanding at end of period	1 year 4 months 28 days	1 year 5 months 4 days

Equity Incentive Schemes and _9

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Non-cash Stock Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 26,271	$ 26,819	$ 31,594
Direct costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	8,557	14,777	17,408
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 17,714	$ 12,042	$ 14,186

Government Grants - Summary (De

Government Grants - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Received	$ 3,539	$ 3,539
Less accumulated amortization	(2,881)	(2,836)
Foreign exchange translation adjustment	228	155
Less current portion	(660,883)	(366,988)
Government Grants
Disaggregation of Revenue [Line Items]
Total government grants	886	858
Less current portion	(48)	(45)
Non-current government grants	$ 838	$ 813

Government Grants - Narrative (

Government Grants - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Revenue Disclosure [Abstract]
Amortization of government grants	$ 45	$ 44	$ 47
Restricted retained earnings	$ 6,000	$ 3,100

Share Capital (Details)

Share Capital (Details) - USD ($)	Dec. 31, 2019	Jul. 22, 2016	Jul. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2015	Jan. 08, 2019	Oct. 03, 2016
Equity, Class of Treasury Stock [Line Items]
Total proceeds from exercise of stock options by employees				$ 13,203,000	$ 21,645,000	$ 16,842,000
Ordinary shares redeemed, value				$ 175,000,000	146,931,000	$ 128,960,000
Buyback Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount			$ 400,000,000						$ 400,000,000
Ordinary shares redeemed (in shares)	35,100		5,316,062	1,235,218		4,026,576	6,198,481
Ordinary shares redeemed, value	$ 5,300,000		$ 400,000,000	$ 175,000,000	$ 141,600,000	$ 372,100,000	$ 457,900,000
Number of shares authorized to be repurchased (in shares)								1,000,000
Employee Stock Option
Equity, Class of Treasury Stock [Line Items]
Options exercised by employees (in shares)				193,417	329,870	408,699
Average exercise price of option per share (USD per share)				$ 68.19	$ 65.54	$ 41.12
Total proceeds from exercise of stock options by employees				$ 13,200,000	$ 21,600,000	$ 16,800,000
Restricted Stock Units (RSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain RSUs previously awarded by the Company (in shares)				144,172	237,119	273,742
Performance Share Unit (PSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain PSUs previously awarded by the Company (in shares)				63,516	118,611	215,826
Maximum | Buyback Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized percentage (in percent)		10.00%

Income Taxes - Components of In

Income Taxes - Components of Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	$ 381,205	$ 426,989	$ 364,614
Ireland
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	280,310	323,726	243,988
United States
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	41,950	21,073	27,499
Other
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before income tax expense	$ 58,945	$ 82,190	$ 93,127

Income Taxes - Components of Pr

Income Taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current tax expense:
Current tax expense	$ 46,948	$ 52,670	$ 40,306
Deferred tax expense/ (benefit):
Deferred tax (benefit)/expense	927	(1,537)	1,652
Income tax expense allocated to continuing operations	47,875	51,133	41,958
Income tax expense was allocated to the following components of other comprehensive income:
Currency impact on long term funding	68	25	119
Fair value of cash flow hedge	0	0	(148)
Total	47,943	51,158	41,929
Ireland
Current tax expense:
Current tax expense	28,963	35,955	28,042
Deferred tax expense/ (benefit):
Deferred tax (benefit)/expense	1,654	2,833	1,054
Income tax expense allocated to continuing operations	30,616	38,788	29,096
United States
Current tax expense:
Current tax expense	3,022	5,073	2,885
Deferred tax expense/ (benefit):
Deferred tax (benefit)/expense	4,577	(3,502)	875
Income tax expense allocated to continuing operations	7,600	1,571	3,761
Other
Current tax expense:
Current tax expense	14,963	11,642	9,379
Deferred tax expense/ (benefit):
Deferred tax (benefit)/expense	$ (5,304)	$ (868)	$ (277)

