ICON Public Limited (ICLR) 6-K 11 May 18 2018 Q1 Current report (foreign) Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2018
Document And Entity Information [Abstract]
Document Type	6-K
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	ICLR
Entity Registrant Name	ICON PLC
Entity Central Index Key	1,060,955
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 272,001	$ 282,859	$ 237,299	$ 192,541
Short term investments - available for sale	81,541	77,589
Accounts receivable, net	393,940	379,501
Unbilled revenue	292,688	268,509
Other receivables	49,563	33,798
Prepayments and other current assets	45,538	34,377
Income taxes receivable	28,232	24,385
Total current assets	1,163,503	1,101,018
Other Assets:
Property, plant and equipment, net	161,060	163,051
Goodwill	776,469	769,058		$ 616,088
Other non-current assets	15,610	15,393
Non-current income taxes receivable	17,530	18,396
Non-current deferred tax asset	18,212	8,074
Intangible assets	67,061	71,628
Total Assets	2,219,445	2,146,618
Current Liabilities:
Accounts payable	11,850	18,590
Payments on account	332,353	298,992
Other liabilities	260,998	233,503
Income taxes payable	22,962	14,973
Total current liabilities	628,163	566,058
Other Liabilities:
Non-current bank credit lines and loan facilities	348,980	348,888
Non-current other liabilities	15,787	17,111
Non-current government grants	977	966
Non-current income taxes payable	13,161	14,879
Non-current deferred tax liability	7,382	7,716
Commitments and contingencies	0	0
Total Liabilities	1,014,450	955,618
Shareholders' Equity:
54,081,601 shares issued and outstanding at December 31, 2017	4,643	4,664
Additional paid‑in capital	491,338	481,337
Other undenominated capital	938	912
Accumulated other comprehensive income	(26,505)	(38,713)
Retained earnings	734,581	742,800
Total Shareholders' Equity	1,204,995	1,191,000
Total Liabilities and Shareholders' Equity	$ 2,219,445	$ 2,146,618

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - € / shares	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Ordinary shares, par value	€ 0.06	€ 0.06
Ordinary shares, shares authorized	100,000,000	100,000,000
Ordinary shares, shares issued	53,793,839	54,081,601
Ordinary shares, shares outstanding	53,793,839	54,081,601

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue:
Revenue	$ 620,125	$ 578,066
Reimbursable expenses	0	(146,112)
Net revenue	620,125	431,954
Costs and expenses:
Direct costs
Direct Costs, Reimbursable Expenses	158,753	0
Direct Costs, Other	271,839	250,459
Selling, general and administrative expense	80,915	81,389
Depreciation and amortization	16,898	14,448
Total costs and expenses	528,405	346,296
Income from operations	91,720	85,658
Interest income	823	555
Interest expense	(3,795)	(3,179)
Income before provision for income taxes	88,748	83,034
Provision for income taxes	(10,650)	(11,625)
Net income	$ 78,098	$ 71,409
Net income per Ordinary Share:
Basic (USD per share)	$ 1.45	$ 1.31
Diluted (USD per share)	$ 1.42	$ 1.29
Weighted average number of Ordinary Shares outstanding:
Basic (in shares)	53,925,560	54,372,931
Diluted (in shares)	54,985,790	55,371,826

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 78,098,000	$ 71,409,000
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	2,000	0
Depreciation expense	11,850,000	10,018,000
Amortization of intangibles	5,048,000	4,430,000
Amortization of government grants	(12,000)	(10,000)
Interest on short term investments	(333,000)	(251,000)
Stock compensation expense	9,392,000	8,809,000
Amortization of gain on interest rate hedge	(230,000)	(230,000)
Amortization of financing costs	390,000	140,000
Deferred taxes	(3,481,000)	(3,593,000)
Changes in assets and liabilities:
Increase in accounts receivable	(12,979,000)	76,478,000
Increase in unbilled revenue	(65,904,000)	(32,944,000)
Increase in other receivables	(2,866,000)	(3,103,000)
Increase in prepayments and other current assets	(10,358,000)	(2,522,000)
Increase in other non-current assets	(179,000)	(526,000)
Increase/(decrease) in payments on account	8,040,000	(6,115,000)
Increase in other current liabilities	27,523,000	22,568,000
Decrease in other non-current liabilities	(1,165,000)	1,082,000
Increase in income taxes payable	4,300,000	8,839,000
Decrease in accounts payable	(6,668,000)	4,848,000
Net cash provided by operating activities	40,468,000	159,327,000
Cash flows from investing activities:
Purchase of property, plant and equipment	(8,304,000)	(8,298,000)
Payments to Acquire Businesses, Gross	(1,645,000)	0
Purchase of short term investments	(10,160,000)	(17,977,000)
Sale of short term investments	6,253,000	4,675,000
Net cash used in investing activities	(13,856,000)	(21,600,000)
Cash flows from financing activities:
Financing costs	(823,000)	0
Proceeds from exercise of equity compensation	727,000	2,780,000
Share issue costs	(4,000)	(7,000)
Repurchase of ordinary shares	(38,208,000)	(96,404,000)
Share repurchase costs	(31,000)	(77,000)
Net cash used in financing activities	(38,339,000)	(93,708,000)
Effect of exchange rate movements on cash	869,000	739,000
Net decrease in cash and cash equivalents	(10,858,000)	44,758,000
Cash and cash equivalents at beginning of period	282,859,000	192,541,000
Cash and cash equivalents at end of period	$ 272,001,000	$ 237,299,000

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Shareholders' Equity and Comprehensive Income - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Other Undenominated Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Cumulative effect adjustment from adoption of ASC 606 | Accounting Standards Update 2014-09 [Member]	$ (48,104)					$ (48,104)
Balance (in shares) at Dec. 31, 2017	54,081,601	54,081,601
Balance (in shares) (Accounting Standards Update 2014-09 [Member]) at Dec. 31, 2017		54,081,601
Beginning balance at Dec. 31, 2017	$ 1,191,000	$ 4,664	$ 481,337	$ 912	$ (38,713)	742,800
Beginning balance (Accounting Standards Update 2014-09 [Member]) at Dec. 31, 2017	1,142,896	$ 4,664	481,337	912	(38,713)	694,696
Net income	78,098					78,098
Currency translation adjustment	10,661				10,635	26
Currency impact of long term funding	1,685				1,685
Tax on currency impact of long term funding	4				4
Unrealized capital gain – investments	(288)				(288)
Amortization of interest rate hedge	(230)				(230)
Fair value of cash flow hedge	402				$ 402
Total comprehensive income	90,332					78,124
Exercise of share options (in shares)		21,574
Exercise of share options	724	$ 2	722
Issue of restricted share units (in shares)		35,776
Issue of restricted share units	3	$ 3
Non-cash stock compensation expense	(4)		(4)
Share issuance costs	9,283		$ 9,283
Share repurchase program (in shares)		(345,112)
Share repurchase program	(38,208)	$ (26)		$ 26		(38,208)
Share repurchase costs	$ (31)					(31)
Balance (in shares) at Mar. 31, 2018	53,793,839
Ending balance at Mar. 31, 2018	$ 1,204,995					$ 734,581

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation These condensed consolidated financial statements which have been prepared in accordance with United States Generally Accepted Accounting Principles (“US GAAP”) have not been audited. The condensed consolidated financial statements reflect all adjustments, which are, in the opinion of management, necessary to present a fair statement of the operating results and financial position for the periods presented. The preparation of the condensed consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect reported amounts and disclosures in the condensed consolidated financial statements. Actual results could differ from those estimates. The condensed consolidated financial statements should be read in conjunction with the accounting policies and notes to the consolidated financial statements included in ICON’s Form 20-F for the year ended December 31, 2017 . Operating results for the three months ended March 31, 2018 are not necessarily indicative of the results that may be expected for the fiscal period ending December 31, 2018 .

Significant Accounting Policies

Significant Accounting Policies Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	Significant accounting policies The Company adopted ASC 606, ‘ Revenue from Contracts with Customers’ , with a date of initial application of January 1, 2018. The revenue recognition accounting policy applied in preparation of the results for the three months ended March 31, 2018 therefore reflect application of ASC 606. ICON has elected to adopt the standard using the cumulative effect transition method. Under this transition method, ICON has applied the new standard as at the date of initial application (i.e. January 1, 2018), without restatement of comparative amounts. The cumulative effect of initially applying the new standard (to revenue, costs and tax) is recorded as an adjustment to the opening balance of equity at the date of initial application. See 'Note 13 - Impact of changes in accounting policies' for details. The comparative information has not been adjusted and therefore continues to be reported under ASC 605, ‘Revenue Recognition’ . The new standard 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. The most significant impact of application of the standard relates to our assessment of performance and percentage of completion in respect of our clinical trial service revenue. Prior to application of ASC 606, the revenue attributable to performance was determined based on both input and output methods of measurement. We have concluded that under the new standard, a clinical trial 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 trial. 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 therefore based on on an input measure being total project costs (inclusive of third party costs) at each reporting period See 'Note 13 - Impact of changes in accounting policies' for details of the impact of application of the provisions of ASC 606 in the three months ended March 31, 2018 and in respect of the position at March 31, 2018.

Revenue

Revenue	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue	Revenue Revenue disaggregated by customer profile is as follows: Three months ended March 31, 2018 March 31, 2017 (in thousands) Top client $ 65,818 $ 104,682 Clients 2-5 164,301 90,265 Clients 6-10 98,539 55,341 Clients 11-25 117,271 70,364 Other 174,196 111,302 Total $ 620,125 $ 431,954 Trade accounts receivables and unbilled revenue are as follows: March 31, 2018 December 31, 2017 (in thousands) Contract assets: Billed services (accounts receivable) $ 403,893 $ 388,431 Unbilled services (unbilled revenue) 292,688 268,509 Trade accounts receivable and unbilled revenue 696,581 656,940 Allowance for doubtful accounts (9,953 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 686,628 $ 648,010 Unbilled services and payments on account were as follows: (in thousands, except percentages) March 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 292,688 $ 268,509 $ 24,179 9.0 % Unearned revenue (payments on account) (332,353 ) (298,992 ) (33,361 ) 11.2 % Net balance $ (39,665 ) $ (30,483 ) $ (9,182 ) 30.1 % Unbilled services as of March 31, 2018 , increased by $24.2 million as compared to December 31, 2017 . Adoption of ASC 606 as at January 1, 2018 resulted in a net reduction in unbilled revenue of $42 million . Unbilled services/revenue balances arise where invoicing or billing is based on the timing of agreed milestones related to service contracts for clinical research. Payments on account increased by $33.4 million over the same period resulting in a decrease of $9.2 million in the net balance of unbilled services and payments on account between December 31, 2017 and March 31, 2018 . These fluctuations are primarily due to timing of payments and invoicing related to the Group's clinical trial management contracts. The bad debt expense recognized on the Group's receivables and unbilled services was de minimis for the three months ended March 31, 2018. As of March 31, 2018 approximately $4.9 billion of revenue is expected to be recognized in the future in respect of unrealized performance obligations. The Company expects to recognize revenue on approximately 40% of the unrealized performance obligation over the next 12 months, with the remainder recognized thereafter over the duration of the customer contracts.

Contract Balances

Contract Balances	3 Months Ended
	Mar. 31, 2018
Contracts Receivable [Abstract]
Contract balances	Revenue Revenue disaggregated by customer profile is as follows: Three months ended March 31, 2018 March 31, 2017 (in thousands) Top client $ 65,818 $ 104,682 Clients 2-5 164,301 90,265 Clients 6-10 98,539 55,341 Clients 11-25 117,271 70,364 Other 174,196 111,302 Total $ 620,125 $ 431,954 Trade accounts receivables and unbilled revenue are as follows: March 31, 2018 December 31, 2017 (in thousands) Contract assets: Billed services (accounts receivable) $ 403,893 $ 388,431 Unbilled services (unbilled revenue) 292,688 268,509 Trade accounts receivable and unbilled revenue 696,581 656,940 Allowance for doubtful accounts (9,953 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 686,628 $ 648,010 Unbilled services and payments on account were as follows: (in thousands, except percentages) March 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 292,688 $ 268,509 $ 24,179 9.0 % Unearned revenue (payments on account) (332,353 ) (298,992 ) (33,361 ) 11.2 % Net balance $ (39,665 ) $ (30,483 ) $ (9,182 ) 30.1 % Unbilled services as of March 31, 2018 , increased by $24.2 million as compared to December 31, 2017 . Adoption of ASC 606 as at January 1, 2018 resulted in a net reduction in unbilled revenue of $42 million . Unbilled services/revenue balances arise where invoicing or billing is based on the timing of agreed milestones related to service contracts for clinical research. Payments on account increased by $33.4 million over the same period resulting in a decrease of $9.2 million in the net balance of unbilled services and payments on account between December 31, 2017 and March 31, 2018 . These fluctuations are primarily due to timing of payments and invoicing related to the Group's clinical trial management contracts. The bad debt expense recognized on the Group's receivables and unbilled services was de minimis for the three months ended March 31, 2018. As of March 31, 2018 approximately $4.9 billion of revenue is expected to be recognized in the future in respect of unrealized performance obligations. The Company expects to recognize revenue on approximately 40% of the unrealized performance obligation over the next 12 months, with the remainder recognized thereafter over the duration of the customer contracts.

Goodwill

Goodwill	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill Three months ended Year ended March 31, 2018 December 31, 2017 (in thousands) Opening balance $ 769,058 $ 616,088 Current period acquisitions (Note 6) — 129,222 Prior period acquisitions (Note 6) (627 ) 1,393 Foreign exchange movement 8,038 22,355 Closing balance $ 776,469 $ 769,058

Business Combinations

Business Combinations	3 Months Ended
	Mar. 31, 2018
Business Combinations [Abstract]
Business Combinations	Business Combinations Acquisitions – Mapi Group On July 27, 2017, a subsidiary of the Company, ICON Clinical Research Limited, acquired Mapi Group. Mapi Group is a leading patient-centered health outcomes research and commercialization company. Cash outflows on acquisition were $145.8 million . The acquisition agreement provided for working capital targets to be achieved. On March 26, 2018, the Company paid $1.6 million in respect of these targets on completion of the working capital review. The acquisition of Mapi has been accounted for as a business combination in accordance with FASB ASC 805 Business Combinations . The Company has made a provisional assessment of the fair value of assets acquired and liabilities assumed as at that date. The table following summarizes the Company’s provisional estimates of the fair values of the assets acquired and liabilities assumed: July 27, 2017 (in thousands) Cash $ 19,649 Property, plant and equipment 3,410 Goodwill* 128,595 Intangible assets** 32,143 Accounts receivable 15,873 Unbilled revenue 8,484 Prepayments and other current assets 3,277 Other receivables 1,430 Income taxes receivable 4,262 Accounts payable (2,994 ) Payments on account (31,237 ) Other liabilities (24,951 ) Non-current other liabilities (1,061 ) Non-current deferred tax liability (11,104 ) Net assets acquired $ 145,776 Cash outflows $ 144,131 Working capital adjustment $ 1,645 Total consideration $ 145,776 *Goodwill represents the acquisition of an established workforce with experience in late phase commercialization, analytics, real world evidence generation and strategic regulatory services in clinical trial services for biologics, drugs and devices. Goodwill related to the business acquired is not tax deductible. **The Company has made an initial estimate of separate intangible assets acquired of $32.1 million , being customer relationships and order book assets. This assessment is under review and will be finalized within 12 months of the date of acquisition. Acquisitions – ClinicalRM On September 15, 2016, a subsidiary of the Company, ICON US Holdings Inc. acquired Clinical Research Management, Inc. (''ClinicalRM'') which resulted in net cash outflow of $52.4 million (including certain payments made on behalf of ClinicalRM totaling $9.2 million ). ClinicalRM is 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. ClinicalRM provide full service and functional research solutions to a broad range of US government agencies. Their extensive expertise extends across basic and applied research, infectious diseases, vaccines development, testing and the response to bio-threats. They have worked in collaboration with government and commercial customers to respond to the threat of global viral epidemics. Further consideration of up to $12.0 million was payable if certain performance milestones are achieved in respect of periods up to December 31, 2017. The fair value of the contingent consideration on acquisition and at March 31, 2017 was estimated at $6 million The evaluation of the performance and forecast performance of ClinicalRM against performance milestones was updated as required at June 30, 2017. Arising from that evaluation, the fair value of the contingent consideration liability was determined as $ Nil , resulting in a net credit of $6 million being recorded within selling, general & administrative expenses in the Statement of Operations. The fair value of the contingent consideration at March 31, 2018 is $ Nil . The acquisition of ClinicalRM has been accounted for as a business combination in accordance with FASB ASC 805 Business Combinations . The table following summarizes the fair values of the assets acquired and liabilities assumed: September 15, 2016 (in thousands) Cash $ 3,168 Property, plant and equipment 939 Goodwill* 35,969 Customer lists 4,012 Order backlog 1,668 Brand 1,409 Accounts receivable 11,431 Unbilled revenue 3,868 Prepayments and other current assets 1,673 Accounts payable (165 ) Other liabilities (5,569 ) Non-current other liabilities (7 ) Net assets acquired $ 58,396 Total consideration $ 58,396 *Goodwill represents the acquisition of an established workforce with experience in preclinical through Phase IV support of clinical research and clinical trial services for biologics, drugs and devices. Goodwill related to the US portion of the business acquired is tax deductible. In finalizing the goodwill on acquisition of CRM in the twelve month period from acquisition, fair value adjustments were made which resulted in an increase to unbilled revenue ( $1.1 million ) and other liabilities ( $1.1 million ) and in a decrease to accounts receivable ( $0.3 million ) and accounts payable ( $0.5 million ). Customer list, order backlog and brand intangible asset values were also finalized.

Restructuring

Restructuring	3 Months Ended
	Mar. 31, 2018
Restructuring and Related Activities [Abstract]
Restructuring and Related Activities Disclosure	Restructuring Restructuring charges No restructuring charge was recognized during the three months ended March 31, 2018 or March 31, 2017 . Prior Period Restructuring charges A restructuring charge of $ 7.8 million was recognized during the year ended December 31, 2017 under a restructuring plan adopted following a review of operations. The restructuring plan reflected resource rationalization across the business to improve resource utilization. No additional charge was recorded during the three months ended March 31, 2018 . Workforce reductions (in thousands) Initial restructuring charge recorded $ 7,753 Utilized (4,656 ) Provision at December 31, 2017 $ 3,097 Utilized (961 ) Provision at March 31, 2018 $ 2,136 A restructuring charge of $ 8.2 million was recognized during the year ended December 31, 2016 under a restructuring plan adopted following a review by the Company of its operations. The restructuring plan includes resource rationalizations in certain areas of the business to improve resource utilization, resulting in a charge of $ 6.2 million and office consolidation resulting in the recognition of an onerous lease obligation of $ 2.0 million during the twelve months ended December 31, 2016 . Workforce Reductions Onerous Lease Total (in thousands) Total provision recognized $ 6,190 $ 1,969 $ 8,159 Utilized (5,734 ) (571 ) (6,305 ) Foreign exchange (63 ) — (63 ) Provision at December 31, 2016 $ 393 $ 1,398 $ 1,791 Utilized (393 ) (1,081 ) (1,474 ) Provision at December 31, 2017 $ — $ 317 $ 317 Utilized — (310 ) (310 ) Provision at March 31, 2018 $ — $ 7 $ 7 A restructuring charge of $ 8.8 million was recognized during the year ended December 31, 2014 . Following the closure of the Company’s European Phase 1 services in 2013, the Company recognized a charge in 2014 in relation to its Manchester, United Kingdom facility; $ 5.6 million in relation to asset impairments and $ 3.2 million in relation to an onerous lease charge associated with this facility. We expect this to be paid by 2024. Onerous Lease Asset Impairment Total (in thousands) Total provision recognized $ 3,167 $ 5,629 $ 8,796 Asset write off — (5,629 ) (5,629 ) Provision at December 31, 2014 $ 3,167 $ — $ 3,167 Utilized (1,167 ) — (1,167 ) Provision at December 31, 2015 $ 2,000 $ — $ 2,000 Utilized (1,359 ) — (1,359 ) Provision at December 31, 2016 $ 641 $ — $ 641 Utilized $ (441 ) $ — $ (441 ) Provision at December 31, 2017 $ 200 $ — $ 200 Utilized $ (92 ) $ — $ (92 ) Provision at March 31, 2018 $ 108 $ — $ 108 At March 31, 2018 , $ 2.1 million is included within other liabilities and $ 0.2 million within non-current other liabilities.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income taxes recognized during the three months ended March 31, 2018 , comprise: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Provision for income taxes $ 10,650 $ 11,625 As at March 31, 2018 the Company maintains a $ 26.0 million liability ( December 31, 2017 : $ 26.1 million ) for unrecognized tax benefit, which is comprised of $ 23.6 million ( December 31, 2017 : $ 23.7 million ) related to items generating unrecognized tax benefits and $ 2.4 million ( December 31, 2017 : $ 2.4 million ) for interest and related penalties to such items. The Company recognizes interest accrued on unrecognized tax benefits as an additional income tax expense. The Company has analyzed the filing positions in all of the significant federal, state and foreign jurisdictions where it is required to file income tax returns, as well as open tax years in these jurisdictions. The only periods subject to examination by the major tax jurisdictions where the Company does business are 2013 through 2017 tax years. The Company does not believe that the outcome of any examination will have a material impact on its financial statements.