Income Taxes - Consolidated Rep

Income Taxes - Consolidated Reported Provision for Income Taxes Differed from Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Taxes at Irish statutory rate of 12.5% (2019:12.5%; 2018:12.5%)	$ 47,651	$ 53,374	$ 45,577
Foreign and other income taxed at higher rates	7,943	7,356	7,649
Research & development tax incentives	(1,243)	(893)	(1,243)
Movement in valuation allowance	3,581	(10)	5,667
Effects of change in tax rates	108	359	(147)
Decrease in unrecognized tax benefits	(1,672)	(1,273)	(5,423)
Impact of stock compensation	(5,150)	(7,383)	(8,301)
Other	(3,343)	(397)	(1,821)
Income tax expense allocated to continuing operations	$ 47,875	$ 51,133	$ 41,958

Income Taxes - Tax Effects of T

Income Taxes - Tax Effects of Temporary Differences That Give Rise to Significant Portions of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities:
Property, plant and equipment	$ 1,359	$ 1,102
Right-of-use-assets	9,402	11,838
Goodwill	31,629	27,590
Intangible assets	13,398	11,805
Other	1,009	6,284
Total deferred tax liabilities recognized	56,797	58,619
Deferred tax assets:
Operating loss and tax credits carryforwards	42,794	37,865
Property, plant and equipment	6,040	5,257
Lease liabilities	9,394	11,754
Accrued expenses and payments on account	24,368	26,380
Stock compensation	3,672	5,009
Deferred compensation	3,184	2,744
Unearned revenue	2,257	3,933
Other	155	604
Total deferred tax assets	91,864	93,546
Valuation allowance for deferred tax assets	(32,768)	(27,721)	$ (27,300)
Deferred tax assets recognized	59,096	65,825
Overall net deferred tax asset	$ 2,299	$ 7,206

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Valuation allowance for deferred tax assets	$ 32,768,000	$ 27,721,000	$ 27,300,000
Net change in the total valuation allowance	5,100,000	400,000
Deferred tax liabilities, undistributed foreign earnings	900,000	5,400,000
Unrecognized tax benefit, potentially expire in 2021	3,200,000
Total unrecognized tax benefits net of potential benefits	19,100,000	20,200,000	21,400,000
Interest and penalties recognized as an expense	600,000	0	$ 1,300,000
Total accrued interest and penalties	500,000	1,100,000
Provision for income taxes
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	3,600,000	0
Other Comprehensive Income
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	1,500,000	$ 400,000
Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Other non-U.S subsidiaries operating loss carryforwards for income tax	42,100,000
Foreign Country | Tax Year 2021 to 2027
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	15,100,000
Foreign Country | Tax Year 2028 to 2037
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	18,300,000
Ireland | Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Ireland subsidiaries additional tax credit carryforward for income tax	11,700,000
United States
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. Federal net operating loss carry forwards currently available for offset	6,400,000
Alternative minimum tax credit carry forwards	800,000
United States | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net operating loss carryforwards	16,579,000
U.S. federal net operating loss carry forwards	15,200,000
Tax credit carry forward	29,100,000
Other non-U.S subsidiaries operating loss carryforwards for income tax	1,363,000
United States | Tax Year 2021 to 2022 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	10,000,000
United States | Tax Year 2023 to 2027 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	4,800,000
United States | Tax Year 2028 to 2036 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	400,000
United States | State NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net operating loss carryforwards	51,141,000
Other non-U.S subsidiaries operating loss carryforwards for income tax	49,000
Other | Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Ireland subsidiaries additional tax credit carryforward for income tax	$ 5,500,000

Income Taxes - Expected Expiry

Income Taxes - Expected Expiry Dates of NOL's (Details) - United States $ in Thousands	Dec. 31, 2020USD ($)
State NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, not subject to expiration	$ 49
Net operating loss carryforwards	51,141
State NOL's | 2022-2035
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	51,092
State NOL's | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	0
Federal NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, not subject to expiration	1,363
Net operating loss carryforwards	16,579
Federal NOL's | 2022-2035
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	13,295
Federal NOL's | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards, subject to expiration	$ 1,921

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at start of year	$ 20,156	$ 21,433	$ 23,720
Increase related to prior year tax positions	401	0	2,084
Decrease related to prior year tax positions	(1,271)	0	(2,915)
Increase related to current year tax positions	2,931	1,588	3,065
Settlements	(369)	(347)	(182)
Lapse of statute of limitations	(2,770)	(2,518)	(4,339)
Unrecognized tax benefits at end of year	$ 19,078	$ 20,156	$ 21,433