Net income per ordinary share

Net income per ordinary share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income per ordinary share	Net income per ordinary share Basic net income per ordinary share has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares. There is no difference in net income used for basic and diluted net income per ordinary share. The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows: Three Months Ended March 31, 2018 March 31, 2017 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 53,925,560 54,372,931 Effect of dilutive share options outstanding 1,060,230 998,895 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 54,985,790 55,371,826

Share-based Awards

Share-based Awards	3 Months Ended
	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Awards	Share-based Awards 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 400,000 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 400,000 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 400,000 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. Share options typically vest over a period of five years from date of grant and expire eight years from date of grant. The maximum contractual term of options outstanding at March 31, 2018 is eight years . The following table summarizes option activity for the three months ended March 31, 2018 : Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Life Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 107,794 $ 115.11 $ 35.10 Exercised (21,574 ) $ 33.53 $ 11.72 Forfeited (5,855 ) $ 39.13 $ 12.74 Outstanding at March 31, 2018 1,251,758 $ 61.57 $ 18.81 4.95 Exercisable at March 31, 2018 594,051 $ 45.17 $ 14.54 3.73 The Company has outstanding options with fair values ranging from $ 8.53 to $ 35.10 per option or a weighted average fair value of $ 13.75 per option. The Company issues ordinary shares for all options exercised. The total amount of fully vested share options which remained outstanding at March 31, 2018 was 594,051 . Fully vested share options at March 31, 2018 have an average remaining contractual term of 3.73 years , an average exercise price of $ 45.17 and a total intrinsic value of $ 43.3 million . The total intrinsic value of options exercised during the three months ended March 31, 2018 was $ 1.8 million ( March 31, 2017 : $ 7.4 million ). The following table summarizes the movement in non-vested share options for the three months ended March 31, 2018 : Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Grant Date Fair Value Non-vested outstanding at December 31, 2017 694,727 $ 68.06 $ 20.03 Granted 107,794 $ 115.11 $ 35.10 Vested (142,109 ) $ 65.43 $ 19.31 Forfeited (2,705 ) $ 56.58 $ 16.84 Non-vested outstanding at March 31, 2018 657,707 $ 76.38 $ 22.67 Fair value of Stock Options Assumptions The weighted average fair value of options granted during the three months ended March 31, 2018 and March 31, 2017 was calculated using the Black-Scholes option pricing model. The weighted average grant date fair values and assumptions used were as follows: Three Months Ended March 31, 2018 March 31, 2017 Weighted average grant date fair value $ 35.10 $ 24.49 Assumptions: Expected volatility 29 % 29 % Dividend yield — % — % Risk-free interest rate 2.63 % 2.05 % Expected life 5 years 5 years Expected volatility is based on the historical volatility of our common stock over a period equal to the expected term of the options; the expected life represents the weighted average period of time that options granted are expected to be outstanding given consideration to vesting schedules and our historical experience of past vesting and termination patterns. The risk-free rate is based on the U.S. government zero-coupon bonds yield curve in effect at time of the grant for periods corresponding with the expected life of the option. Restricted Share Units and Performance Share Units On July 21, 2008, the Company adopted the 2008 Employees Restricted Share Unit Plan (the “2008 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. An aggregate of 1.0 million ordinary shares have been reserved for issuance under the 2008 RSU Plan. 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. The Company has awarded RSUs and PSUs to certain key individuals of the Group. The following table summarizes RSU and PSU activity for the three months ended March 31, 2018 : PSU Outstanding Number of Shares PSU Weighted Average Grant Date Fair Value PSU Weighted Average Remaining Contractual Life RSU Outstanding Number of Shares RSU Weighted Average Grant Date Fair Value RSU Weighted Average Remaining Contractual Life Outstanding at December 31, 2017 511,026 $ 72.07 0.93 715,970 $ 72.65 1.28 Granted 65,736 $ 115.11 36,838 $ 115.11 Shares vested — $ — (35,776 ) $ 75.05 Forfeited (16,666 ) $ 66.18 (16,424 ) $ 71.11 Outstanding at March 31, 2018 560,096 $ 76.79 0.95 700,608 $ 74.79 1.14 The fair value of RSUs vested for the three months ended March 31, 2018 totaled $ 2.7 million (full year 2017: $ 16.6 million ). No PSUs vested for the three months ended March 31, 2018 . The fair value of PSUs vested for the full year 2017 was $ 15.0 million ). The PSUs vest based on service and specified EPS targets over the period 2015 – 2018, 2016 – 2019, 2017 – 2020 and 2018 – 2021. Since 2013, 301,932 PSUs (net of forfeitures) have been granted. Depending on the amount of EPS from 2015 to 2021 , up to an additional 258,164 PSUs may also be granted Non-cash stock compensation expense Non-cash stock compensation expense for the three months ended March 31, 2018 has been allocated as follows: Three Months Ended March 31, 2018 March 31, 2017 (In thousands) Direct costs $ 5,175 $ 4,854 Selling, general and administrative 4,217 3,955 $ 9,392 $ 8,809 Total non-cash stock compensation expense not yet recognized at March 31, 2018 amounted to $ 53.7 million . The weighted average period over which this is expected to be recognized is 2.4 years . The amendments required by Accounting Standards Update (‘ASU’) 2016-09 ‘Improvements to Employee Share-Based Payment Accounting’ require the Company to record all tax effects related to share-based payments through the income statement rather than additional paid in capital. The Company applied the updated standard prospectively during the year ended December 31, 2017 .

Share Capital

Share Capital	3 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Share Capital	Share Capital On October 3, 2016, the Company commenced a previously announced share buyback program of up to $ 400 million . The Company can acquire up to 10% of 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. The buyback program gives a broker authority to acquire the Company’s ordinary shares from time to time on the open market in accordance with agreed terms and limitations. The acquisition of shares pursuant to the buyback program was effected by way of redemption and cancellation of the shares, in accordance with the Constitution of the Company. During the three months ended March 31, 2018 345,112 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $ 38.2 million . At March 31, 2018 a total of 3,363,526 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $ 281.3 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 an other undenominated capital reserve as required under Irish Company Law.

Business Segment Information

Business Segment Information	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Segment Reporting Disclosure	Business Segment Information The Company determines and presents operating segments based on the information that is internally provided to the chief operating decision maker, together the (‘CODM’) in accordance with FASB ASC 280-10 Disclosures about Segments of an Enterprises and Related Information . The Chief Executive Officer, Chief Financial Officer and Chief Operating Officer, were together considered the Company’s CODM in the period up to and including March 1, 2017. On March 1, 2017, Mr. Ciaran Murray transitioned from his role as Chief Executive Officer to the role of Executive Chairman of the Board of Directors and Dr. Steve Cutler was appointed as Chief Executive Officer. As of March 1, 2017, the Company determined that the CODM was comprised of the Chief Executive Officer and the Chief Financial Officer. The Company determines and presents operating segments based on the information that is provided to the CODM. The Company operates as one single business segment, which is the provision of outsourced development services on a global basis to the pharmaceutical, biotechnology and medical devices industries. There have been no changes to the basis of segmentation or the measurement basis for the segment results in the period. 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 is generally awarded projects based upon responses to requests for proposals received from companies in the pharmaceutical, biotechnology and medical device industries or work orders executed under our strategic partnership arrangements. Contracts with customers are generally entered into centrally, in most cases with ICON Clinical Research Limited (“ICON Ireland”), the Company’s principal operating subsidiary in Ireland. Revenues, which consist primarily of fees earned under these contracts, are allocated to individual entities within the Group, based on where the work is performed in accordance with the Company’s global transfer pricing model. ICON Ireland acts as the group entrepreneur under the Company’s global transfer pricing model given its role in the development and management of the group, it’s ownership of key intellectual property and customer relationships, its key role in the mitigation of risks faced by the group and its responsibility for maintaining the Company’s global network. ICON Ireland enters into the majority of the Company’s customer contracts. ICON Ireland remunerates other operating entities in the 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 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. The geographic split of revenue disclosed for each region outside Ireland is the cost plus revenue attributable to these entities. The residual revenues of the Group, once each ICON entity has been paid its respective intercompany service fee, generally fall to be retained by ICON Ireland. As such revenues and income from operations in Ireland are a function of this global transfer pricing model and comprise revenues of the Group after deducting the 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, Belgium, France, Germany, Italy, Spain, The Netherlands, Sweden, Turkey, Poland, Czech Republic, Latvia, Russia, Ukraine, Hungary, Israel, Romania, Switzerland, Canada, Mexico, Brazil, Colombia, Argentina, Chile, Peru, India, China, South Korea, Japan, Thailand, Taiwan, Singapore, The Philippines, Australia, New Zealand, and South Africa. The geographical distribution of the Company’s segment measures as at March 31, 2018 and December 31, 2017 and for the three months ended March 31, 2018 and March 31, 2017 is as follows: a) The distribution of revenue by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 235,110 $ 232,348 Rest of Europe 95,040 72,557 U.S. 225,670 223,082 Rest of World 64,305 50,079 Total $ 620,125 $ 578,066 * All sales shown for Ireland are export sales. ** 2017 restated for gross revenue. b) The distribution of income from operations, by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 69,810 $ 63,968 Rest of Europe 5,168 3,699 U.S. 11,802 10,499 Rest of World 4,940 7,492 Total $ 91,720 $ 85,658 c) The distribution of property, plant and equipment, net, by geographical area was as follows: March 31, 2018 December 31, 2017 (in thousands) Ireland $ 112,239 $ 111,329 Rest of Europe 8,935 9,026 U.S. 26,654 27,797 Rest of World 13,232 14,899 Total $ 161,060 $ 163,051 d) The distribution of depreciation and amortization by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 7,885 $ 5,972 Rest of Europe 1,539 1,658 U.S. 6,299 5,948 Rest of World 1,175 870 Total $ 16,898 $ 14,448 e) The distribution of total assets by geographical area was as follows: March 31, 2018 December 31, 2017 (in thousands) Ireland $ 957,886 $ 880,378 Rest of Europe 506,356 504,418 U.S. 643,831 650,681 Rest of World 111,372 111,141 Total $ 2,219,445 $ 2,146,618

Impact of change in accounting

Impact of change in accounting policies	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Impact fo change in accounting policy	Impact of change in accounting policies - revenue recognition The Company adopted ASC 606, ‘ Revenue from Contracts with Customers’ , with a date of initial application of January 1, 2018. The revenue recognition accounting policy applied in preparation of the results for the three months ended March 31, 2018 therefore reflect application of ASC 606. ICON has elected to adopt the standard using the cumulative effect transition method. Under this transition method, ICON has applied the new standard as at the date of initial application (i.e. January 1, 2018), without restatement of comparative amounts. The cumulative effect of initially applying the new standard (to revenue, costs and tax) is recorded as an adjustment to the opening balance of equity at the date of initial application. See 'Note 13 - Impact of changes in accounting policies' for details. The comparative information has not been adjusted and therefore continues to be reported under ASC 605, ‘Revenue Recognition’ . The new standard 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. The most significant impact of application of the standard relates to our assessment of performance and percentage of completion in respect of our clinical trial service revenue. Prior to application of ASC 606, the revenue attributable to performance was determined based on both input and output methods of measurement. We have concluded that under the new standard, a clinical trial service is a single performance obligation satisfied over time i.e. the full 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) 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 therefore based on on an input measure being total project costs (inclusive of third party costs) at each reporting period The tables on the pages following summarize the impact of adopting ASC 606 on the consolidated financial statements for the three months ended March 31, 2018 . ICON plc CONDENSED CONSOLIDATED BALANCE SHEETS AS AT MARCH 31, 2018 (UNAUDITED) March 31, 2018 As reported Adjustments Balance without adoption of Topic 606 ASSETS (in thousands) Current Assets: Cash and cash equivalents $ 272,001 — $ 272,001 Short term investments - available for sale 81,541 — 81,541 Accounts receivable, net 393,940 — 393,940 Unbilled revenue 292,688 43,215 335,903 Other receivables 49,563 (12,268 ) 37,295 Prepayments and other current assets 45,538 — 45,538 Income taxes receivable 28,232 — 28,232 Total current assets 1,163,503 30,947 1,194,450 Other Assets: Property, plant and equipment, net 161,060 — 161,060 Goodwill 776,469 — 776,469 Non-current other assets 15,610 — 15,610 Non-current income taxes receivable 17,530 — 17,530 Non-current deferred tax asset 18,212 (6,574 ) 11,638 Intangible assets 67,061 — 67,061 Total Assets $ 2,219,445 $ 24,373 $ 2,243,818 LIABILITIES AND SHAREHOLDERS’ EQUITY Current Liabilities: Accounts payable $ 11,850 $ — $ 11,850 Payments on account 332,353 (25,100 ) 307,253 Other liabilities 260,998 260,998 Income taxes payable 22,962 432 23,394 Total current liabilities 628,163 (24,668 ) 603,495 Other Liabilities: Non-current bank credit lines and loan facilities 348,980 — 348,980 Non-current other liabilities 15,787 — 15,787 Non-current government grants 977 — 977 Non-current income taxes payable 13,161 — 13,161 Non-current deferred tax liability 7,382 — 7,382 Commitments and contingencies — — — Total Liabilities 1,014,450 (24,668 ) 989,782 Shareholders' Equity: Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 53,793,839 shares issued and outstanding at March 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017 4,643 — 4,643 Additional paid‑in capital 491,338 — 491,338 Other undenominated capital 938 — 938 Accumulated other comprehensive income (26,505 ) — (26,505 ) Retained earnings 734,581 49,041 783,622 Total Shareholders' Equity 1,204,995 49,041 1,254,036 Total Liabilities and Shareholders' Equity $ 2,219,445 $ 24,373 $ 2,243,818 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS FOR THE THREE MONTHS ENDED MARCH 31, 2018 (UNAUDITED) Three Months Ended As reported Adjustments Balance without adoption of Topic 606 (in thousands except share and per share data) Revenue: Revenue $ 620,125 $ 1,231 $ 621,356 Reimbursable expenses — (158,753 ) (158,753 ) 620,125 (157,522 ) 462,603 Costs and expenses: Direct costs - Reimbursable expenses 158,753 (158,753 ) — - Other direct costs 271,839 — 271,839 Selling, general and administrative expense 80,915 160 81,075 Depreciation and amortization 16,898 — 16,898 Total costs and expenses 528,405 (158,593 ) 369,812 Income from operations 91,720 1,071 92,791 Interest income 823 — 823 Interest expense (3,795 ) — (3,795 ) Income before provision for income taxes 88,748 1,071 89,819 Provision for income taxes (10,650 ) (134 ) (10,784 ) Net income $ 78,098 $ 937 $ 79,035 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS FOR THE THREE MONTHS ENDED MARCH 31, 2018 (UNAUDITED) Three Months Ended March 31, 2018 As Reported Adjustments Balance without adoption of Topic 606 (in thousands) Cash flows from operating activities: Net income $ 78,098 $ 937 $ 79,035 Adjustments to reconcile net income to net cash provided by operating activities: Loss on disposal of property, plant and equipment 2 — 2 Depreciation expense 11,850 — 11,850 Amortization of intangibles 5,048 — 5,048 Amortization of government grants (12 ) — (12 ) Interest on short term investments (333 ) — (333 ) Stock compensation expense 9,392 — 9,392 Amortization of gain on interest rate hedge (230 ) — (230 ) Amortization of financing costs 390 — 390 Deferred taxes (3,481 ) 6,574 3,093 Changes in assets and liabilities: Increase in accounts receivable (12,979 ) — (12,979 ) Increase in unbilled revenue (65,904 ) 4,889 (61,015 ) Increase in other receivables (2,866 ) 12,268 9,402 Increase in prepayments and other current assets (10,358 ) — (10,358 ) Increase in other non-current assets (179 ) — (179 ) Decrease in payments on account 8,040 (25,100 ) (17,060 ) Increase in other current liabilities 27,523 — 27,523 Decrease in other non-current liabilities (1,165 ) — (1,165 ) Increase in income taxes payable 4,300 432 4,732 Decrease in accounts payable (6,668 ) — (6,668 ) Net cash provided by operating activities 40,468 — 40,468 Cash flows from investing activities: Purchase of property, plant and equipment (8,304 ) — (8,304 ) Purchase of subsidiary undertakings (1,645 ) — (1,645 ) Purchase of short term investments (10,160 ) — (10,160 ) Sale of short term investments 6,253 — 6,253 Net cash used in investing activities (13,856 ) — (13,856 ) Cash flows from financing activities: Financing costs (823 ) — (823 ) Proceeds from exercise of equity compensation 727 — 727 Share issue costs (4 ) — (4 ) Repurchase of ordinary shares (38,208 ) — (38,208 ) Share repurchase costs (31 ) — (31 ) Net cash provided by financing activities (38,339 ) — (38,339 ) Effect of exchange rate movements on cash 869 869 Net decrease in cash and cash equivalents (10,858 ) — (10,858 ) Cash and cash equivalents at beginning of period 282,859 282,859 Cash and cash equivalents at end of period $ 272,001 $ — $ 272,001 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME (UNAUDITED) Shares Amount Additional Paid-in Capital Other Undenominated Capital Accumulated Other Comprehensive Income Retained Earnings Total (dollars in thousands, except share data) Balance at December 31, 2017 54,081,601 $ 4,664 $ 481,337 $ 912 $ (38,713 ) $ 742,800 $ 1,191,000 Comprehensive income: Net income (as reported) — — — — — 78,098 78,098 Impact of change in accounting policy — — — — — 937 937 Currency translation adjustment — — — — 10,635 26 10,661 Currency impact of long term funding — — — — 1,685 — 1,685 Tax on currency impact of long term funding — — — — 4 — 4 Unrealized capital gain – investments — — — — (288 ) — (288 ) Amortization of interest rate hedge — — — — (230 ) — (230 ) Fair value of cash flow hedge — — — — 402 — 402 Total comprehensive income — — — — 12,208 79,061 91,269 Exercise of share options 21,574 2 722 — — — 724 Issue of restricted share units 35,776 3 — — — — 3 Non-cash stock compensation expense — — 9,283 — — — 9,283 Share issuance costs — — (4 ) — — — (4 ) Share repurchase program (345,112 ) (26 ) — 26 — (38,208 ) (38,208 ) Share repurchase costs — — — — — (31 ) (31 ) Balance at March 31, 2018 (without the adoption of ASC 606) 53,793,839 $ 4,643 $ 491,338 $ 938 $ (26,505 ) $ 783,622 $ 1,254,036

Significant Accounting Polici20

Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
New Accounting Pronouncements	The Company adopted ASC 606, ‘ Revenue from Contracts with Customers’ , with a date of initial application of January 1, 2018. The revenue recognition accounting policy applied in preparation of the results for the three months ended March 31, 2018 therefore reflect application of ASC 606. ICON has elected to adopt the standard using the cumulative effect transition method. Under this transition method, ICON has applied the new standard as at the date of initial application (i.e. January 1, 2018), without restatement of comparative amounts. The cumulative effect of initially applying the new standard (to revenue, costs and tax) is recorded as an adjustment to the opening balance of equity at the date of initial application. See 'Note 13 - Impact of changes in accounting policies' for details. The comparative information has not been adjusted and therefore continues to be reported under ASC 605, ‘Revenue Recognition’ . The new standard 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. The most significant impact of application of the standard relates to our assessment of performance and percentage of completion in respect of our clinical trial service revenue. Prior to application of ASC 606, the revenue attributable to performance was determined based on both input and output methods of measurement. We have concluded that under the new standard, a clinical trial 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 trial. 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 therefore based on on an input measure being total project costs (inclusive of third party costs) at each reporting period See 'Note 13 - Impact of changes in accounting policies' for details of the impact of application of the provisions of ASC 606 in the three months ended March 31, 2018 and in respect of the position at March 31, 2018.