Restructuring charges - Summary

Restructuring charges - Summary of Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 18,089	$ 0	$ 12,490
Resource rationalization, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring charges	18,089
Utilization	(8,296)
Foreign exchange movement	325
Restructuring Reserve	10,118
Workforce Reductions | Resource rationalization, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring charges	11,391
Utilization	(6,987)
Foreign exchange movement	0
Restructuring Reserve	4,404
Onerous lease | Resource rationalization, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6,698
Utilization	(1,309)
Foreign exchange movement	325
Restructuring Reserve	$ 5,714

Restructuring charges - Narrati

Restructuring charges - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 18,089	$ 0	$ 12,490
Impairment of right-of-use assets	5,411	$ 0	0
Restructuring reserve, current	7,200
Restructuring reserve, noncurrent	3,500
Resource Rationalizations 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges			12,490
Resource Rationalizations 2018 | Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring charges	11,400		9,684
Resource Rationalizations 2018 | Onerous lease
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6,700		$ 2,806
Resource rationalization, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring charges	18,089
Resource rationalization, 2020 | Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring charges	11,391
Resource rationalization, 2020 | Onerous lease
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6,698
Resource rationalization, 2020 | Other restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,300

Restructuring charges - Future

Restructuring charges - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2020USD ($)
Operating Lease - Future Minimum Lease Payments [Line Items]
2021	$ 26,164
2022	21,510
2023	16,108
2024	10,165
2025	6,046
Thereafter	10,490
Total future minimum lease payments	90,483
Lease imputed interest	(5,348)
Operating lease liabilities	85,135
Onerous lease | Resource rationalization, 2020
Operating Lease - Future Minimum Lease Payments [Line Items]
2021	2,602
2022	1,896
2023	999
2024	139
2025	139
Thereafter	402
Total future minimum lease payments	6,177
Lease imputed interest	(463)
Operating lease liabilities	$ 5,714

Restructuring charges - Prior P

Restructuring charges - Prior Period Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Restructuring charges	$ 18,089	$ 0	$ 12,490
Resource Rationalizations 2018
Restructuring Reserve [Roll Forward]
Restructuring charges			12,490
Utilized	(1,007)	(4,782)	(6,071)
Restructuring Reserve	630	1,637	6,419
Resource Rationalizations 2018 | Workforce Reductions
Restructuring Reserve [Roll Forward]
Restructuring charges	11,400		9,684
Utilized	(731)	(3,554)	(5,399)
Restructuring Reserve	0	731	4,285
Resource Rationalizations 2018 | Onerous lease
Restructuring Reserve [Roll Forward]
Restructuring charges	6,700		2,806
Utilized	(276)	(1,228)	(672)
Restructuring Reserve	$ 630	$ 906	$ 2,134

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Non-cancelable operating leases for facilities expiration period	12 years

Disaggregation of Revenue (Deta

Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 2,797,288	$ 2,805,839	$ 2,595,777
Top client
Disaggregation of Revenue [Line Items]
Revenue	337,904	350,287	352,335
Clients 2-5
Disaggregation of Revenue [Line Items]
Revenue	754,906	704,963	671,723
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue	350,865	347,832	385,741
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue	501,643	529,713	461,351
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 851,970	$ 873,044	$ 724,627

Accounts receivable, unbilled_3

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Summary of Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Billed services (accounts receivable)	$ 722,420	$ 535,088
Unbilled services (unbilled revenue)	428,684	422,769
Unbilled services (unbilled revenue)	1,151,104	957,857
Allowance for credit losses (note 18)	(7,149)	(7,380)	$ (8,889)
Accounts receivable and unbilled revenue, net	1,143,955	950,477
Unearned revenue (payments on account)	(660,883)	(366,988)
Net balance	(232,199)	$ 55,781
Change in unbilled receivables	5,915
Change in unearned revenue (payments on account)	(293,895)
Change in advance payments netted against unbilled contracts receivable	$ (287,980)
Change in unbilled receivables (in percent)	1.40%
Change in unearned revenue (payments on account) (in percent)	80.10%
Change in advance payments netted against unbilled contracts receivable (in percent)	516.30%

Accounts receivable, unbilled_4

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Amounts due to third parties for reimbursable expenses	$ 138,200	$ 142,600
Change in unbilled receivables	(5,915)
Change in unearned revenue (payments on account)	(293,895)
Change in advance payments netted against unbilled contracts receivable	287,980
Bad debt expense	$ 2,900	$ 400