Revenue (Tables)

Revenue (Tables)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Revenue disaggregated by customer profile is as follows: Three months ended March 31, 2018 March 31, 2017 (in thousands) Top client $ 65,818 $ 104,682 Clients 2-5 164,301 90,265 Clients 6-10 98,539 55,341 Clients 11-25 117,271 70,364 Other 174,196 111,302 Total $ 620,125 $ 431,954

Contract Balances (Tables)

Contract Balances (Tables)	3 Months Ended
	Mar. 31, 2018
Contracts Receivable [Abstract]
Schedule of Contracts with Customers, Asset and Liabilities	Trade accounts receivables and unbilled revenue are as follows: March 31, 2018 December 31, 2017 (in thousands) Contract assets: Billed services (accounts receivable) $ 403,893 $ 388,431 Unbilled services (unbilled revenue) 292,688 268,509 Trade accounts receivable and unbilled revenue 696,581 656,940 Allowance for doubtful accounts (9,953 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 686,628 $ 648,010 Unbilled services and payments on account were as follows: (in thousands, except percentages) March 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 292,688 $ 268,509 $ 24,179 9.0 % Unearned revenue (payments on account) (332,353 ) (298,992 ) (33,361 ) 11.2 % Net balance $ (39,665 ) $ (30,483 ) $ (9,182 ) 30.1 %

Goodwill (Tables)

Goodwill (Tables)	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	Three months ended Year ended March 31, 2018 December 31, 2017 (in thousands) Opening balance $ 769,058 $ 616,088 Current period acquisitions (Note 6) — 129,222 Prior period acquisitions (Note 6) (627 ) 1,393 Foreign exchange movement 8,038 22,355 Closing balance $ 776,469 $ 769,058

Business Combinations (Tables)

Business Combinations (Tables)	3 Months Ended
	Mar. 31, 2018
Mapi Group [Member]
Schedule of summary of recognized identified assets acquired and liabilities assumed	The table following summarizes the Company’s provisional estimates of the fair values of the assets acquired and liabilities assumed: July 27, 2017 (in thousands) Cash $ 19,649 Property, plant and equipment 3,410 Goodwill* 128,595 Intangible assets** 32,143 Accounts receivable 15,873 Unbilled revenue 8,484 Prepayments and other current assets 3,277 Other receivables 1,430 Income taxes receivable 4,262 Accounts payable (2,994 ) Payments on account (31,237 ) Other liabilities (24,951 ) Non-current other liabilities (1,061 ) Non-current deferred tax liability (11,104 ) Net assets acquired $ 145,776 Cash outflows $ 144,131 Working capital adjustment $ 1,645 Total consideration $ 145,776 *Goodwill represents the acquisition of an established workforce with experience in late phase commercialization, analytics, real world evidence generation and strategic regulatory services in clinical trial services for biologics, drugs and devices. Goodwill related to the business acquired is not tax deductible. **The Company has made an initial estimate of separate intangible assets acquired of $32.1 million , being customer relationships and order book assets. This assessment is under review and will be finalized within 12 months of the date of acquisition.
Clinical Research Management Inc.
Schedule of summary of recognized identified assets acquired and liabilities assumed	The acquisition of ClinicalRM has been accounted for as a business combination in accordance with FASB ASC 805 Business Combinations . The table following summarizes the fair values of the assets acquired and liabilities assumed: September 15, 2016 (in thousands) Cash $ 3,168 Property, plant and equipment 939 Goodwill* 35,969 Customer lists 4,012 Order backlog 1,668 Brand 1,409 Accounts receivable 11,431 Unbilled revenue 3,868 Prepayments and other current assets 1,673 Accounts payable (165 ) Other liabilities (5,569 ) Non-current other liabilities (7 ) Net assets acquired $ 58,396 Total consideration $ 58,396 *Goodwill represents the acquisition of an established workforce with experience in preclinical through Phase IV support of clinical research and clinical trial services for biologics, drugs and devices. Goodwill related to the US portion of the business acquired is tax deductible.

Restructuring (Tables)

Restructuring (Tables)	3 Months Ended
	Mar. 31, 2018
Restructuring and Related Activities [Abstract]
Details of Movement in Restructuring Provisions Recognized	Workforce reductions (in thousands) Initial restructuring charge recorded $ 7,753 Utilized (4,656 ) Provision at December 31, 2017 $ 3,097 Utilized (961 ) Provision at March 31, 2018 $ 2,136
Restructuring Charges	Workforce Reductions Onerous Lease Total (in thousands) Total provision recognized $ 6,190 $ 1,969 $ 8,159 Utilized (5,734 ) (571 ) (6,305 ) Foreign exchange (63 ) — (63 ) Provision at December 31, 2016 $ 393 $ 1,398 $ 1,791 Utilized (393 ) (1,081 ) (1,474 ) Provision at December 31, 2017 $ — $ 317 $ 317 Utilized — (310 ) (310 ) Provision at March 31, 2018 $ — $ 7 $ 7 A restructuring charge of $ 8.8 million was recognized during the year ended December 31, 2014 . Following the closure of the Company’s European Phase 1 services in 2013, the Company recognized a charge in 2014 in relation to its Manchester, United Kingdom facility; $ 5.6 million in relation to asset impairments and $ 3.2 million in relation to an onerous lease charge associated with this facility. We expect this to be paid by 2024. Onerous Lease Asset Impairment Total (in thousands) Total provision recognized $ 3,167 $ 5,629 $ 8,796 Asset write off — (5,629 ) (5,629 ) Provision at December 31, 2014 $ 3,167 $ — $ 3,167 Utilized (1,167 ) — (1,167 ) Provision at December 31, 2015 $ 2,000 $ — $ 2,000 Utilized (1,359 ) — (1,359 ) Provision at December 31, 2016 $ 641 $ — $ 641 Utilized $ (441 ) $ — $ (441 ) Provision at December 31, 2017 $ 200 $ — $ 200 Utilized $ (92 ) $ — $ (92 ) Provision at March 31, 2018 $ 108 $ — $ 108

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense	Income taxes recognized during the three months ended March 31, 2018 , comprise: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Provision for income taxes $ 10,650 $ 11,625

Net income per ordinary share (

Net income per ordinary share (Tables)	3 Months Ended
	Mar. 31, 2018
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: Three Months Ended March 31, 2018 March 31, 2017 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 53,925,560 54,372,931 Effect of dilutive share options outstanding 1,060,230 998,895 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 54,985,790 55,371,826

Share-based Awards (Tables)

Share-based Awards (Tables)	3 Months Ended
	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of share option activity	The following table summarizes option activity for the three months ended March 31, 2018 : Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Life Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 107,794 $ 115.11 $ 35.10 Exercised (21,574 ) $ 33.53 $ 11.72 Forfeited (5,855 ) $ 39.13 $ 12.74 Outstanding at March 31, 2018 1,251,758 $ 61.57 $ 18.81 4.95 Exercisable at March 31, 2018 594,051 $ 45.17 $ 14.54 3.73
Schedule of movement in non-vested share options	The following table summarizes the movement in non-vested share options for the three months ended March 31, 2018 : Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Grant Date Fair Value Non-vested outstanding at December 31, 2017 694,727 $ 68.06 $ 20.03 Granted 107,794 $ 115.11 $ 35.10 Vested (142,109 ) $ 65.43 $ 19.31 Forfeited (2,705 ) $ 56.58 $ 16.84 Non-vested outstanding at March 31, 2018 657,707 $ 76.38 $ 22.67
Schedule of weighted average fair values and assumptions used	The weighted average grant date fair values and assumptions used were as follows: Three Months Ended March 31, 2018 March 31, 2017 Weighted average grant date fair value $ 35.10 $ 24.49 Assumptions: Expected volatility 29 % 29 % Dividend yield — % — % Risk-free interest rate 2.63 % 2.05 % Expected life 5 years 5 years
Schedule of RSU and PSU activity	The following table summarizes RSU and PSU activity for the three months ended March 31, 2018 : PSU Outstanding Number of Shares PSU Weighted Average Grant Date Fair Value PSU Weighted Average Remaining Contractual Life RSU Outstanding Number of Shares RSU Weighted Average Grant Date Fair Value RSU Weighted Average Remaining Contractual Life Outstanding at December 31, 2017 511,026 $ 72.07 0.93 715,970 $ 72.65 1.28 Granted 65,736 $ 115.11 36,838 $ 115.11 Shares vested — $ — (35,776 ) $ 75.05 Forfeited (16,666 ) $ 66.18 (16,424 ) $ 71.11 Outstanding at March 31, 2018 560,096 $ 76.79 0.95 700,608 $ 74.79 1.14
Schedule of non-cash stock compensation expense	Non-cash stock compensation expense for the three months ended March 31, 2018 has been allocated as follows: Three Months Ended March 31, 2018 March 31, 2017 (In thousands) Direct costs $ 5,175 $ 4,854 Selling, general and administrative 4,217 3,955 $ 9,392 $ 8,809

Business Segment Information (T

Business Segment Information (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Distribution of Net Revenue by Geographical Area	a) The distribution of revenue by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 235,110 $ 232,348 Rest of Europe 95,040 72,557 U.S. 225,670 223,082 Rest of World 64,305 50,079 Total $ 620,125 $ 578,066 * All sales shown for Ireland are export sales.
Distribution of Income from Operations by Geographical Area	b) The distribution of income from operations, by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 69,810 $ 63,968 Rest of Europe 5,168 3,699 U.S. 11,802 10,499 Rest of World 4,940 7,492 Total $ 91,720 $ 85,658
Distribution of Property, Plant and Equipment, Net, by Geographical Area	The distribution of property, plant and equipment, net, by geographical area was as follows: March 31, 2018 December 31, 2017 (in thousands) Ireland $ 112,239 $ 111,329 Rest of Europe 8,935 9,026 U.S. 26,654 27,797 Rest of World 13,232 14,899 Total $ 161,060 $ 163,051
Distribution of Depreciation and Amortization by Geographical Area	The distribution of depreciation and amortization by geographical area was as follows: Three Months Ended March 31, 2018 March 31, 2017 (in thousands) Ireland $ 7,885 $ 5,972 Rest of Europe 1,539 1,658 U.S. 6,299 5,948 Rest of World 1,175 870 Total $ 16,898 $ 14,448
Distribution of Total Assets by Geographical Area	The distribution of total assets by geographical area was as follows: March 31, 2018 December 31, 2017 (in thousands) Ireland $ 957,886 $ 880,378 Rest of Europe 506,356 504,418 U.S. 643,831 650,681 Rest of World 111,372 111,141 Total $ 2,219,445 $ 2,146,618

Impact of change in accountin30

Impact of change in accounting policies (Tables)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The tables on the pages following summarize the impact of adopting ASC 606 on the consolidated financial statements for the three months ended March 31, 2018 . ICON plc CONDENSED CONSOLIDATED BALANCE SHEETS AS AT MARCH 31, 2018 (UNAUDITED) March 31, 2018 As reported Adjustments Balance without adoption of Topic 606 ASSETS (in thousands) Current Assets: Cash and cash equivalents $ 272,001 — $ 272,001 Short term investments - available for sale 81,541 — 81,541 Accounts receivable, net 393,940 — 393,940 Unbilled revenue 292,688 43,215 335,903 Other receivables 49,563 (12,268 ) 37,295 Prepayments and other current assets 45,538 — 45,538 Income taxes receivable 28,232 — 28,232 Total current assets 1,163,503 30,947 1,194,450 Other Assets: Property, plant and equipment, net 161,060 — 161,060 Goodwill 776,469 — 776,469 Non-current other assets 15,610 — 15,610 Non-current income taxes receivable 17,530 — 17,530 Non-current deferred tax asset 18,212 (6,574 ) 11,638 Intangible assets 67,061 — 67,061 Total Assets $ 2,219,445 $ 24,373 $ 2,243,818 LIABILITIES AND SHAREHOLDERS’ EQUITY Current Liabilities: Accounts payable $ 11,850 $ — $ 11,850 Payments on account 332,353 (25,100 ) 307,253 Other liabilities 260,998 260,998 Income taxes payable 22,962 432 23,394 Total current liabilities 628,163 (24,668 ) 603,495 Other Liabilities: Non-current bank credit lines and loan facilities 348,980 — 348,980 Non-current other liabilities 15,787 — 15,787 Non-current government grants 977 — 977 Non-current income taxes payable 13,161 — 13,161 Non-current deferred tax liability 7,382 — 7,382 Commitments and contingencies — — — Total Liabilities 1,014,450 (24,668 ) 989,782 Shareholders' Equity: Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 53,793,839 shares issued and outstanding at March 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017 4,643 — 4,643 Additional paid‑in capital 491,338 — 491,338 Other undenominated capital 938 — 938 Accumulated other comprehensive income (26,505 ) — (26,505 ) Retained earnings 734,581 49,041 783,622 Total Shareholders' Equity 1,204,995 49,041 1,254,036 Total Liabilities and Shareholders' Equity $ 2,219,445 $ 24,373 $ 2,243,818 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS FOR THE THREE MONTHS ENDED MARCH 31, 2018 (UNAUDITED) Three Months Ended As reported Adjustments Balance without adoption of Topic 606 (in thousands except share and per share data) Revenue: Revenue $ 620,125 $ 1,231 $ 621,356 Reimbursable expenses — (158,753 ) (158,753 ) 620,125 (157,522 ) 462,603 Costs and expenses: Direct costs - Reimbursable expenses 158,753 (158,753 ) — - Other direct costs 271,839 — 271,839 Selling, general and administrative expense 80,915 160 81,075 Depreciation and amortization 16,898 — 16,898 Total costs and expenses 528,405 (158,593 ) 369,812 Income from operations 91,720 1,071 92,791 Interest income 823 — 823 Interest expense (3,795 ) — (3,795 ) Income before provision for income taxes 88,748 1,071 89,819 Provision for income taxes (10,650 ) (134 ) (10,784 ) Net income $ 78,098 $ 937 $ 79,035 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS FOR THE THREE MONTHS ENDED MARCH 31, 2018 (UNAUDITED) Three Months Ended March 31, 2018 As Reported Adjustments Balance without adoption of Topic 606 (in thousands) Cash flows from operating activities: Net income $ 78,098 $ 937 $ 79,035 Adjustments to reconcile net income to net cash provided by operating activities: Loss on disposal of property, plant and equipment 2 — 2 Depreciation expense 11,850 — 11,850 Amortization of intangibles 5,048 — 5,048 Amortization of government grants (12 ) — (12 ) Interest on short term investments (333 ) — (333 ) Stock compensation expense 9,392 — 9,392 Amortization of gain on interest rate hedge (230 ) — (230 ) Amortization of financing costs 390 — 390 Deferred taxes (3,481 ) 6,574 3,093 Changes in assets and liabilities: Increase in accounts receivable (12,979 ) — (12,979 ) Increase in unbilled revenue (65,904 ) 4,889 (61,015 ) Increase in other receivables (2,866 ) 12,268 9,402 Increase in prepayments and other current assets (10,358 ) — (10,358 ) Increase in other non-current assets (179 ) — (179 ) Decrease in payments on account 8,040 (25,100 ) (17,060 ) Increase in other current liabilities 27,523 — 27,523 Decrease in other non-current liabilities (1,165 ) — (1,165 ) Increase in income taxes payable 4,300 432 4,732 Decrease in accounts payable (6,668 ) — (6,668 ) Net cash provided by operating activities 40,468 — 40,468 Cash flows from investing activities: Purchase of property, plant and equipment (8,304 ) — (8,304 ) Purchase of subsidiary undertakings (1,645 ) — (1,645 ) Purchase of short term investments (10,160 ) — (10,160 ) Sale of short term investments 6,253 — 6,253 Net cash used in investing activities (13,856 ) — (13,856 ) Cash flows from financing activities: Financing costs (823 ) — (823 ) Proceeds from exercise of equity compensation 727 — 727 Share issue costs (4 ) — (4 ) Repurchase of ordinary shares (38,208 ) — (38,208 ) Share repurchase costs (31 ) — (31 ) Net cash provided by financing activities (38,339 ) — (38,339 ) Effect of exchange rate movements on cash 869 869 Net decrease in cash and cash equivalents (10,858 ) — (10,858 ) Cash and cash equivalents at beginning of period 282,859 282,859 Cash and cash equivalents at end of period $ 272,001 $ — $ 272,001 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME (UNAUDITED) Shares Amount Additional Paid-in Capital Other Undenominated Capital Accumulated Other Comprehensive Income Retained Earnings Total (dollars in thousands, except share data) Balance at December 31, 2017 54,081,601 $ 4,664 $ 481,337 $ 912 $ (38,713 ) $ 742,800 $ 1,191,000 Comprehensive income: Net income (as reported) — — — — — 78,098 78,098 Impact of change in accounting policy — — — — — 937 937 Currency translation adjustment — — — — 10,635 26 10,661 Currency impact of long term funding — — — — 1,685 — 1,685 Tax on currency impact of long term funding — — — — 4 — 4 Unrealized capital gain – investments — — — — (288 ) — (288 ) Amortization of interest rate hedge — — — — (230 ) — (230 ) Fair value of cash flow hedge — — — — 402 — 402 Total comprehensive income — — — — 12,208 79,061 91,269 Exercise of share options 21,574 2 722 — — — 724 Issue of restricted share units 35,776 3 — — — — 3 Non-cash stock compensation expense — — 9,283 — — — 9,283 Share issuance costs — — (4 ) — — — (4 ) Share repurchase program (345,112 ) (26 ) — 26 — (38,208 ) (38,208 ) Share repurchase costs — — — — — (31 ) (31 ) Balance at March 31, 2018 (without the adoption of ASC 606) 53,793,839 $ 4,643 $ 491,338 $ 938 $ (26,505 ) $ 783,622 $ 1,254,036

Revenue (Details)

Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 620,125	$ 431,954
Top client
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	65,818	104,682
Clients 2-5
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	164,301	90,265
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	98,539	55,341
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	117,271	70,364
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 174,196	$ 111,302

Contract Balances Contract Asse

Contract Balances Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Billed services (accounts receivable)	$ 403,893		$ 388,431
Unbilled revenue	292,688		268,509
Trade accounts receivable and unbilled revenue	696,581		656,940
Allowance for doubtful accounts	(9,953)		(8,930)
Trade accounts receivable and unbilled revenue, net	686,628		648,010
Change in unbilled receivables	$ 24,179
Change in unbilled receivables, percent	9.00%
Change in customer advances and deposits	$ 33,361
Unearned revenue (payments on account)	$ (332,353)		(298,992)
Change in customer advances and deposits, percent	11.20%
Net balance	$ (39,665)		$ (30,483)
Change in advance payments netted against unbilled contracts receivable	$ 9,182
Change in advance payments netted against unbilled contracts receivable, percent	30.10%
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled revenue	$ 43,215	$ 42,000

Contract Balances Revenue remai

Contract Balances Revenue remaining performance obligations (Details) $ in Billions	3 Months Ended
	Mar. 31, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, percent	40.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 4.9
Revenue, remaining performance obligation, expected timing of satisfaction, period

Goodwill (Detail)

Goodwill (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Opening balance	$ 769,058	$ 616,088
Current period acquisitions (Note 6)	0	129,222
Prior period acquisitions (Note 6)	(627)	1,393
Foreign exchange movement	8,038	22,355
Closing balance	$ 776,469	$ 769,058

Fair value of assets acquired a

Fair value of assets acquired and liabilities assumed of Mapi Group (Detail) - USD ($)	Jul. 27, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 776,469,000		$ 769,058,000	$ 616,088,000
Cash outflows		$ 1,645,000	$ 0
Mapi Group [Member]
Business Acquisition [Line Items]
Cash	$ 19,649,000
Property, plant and equipment	3,410,000
Goodwill	128,595,000
Intangible assets	32,143,000
Accounts receivable	15,873,000
Unbilled revenue	8,484,000
Prepayments and other current assets	3,277,000
Other receivables	1,430,000
Income taxes receivable	4,262,000
Accounts payable	(2,994,000)
Payments on account	(31,237,000)
Other liabilities	(24,951,000)
Non-current other liabilities	(1,061,000)
Non-current deferred tax liability	(11,104,000)
Net assets acquired	145,776,000
Cash outflows	144,131,000
Working capital adjustment	1,645,000
Total consideration	$ 145,776,000