Accounts receivable, unbilled_5

Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) - Performance Obligation (Details) $ in Billions	Dec. 31, 2020USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, amount	$ 6.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation (in percent)	42.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	12 months

Provision for Credit Losses (De

Provision for Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Opening provision	$ 7,380	$ 8,889
Amounts used during the year	(2,561)	0
Amounts provided during the year	2,692	1,691
Amounts released during the year	(510)	(3,226)
Foreign exchange	148	26
Closing provision	$ 7,149	$ 7,380

Business Segment and Geograph_3

Business Segment and Geographical Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2020segment
Segment Reporting [Abstract]
Number of operating segments	1

Business Segment and Geograph_4

Business Segment and Geographical Information - Distribution of Net Revenue by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue	$ 2,797,288	$ 2,805,839	$ 2,595,777
Ireland
Segment Reporting Information [Line Items]
Revenue	1,181,292	1,252,834	1,066,200
Rest of Europe
Segment Reporting Information [Line Items]
Revenue	416,884	388,916	379,883
U.S.
Segment Reporting Information [Line Items]
Revenue	925,563	892,497	894,978
Other
Segment Reporting Information [Line Items]
Revenue	$ 273,549	$ 271,592	$ 254,716

Business Segment and Geograph_5

Business Segment and Geographical Information - Distribution of Income from Operations, Including Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Income from operations	$ 391,500	$ 433,406	$ 373,357
Ireland
Segment Reporting Information [Line Items]
Income from operations	276,478	314,287	257,089
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations	35,765	37,997	36,280
U.S.
Segment Reporting Information [Line Items]
Income from operations	58,018	60,272	58,561
Other
Segment Reporting Information [Line Items]
Income from operations	$ 21,239	$ 20,850	$ 21,427

Business Segment and Geograph_6

Business Segment and Geographical Information - Distribution of Income from Operations, Excluding Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 409,589	$ 433,406	$ 385,847
Ireland
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	295,360	314,287	269,196
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	35,402	37,997	36,904
U.S.
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	57,680	60,272	58,340
Other
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 21,147	$ 20,850	$ 21,407

Business Segment and Geograph_7

Business Segment and Geographical Information - Distribution of Long-lived Assets, Net, by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Long-lived assets	$ 258,904	$ 270,064
Ireland
Segment Reporting Information [Line Items]
Long-lived assets	118,361	110,522
Rest of Europe
Segment Reporting Information [Line Items]
Long-lived assets	36,723	41,970
U.S.
Segment Reporting Information [Line Items]
Long-lived assets	65,152	72,578
Other
Segment Reporting Information [Line Items]
Long-lived assets	$ 38,668	$ 44,994

Business Segment and Geograph_8

Business Segment and Geographical Information - Distribution of Depreciation and Amortization by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Depreciation amortization and right-of-use assets amortization	$ 100,017	$ 91,922	$ 65,916
Ireland
Segment Reporting Information [Line Items]
Depreciation amortization and right-of-use assets amortization	33,516	30,635	34,721
Rest of Europe
Segment Reporting Information [Line Items]
Depreciation amortization and right-of-use assets amortization	18,569	14,370	5,331
U.S.
Segment Reporting Information [Line Items]
Depreciation amortization and right-of-use assets amortization	36,060	33,922	21,605
Other
Segment Reporting Information [Line Items]
Depreciation amortization and right-of-use assets amortization	$ 11,872	$ 12,995	$ 4,259

Business Segment and Geograph_9

Business Segment and Geographical Information - Distribution of Total Assets by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 3,435,606	$ 2,907,512
Ireland
Segment Reporting Information [Line Items]
Assets	1,675,980	1,323,181
Rest of Europe
Segment Reporting Information [Line Items]
Assets	671,218	660,797
U.S.
Segment Reporting Information [Line Items]
Assets	909,202	755,271
Other
Segment Reporting Information [Line Items]
Assets	$ 179,206	$ 168,263