Fair values of assets acquired

Fair values of assets acquired and liabilities assumed of Clinical RM (Detail) - USD ($) $ in Thousands	Sep. 15, 2016	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 776,469	$ 769,058	$ 616,088
Clinical Research Management Inc.
Business Acquisition [Line Items]
Cash	$ 3,168
Property, plant and equipment	939
Goodwill	35,969
Accounts receivable	11,431
Unbilled revenue	3,868
Prepayments and other current assets	1,673
Accounts payable	(165)
Other liabilities	(5,569)
Non-current other liabilities	(7)
Net assets acquired	58,396
Total consideration	58,396
Clinical Research Management Inc. | Order Backlog
Business Acquisition [Line Items]
Intangible assets	1,668
Clinical Research Management Inc. | Customer Lists
Business Acquisition [Line Items]
Intangible assets	4,012
Clinical Research Management Inc. | Brand
Business Acquisition [Line Items]
Intangible assets	$ 1,409

Business Combinations Narrative

Business Combinations Narrative (Details) - USD ($)	Jul. 27, 2017	Sep. 15, 2016	Jun. 30, 2017	Sep. 30, 2017	Mar. 31, 2018	Mar. 31, 2017
Mapi Group [Member]
Business Acquisition [Line Items]
Cash outflows on acquisition	$ 145,800,000
Working capital adjustment	1,645,000
Estimate of intangible assets acquired	$ 32,143,000
Clinical Research Management Inc.
Business Acquisition [Line Items]
Cash outflows on acquisition		$ 52,400,000
Certain payments made on behalf of business acquired		9,200,000
Contingent consideration		$ 12,000,000
Fair value of contingent consideration liability			$ 0		$ 0	$ 6,000,000
Fair value adjustments made to unbilled revenue				$ 1,100,000
Fair value adjustments made to AR acquired				300,000
Fair value of adjustments made to accounts payable				500,000
Fair value adjustments made to other liabilities				$ 1,100,000
Clinical Research Management Inc. | Selling, General and Administrative Expenses [Member]
Business Acquisition [Line Items]
Contingent consideration recorded in SG&A costs			$ 6,000,000

Movement in Restructuring Provi

Movement in Restructuring Provisions (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restructuring Reserve [Roll Forward]
Beginning provision	$ 200,000	$ 641,000	$ 2,000,000	$ 3,167,000
Restructuring	0	7,800,000			$ 8,796,000
Utilization	(92,000)	(441,000)	(1,359,000)	(1,167,000)
Closing provision	108,000	200,000	$ 641,000	$ 2,000,000	$ 3,167,000
Workforce Reduction [Member]
Restructuring Reserve [Roll Forward]
Beginning provision	3,097,000
Restructuring		7,753,000
Utilization	(961,000)	(4,656,000)
Closing provision	$ 2,136,000	$ 3,097,000

Movement in Restructuring Pro39

Movement in Restructuring Provisions incl. Resource Rationalizations (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restructuring Reserve [Roll Forward]
Beginning provision	$ 200,000	$ 641,000	$ 2,000,000	$ 3,167,000
Total provision recognized	0	7,800,000			$ 8,796,000
Utilized	(92,000)	(441,000)	(1,359,000)	(1,167,000)
Closing provision	108,000	200,000	641,000	$ 2,000,000	$ 3,167,000
Resource Rationalizations
Restructuring Reserve [Roll Forward]
Beginning provision	317,000	1,791,000
Total provision recognized			8,159,000
Utilized	(310,000)	(1,474,000)	(6,305,000)
Foreign exchange			(63,000)
Closing provision	7,000	317,000	1,791,000
Workforce Reduction
Restructuring Reserve [Roll Forward]
Beginning provision	3,097,000
Total provision recognized		7,753,000
Utilized	(961,000)	(4,656,000)
Closing provision	2,136,000	3,097,000
Workforce Reduction | Resource Rationalizations
Restructuring Reserve [Roll Forward]
Beginning provision	0	393,000
Total provision recognized			6,190,000
Utilized	0	(393,000)	(5,734,000)
Foreign exchange			(63,000)
Closing provision	0	0	393,000
Onerous Lease | Resource Rationalizations
Restructuring Reserve [Roll Forward]
Beginning provision	317,000	1,398,000
Total provision recognized			1,969,000
Utilized	(310,000)	(1,081,000)	(571,000)
Foreign exchange			0
Closing provision	$ 7,000	$ 317,000	$ 1,398,000

Restructuring Movement in Restr

Restructuring Movement in Restructuring Provisions, Manchester Facility (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restructuring Reserve [Roll Forward]
Beginning provision	$ 200,000	$ 641,000	$ 2,000,000	$ 3,167,000
Total provision recognized	0	7,800,000			$ 8,796,000
Asset write off					(5,629,000)
Utilized	(92,000)	(441,000)	(1,359,000)	(1,167,000)
Closing provision	108,000	200,000	641,000	2,000,000	3,167,000
Onerous Lease [Member] | Manchester Facility [Member]
Restructuring Reserve [Roll Forward]
Beginning provision	200,000	641,000	2,000,000	3,167,000
Total provision recognized					3,167,000
Asset write off					0
Utilized	(92,000)	(441,000)	(1,359,000)	(1,167,000)
Closing provision	108,000	200,000	641,000	2,000,000	3,167,000
Asset Impairment [Member] | Manchester Facility [Member]
Restructuring Reserve [Roll Forward]
Beginning provision	0	0	0	0
Total provision recognized					5,629,000
Asset write off					(5,629,000)
Utilized	0	0	0	0
Closing provision	$ 0	$ 0	$ 0	$ 0	$ 0

Restructuring Narrative (Detail

Restructuring Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Restructuring Cost and Reserve [Line Items]
Restructuring	$ 0	$ 7,800,000		$ 8,796,000
Asset impairment charges				5,629,000
Resource Rationalizations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring			$ 8,159,000
Workforce Reduction [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring		$ 7,753,000
Workforce Reduction [Member] | Resource Rationalizations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring			6,190,000
Onerous Lease [Member] | Resource Rationalizations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring			$ 1,969,000
Onerous Lease [Member] | Manchester Facility [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring				3,167,000
Asset impairment charges				0
Asset Impairment [Member] | Manchester Facility [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring				5,629,000
Asset impairment charges				$ 5,629,000
Other Current Liabilities [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, current	2,100,000
Other Noncurrent Liabilities [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, noncurrent	$ 200,000

Income Taxes Recognized During

Income Taxes Recognized During Period (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 10,650	$ 11,625

Income Taxes Narrative (Details

Income Taxes Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Liability for unrecognized tax benefit	$ 26	$ 26.1
Items generating unrecognized tax benefits	23.6	23.7
Interest and related penalties	$ 2.4	$ 2.4
Earliest Tax Year
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Tax year subject to examination by major tax jurisdictions	2,013
Latest Tax Year
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Tax year subject to examination by major tax jurisdictions	2,017

Reconciliation of Number of Sha

Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Weighted average number of ordinary shares outstanding for basic net income per ordinary share	53,925,560	54,372,931
Effect of dilutive share options outstanding	1,060,230	998,895
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share	54,985,790	55,371,826

Share-based Awards - Narrative

Share-based Awards - Narrative (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 14, 2017	May 11, 2015	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 31, 2018
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Limit of shares issued or to be issued pursuant to options granted			1,000,000
Non-cash stock compensation expense not yet recognized			$ 53.7			$ 53.7
Unrecognized stock-based compensation expense, weighted average period (years)			2 years 4 months 11 days
Restricted Stock Units (RSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 2.7		$ 16.6
Stock units granted			36,838
Performance Share Unit (PSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested					$ 15
Stock units granted			65,736
PSUs Based on Service and EPS Targets
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units granted						301,932
PSUs Based on Service and EPS Targets | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Cumulative EPS vesting period			2,015
PSUs Based on Service and EPS Targets | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units to be granted			258,164			258,164
Cumulative EPS vesting period			2,021
Employee Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance			6,000,000			6,000,000
Employee Stock Plan, 2008 Plan | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Additional number of ordinary shares which have been reserved for issuance	400,000
Percentage of option price for fair value of ordinary share			100.00%
Employee Stock Plan, 2008 Plan | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Additional number of ordinary shares which have been reserved for issuance	1,000,000
Employee Stock Plan, 2003 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance			6,000,000			6,000,000
Maximum number of award as percentage of shares outstanding			10.00%
Employee Stock Plan, 2003 Plan | Individual Employee
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance			400,000			400,000
Employee Stock Option
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Shares vesting period			5 years
Shares expiration period			8 years
Weighted average contractual term of options outstanding			4 years 11 months 12 days
Weighted average fair value per option			$ 13.75
Exercisable - number of shares			594,051			594,051
Exercisable - weighted average remaining contractual life			3 years 8 months 23 days
Exercisable - weighted average exercise price			$ 45.17			$ 45.17
Fully vested total intrinsic value			$ 43.3			$ 43.3
Intrinsic value of option exercised			$ 1.8	$ 7.4
Employee Stock Option | Minimum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Options outstanding fair value			$ 8.53			$ 8.53
Employee Stock Option | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Weighted average contractual term of options outstanding			8 years
Options outstanding fair value			$ 35.10			$ 35.10
Restricted Stock Units 2008
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance			1,000,000			1,000,000
Restricted Stock Units 2013
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Additional number of ordinary shares which have been reserved for issuance		2,500,000
Ordinary shares which have been reserved for issuance		4,100,000

Summary of Stock Option Activit

Summary of Stock Option Activity (Detail) - Employee Stock Option - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Options Outstanding Number of Shares
Outstanding at beginning of period (in shares)	1,171,393
Granted (in shares)	107,794
Exercised (in shares)	(21,574)
Forfeited (in shares)	(5,855)
Outstanding at end of period (in shares)	1,251,758
Exercisable at end of period	594,051
Weighted Average Exercise Price
Outstanding at beginning of period (USD per share)	$ 56.02
Granted (USD per share)	115.11
Exercised (USD per share)	33.53
Forfeited (USD per share)	39.13
Outstanding at end of period (USD per share)	61.57
Exercisable at end of period	45.17
Weighted Average Fair Value
Outstanding at beginning of period (USD per share)	17.15
Granted (USD per share)	35.10	$ 24.49
Exercised (USD per share)	11.72
Forfeited (USD per share)	12.74
Outstanding at end of period (USD per share)	18.81
Exercisable at end of period	$ 14.54
Weighted Average Remaining Contractual Life
Outstanding at end of period	4 years 11 months 12 days
Exercisable at end of period	3 years 8 months 23 days