Business Segment and Geograp_10

Business Segment and Geographical Information - Distribution of Capital Expenditure by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Capital expenditures	$ 63,155	$ 80,145	$ 48,397
Ireland
Segment Reporting Information [Line Items]
Capital expenditures	29,309	32,526	30,942
Rest of Europe
Segment Reporting Information [Line Items]
Capital expenditures	9,298	13,971	2,590
U.S.
Segment Reporting Information [Line Items]
Capital expenditures	14,347	18,638	9,311
Other
Segment Reporting Information [Line Items]
Capital expenditures	$ 10,201	$ 15,010	$ 5,554

Business Segment and Geograp_11

Business Segment and Geographical Information - Company's Net Revenue for Major Clients (Details) - Customer Concentration Risk - Net Revenue	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Client A
Revenue, Major Customer [Line Items]
Clients which represented 10% or more of the company's net revenue (in percent)	12.00%	12.00%	14.00%
Client B
Revenue, Major Customer [Line Items]
Clients which represented 10% or more of the company's net revenue (in percent)	10.00%	10.00%	10.00%

Business Segment and Geograp_12

Business Segment and Geographical Information - Distribution of Interest Income by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest income	$ 2,724	$ 6,859	$ 4,759
Ireland
Segment Reporting Information [Line Items]
Interest income	1,620	3,674	2,620
Rest of Europe
Segment Reporting Information [Line Items]
Interest income	570	2,242	1,750
U.S.
Segment Reporting Information [Line Items]
Interest income	22	111	17
Other
Segment Reporting Information [Line Items]
Interest income	$ 512	$ 832	$ 372

Business Segment and Geograp_13

Business Segment and Geographical Information - Distribution of Income Tax Charge by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Income tax charge	$ 47,875	$ 51,133	$ 41,958
Ireland
Segment Reporting Information [Line Items]
Income tax charge	30,616	38,788	29,096
Rest of Europe
Segment Reporting Information [Line Items]
Income tax charge	4,609	4,234	(434)
U.S.
Segment Reporting Information [Line Items]
Income tax charge	7,600	1,571	3,761
Other
Segment Reporting Information [Line Items]
Income tax charge	$ 5,050	$ 6,540	$ 9,535

Supplemental Disclosure of Ca_3

Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 13,062	$ 13,059	$ 13,060
Cash paid for income taxes (net of refunds)	$ 27,604	$ 29,836	$ 18,558

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation adjustments	$ (15,894)	$ (64,023)
Currency impact on long term funding	(9,329)	(7,726)
Actuarial loss on defined benefit pension plan (note 9)	(9,364)	(5,226)
Unrealized capital gain – investments (note 3)	0	231
Loss on interest rate hedge	(905)	0
Amortization of loss on interest rate hedge	15	0
Total	(35,477)	(75,819)
Interest Rate Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Realized gain on interest rate hedge	4,658	4,658
Amortization of gain on interest rate hedge	$ (4,658)	$ (3,733)

Operating Leases - Lease Costs

Operating Leases - Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease costs	$ 31,088	$ 34,759
Income from sub-leases	(940)	(1,761)
Net operating lease costs	$ 30,148	$ 32,998

Operating Leases Operating Leas

Operating Leases Operating Leases - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	$ 30,148,000	$ 32,998,000
Costs related to variable lease payments	0	0
Operating lease right-of-use assets	84,561,000	104,977,000
Operating lease liabilities	85,135,000
Right-of-use assets obtained in exchange for lease obligations	12,100,000	29,500,000
Impairment of right-of-use assets	$ 5,411,000	$ 0	$ 0
Weighted average remaining lease term	4 years 5 months 12 days	5 years 2 months 8 days
Weighted average discount rate (in percent)	2.53%	2.91%
Operating lease liabilities	$ 24,334,000	$ 28,320,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Selling, general and administrative
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	$ 27,600,000	$ 30,500,000
Direct costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net operating lease costs	2,500,000	2,500,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets				$ 106,500,000
Operating lease liabilities				$ 106,500,000
Other current liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities	$ 24,300,000	$ 28,300,000

Operating Leases Operating Le_2

Operating Leases Operating Leases - Operating Lease Maturity (Details) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 26,164
2022	21,510
2023	16,108
2024	10,165
2025	6,046
Thereafter	10,490
Total future minimum lease payments	90,483
Lease imputed interest	(5,348)
Total	$ 85,135

Debt - Senior Notes (Details)

Debt - Senior Notes (Details) $ in Thousands	Dec. 08, 2020USD ($)tranche	Dec. 15, 2015USD ($)	Jul. 31, 2020USD ($)	Nov. 30, 2015USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
Short-term debt					$ 0	$ 349,640
Issue of senior notes, number of tranches | tranche	2
Debt instrument, term					5 years
Loss on settlement of interest rate hedge			$ 900		$ 905	0	$ 0
Senior Notes
Debt Instrument [Line Items]
Short-term debt					350,000	350,000
Debt instrument, face amount					$ 350,000	$ 350,000
Senior Notes | Senior Notes 2015
Debt Instrument [Line Items]
Debt instrument, face amount		$ 350,000
Debt instrument, interest rate (in percent)		3.64%
Debt instrument, term		5 years
Cash proceeds from interest rate hedge				$ 4,600
Senior Notes | Senior Notes 2020
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt	$ 350,000
Debt instrument, interest rate (in percent)	2.32%
Senior Notes | Senior A Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 275,000
Debt instrument, interest rate (in percent)	2.43%
Senior Notes | Senior B Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 75,000
Debt instrument, interest rate (in percent)	2.43%

Related Parties (Details)

Related Parties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Ownership percentage	49.00%
Payments to acquire equity method investments	$ 2,450	$ 0	$ 0
Majority investor, ownership percentage	51.00%
Oncacare
Related Party Transaction [Line Items]
Equity method investment, aggregate cost	$ 4,900
Ownership percentage	49.00%
Subsidiaries | DS Biopharma Limited
Related Party Transaction [Line Items]
Revenue earned from related parties	$ 321	95
Amounts due from related parties	$ 41	$ 36

Net Income Per Ordinary Share -

Net Income Per Ordinary Share - Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares)	52,859,911	53,859,537	54,118,764
Effect of dilutive share options outstanding (in shares)	423,674	473,924	671,899
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares)	53,283,585	54,333,461	54,790,663

Net Income Per Ordinary Share_2

Net Income Per Ordinary Share - Reconciliation of Net Income Attributable to the Group Per the Statement of Operating Income and Net Income Used For Net Income Per Ordinary Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income attributable to the Group	$ 332,331	$ 373,986	$ 322,656
Noncontrolling interest adjustment to redemption amount	(4,522)	(5,048)	0
Net income attributable to the Group (including NCI redemption adjustment)	$ 327,809	$ 368,938	$ 322,656
Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment):
Basic (USD per share)	$ 6.20	$ 6.85	$ 5.96
Diluted (USD per share)	$ 6.15	$ 6.79	$ 5.89

Subsequent Events (Details)

Subsequent Events (Details) $ in Millions	3 Months Ended
	Sep. 30, 2021USD ($)	Feb. 24, 2021USD ($)
Forecast | PRAH Health Sciences, Inc
Subsequent Event [Line Items]
Total consideration	$ 12,000
Merger agreement cash per share issued	80
Merger agreement ratio of shares issued per share owned	0.4125
Forecast | PRAH Health Sciences, Inc | ICON plc And PRAH Health Sciences, Inc
Subsequent Event [Line Items]
Ownership percentage	66.00%
Forecast | PRAH Health Sciences, Inc | ICON plc And PRAH Health Sciences, Inc | PRAH Health Sciences, Inc
Subsequent Event [Line Items]
Non-controlling owners, ownership percentage	34.00%
Subsequent Event | Senior Secured Revolving Credit Facility | Revolving Credit Facility
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 300
Subsequent Event | Senior Secured Bridge Loan Facility | Secured Debt
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 6,060

Impact of Change in Accountin_2

Impact of Change in Accounting Policies (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities	$ 85,135,000
Operating lease right-of-use assets	84,561,000	$ 104,977,000
Shareholders' equity	1,850,236,000	1,618,055,000		$ 1,354,281,000	$ 1,191,000,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities			$ 106,500,000
Operating lease right-of-use assets			106,500,000
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 1,262,895,000	$ 1,110,226,000		$ 888,326,000	$ 742,800,000
Retained Earnings | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity			$ 0

Uncategorized Items - iclr-2020

Label	Element	Value
Accounting Standards Update [Extensible List]	us-gaap_AccountingStandardsUpdateExtensibleList	Accounting Standards Update 2016-02 [Member]