Summary of Movement in Non-Vest

Summary of Movement in Non-Vested Share Options (Detail) - Employee Stock Option - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Options Outstanding Number of Shares
Beginning Balance (in shares)	694,727
Granted (in shares)	107,794
Vested (in shares)	(142,109)
Forfeited (in shares)	(2,705)
Ending Balance (in shares)	657,707
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 68.06
Granted (USD per share)	115.11
Vested (USD per share)	65.43
Forfeited (USD per share)	56.58
Ending balance (USD per share)	76.38
Weighted Average Fair Value
Beginning Balance (USD per share)	20.03
Granted (USD per share)	35.10	$ 24.49
Vested (USD per share)	19.31
Forfeited (USD per share)	16.84
Ending Balance (USD per share)	$ 22.67

Schedule of Weighted Average Fa

Schedule of Weighted Average Fair Values and Assumptions Used (Detail) - Employee Stock Option - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value	$ 35.10	$ 24.49
Assumptions:
Expected volatility	29.00%	29.00%
Dividend yield	0.00%	0.00%
Risk-free interest rate	2.63%	2.05%
Expected life	5 years	5 years

Summary of RSU and PSU Activity

Summary of RSU and PSU Activity (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Performance Share Unit (PSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	511,026
Granted (in shares)	65,736
Shares vested (in shares)	0
Forfeited (in shares)	(16,666)
Outstanding at ending of period (in share)	560,096	511,026
Weighted Average Fair Value
Outstanding at beginning of period (USD per share)	$ 72.07
Granted (USD per share)	115.11
Shares vested (USD per share)	0
Forfeited (USD per share)	66.18
Outstanding at end of period (USD per share)	$ 76.79	$ 72.07
Weighted Average Remaining Contractual Life
Outstanding at end of period	11 months 12 days	11 months 5 days
Restricted Stock Units (RSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	715,970
Granted (in shares)	36,838
Shares vested (in shares)	(35,776)
Forfeited (in shares)	(16,424)
Outstanding at ending of period (in share)	700,608	715,970
Weighted Average Fair Value
Outstanding at beginning of period (USD per share)	$ 72.65
Granted (USD per share)	115.11
Shares vested (USD per share)	75.05
Forfeited (USD per share)	71.11
Outstanding at end of period (USD per share)	$ 74.79	$ 72.65
Weighted Average Remaining Contractual Life
Outstanding at end of period	1 year 1 month 21 days	1 year 3 months 11 days

Schedule of Non-cash Stock Comp

Schedule of Non-cash Stock Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 9,392	$ 8,809
Direct costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	5,175	4,854
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 4,217	$ 3,955

Share Capital Narrative (Detail

Share Capital Narrative (Details) - USD ($)	Mar. 31, 2018	Oct. 03, 2016	Mar. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Ordinary shares repurchased	3,363,526		345,112
Ordinary shares repurchased, value	$ 281,300,000		$ 38,200,000
Buyback Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount		$ 400,000,000
Buyback Program | Maximum
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized percentage		10.00%

Distribution of Net Revenue by

Distribution of Net Revenue by Geographical Area (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenue	$ 620,125	$ 578,066
Ireland
Segment Reporting Information [Line Items]
Revenue	235,110	232,348
Rest of Europe
Segment Reporting Information [Line Items]
Revenue	95,040	72,557
U.S.
Segment Reporting Information [Line Items]
Revenue	225,670	223,082
Rest of World
Segment Reporting Information [Line Items]
Revenue	$ 64,305	$ 50,079

Distribution Income from Operat

Distribution Income from Operations, including Restructuring, by Geographical Area (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Income from operations	$ 91,720	$ 85,658
Ireland
Segment Reporting Information [Line Items]
Income from operations	69,810	63,968
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations	5,168	3,699
U.S.
Segment Reporting Information [Line Items]
Income from operations	11,802	10,499
Rest of World
Segment Reporting Information [Line Items]
Income from operations	$ 4,940	$ 7,492

Distribution of Property, Plant

Distribution of Property, Plant and Equipment, Net, by Geographical Area (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 161,060	$ 163,051
Ireland
Segment Reporting Information [Line Items]
Property, plant and equipment, net	112,239	111,329
Rest of Europe
Segment Reporting Information [Line Items]
Property, plant and equipment, net	8,935	9,026
U.S.
Segment Reporting Information [Line Items]
Property, plant and equipment, net	26,654	27,797
Rest of World
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 13,232	$ 14,899

Distribution of Depreciation an

Distribution of Depreciation and Amortization by Geographical Area (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Depreciation and amortization	$ 16,898	$ 14,448
Ireland
Segment Reporting Information [Line Items]
Depreciation and amortization	7,885	5,972
Rest of Europe
Segment Reporting Information [Line Items]
Depreciation and amortization	1,539	1,658
U.S.
Segment Reporting Information [Line Items]
Depreciation and amortization	6,299	5,948
Rest of World
Segment Reporting Information [Line Items]
Depreciation and amortization	$ 1,175	$ 870

Distribution of Total Assets by

Distribution of Total Assets by Geographical Area (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 2,219,445	$ 2,146,618
Ireland
Segment Reporting Information [Line Items]
Assets	957,886	880,378
Rest of Europe
Segment Reporting Information [Line Items]
Assets	506,356	504,418
U.S.
Segment Reporting Information [Line Items]
Assets	643,831	650,681
Rest of World
Segment Reporting Information [Line Items]
Assets	$ 111,372	$ 111,141

Business Segment Information Na

Business Segment Information Narrative (Details)	3 Months Ended
	Mar. 31, 2018segment
Segment Reporting [Abstract]
Number of operating segments	1

Impact of change in accountin58

Impact of change in accounting policies Balance sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 272,001		$ 282,859	$ 237,299	$ 192,541
Short term investments - available for sale	81,541		77,589
Accounts receivable, net	393,940		379,501
Unbilled revenue	292,688		268,509
Other receivables	49,563		33,798
Prepayments and other current assets	45,538		34,377
Income taxes receivable	28,232		24,385
Total current assets	1,163,503		1,101,018
Other Assets:
Property, plant and equipment, net	161,060		163,051
Goodwill	776,469		769,058		$ 616,088
Non-current other assets	15,610		15,393
Non-current income taxes receivable	17,530		18,396
Non-current deferred tax asset	18,212		8,074
Intangible assets	67,061		71,628
Total Assets	2,219,445		2,146,618
Current Liabilities:
Accounts payable	11,850		18,590
Payments on account	332,353		298,992
Other liabilities	260,998		233,503
Income taxes payable	22,962		14,973
Total current liabilities	628,163		566,058
Other Liabilities:
Non-current bank credit lines and loan facilities	348,980		348,888
Non-current other liabilities	15,787		17,111
Non-current government grants	977		966
Non-current income taxes payable	13,161		14,879
Non-current deferred tax liability	7,382		7,716
Commitments and contingencies	0		0
Total Liabilities	1,014,450		955,618
Shareholders' Equity:
Common Stock, Value, Issued	4,643		4,664
Additional paid‑in capital	491,338		481,337
Other undenominated capital	938		912
Accumulated other comprehensive income	(26,505)		(38,713)
Retained earnings	734,581		742,800
Total Shareholders' Equity	1,204,995		1,191,000
Total Liabilities and Shareholders' Equity	2,219,445		2,146,618
Accounting Standards Update 2014-09 [Member]
Shareholders' Equity:
Total Shareholders' Equity			1,142,896
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Current Assets:
Cash and cash equivalents	0
Short term investments - available for sale	0
Accounts receivable, net	0
Unbilled revenue	43,215	$ 42,000
Other receivables	(12,268)
Prepayments and other current assets	0
Income taxes receivable	0
Total current assets	30,947
Other Assets:
Property, plant and equipment, net	0
Goodwill	0
Non-current other assets	0
Non-current income taxes receivable	0
Non-current deferred tax asset	(6,574)
Intangible assets	0
Total Assets	24,373
Current Liabilities:
Accounts payable	0
Payments on account	(25,100)
Other liabilities
Income taxes payable	432
Total current liabilities	(24,668)
Other Liabilities:
Non-current bank credit lines and loan facilities	0
Non-current other liabilities	0
Non-current government grants	0
Non-current income taxes payable	0
Non-current deferred tax liability	0
Commitments and contingencies	0
Total Liabilities	(24,668)
Shareholders' Equity:
Common Stock, Value, Issued	0
Additional paid‑in capital	0
Other undenominated capital	0
Accumulated other comprehensive income	0
Retained earnings	49,041
Total Shareholders' Equity	49,041
Total Liabilities and Shareholders' Equity	24,373
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Shareholders' Equity:
Total Shareholders' Equity	1,254,036
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Current Assets:
Cash and cash equivalents	272,001		$ 282,859
Short term investments - available for sale	81,541
Accounts receivable, net	393,940
Unbilled revenue	335,903
Other receivables	37,295
Prepayments and other current assets	45,538
Income taxes receivable	28,232
Total current assets	1,194,450
Other Assets:
Property, plant and equipment, net	161,060
Goodwill	776,469
Non-current other assets	15,610
Non-current income taxes receivable	17,530
Non-current deferred tax asset	11,638
Intangible assets	67,061
Total Assets	2,243,818
Current Liabilities:
Accounts payable	11,850
Payments on account	307,253
Other liabilities	260,998
Income taxes payable	23,394
Total current liabilities	603,495
Other Liabilities:
Non-current bank credit lines and loan facilities	348,980
Non-current other liabilities	15,787
Non-current government grants	977
Non-current income taxes payable	13,161
Non-current deferred tax liability	7,382
Commitments and contingencies	0
Total Liabilities	989,782
Shareholders' Equity:
Common Stock, Value, Issued	4,643
Additional paid‑in capital	491,338
Other undenominated capital	938
Accumulated other comprehensive income	(26,505)
Retained earnings	783,622
Total Shareholders' Equity	1,254,036
Total Liabilities and Shareholders' Equity	$ 2,243,818

Impact of change in accountin59

Impact of change in accounting policies Statement of operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues [Abstract]
Revenue	$ 620,125	$ 578,066
Reimbursable expenses	0	(146,112)
Revenue, Net	620,125	431,954
Costs and expenses [Abstract]
Direct costs
Direct Costs, Reimbursable Expenses	158,753	0
Direct Costs, Other	271,839	250,459
Selling, general and administrative expense	80,915	81,389
Depreciation and amortization	16,898	14,448
Total costs and expenses	528,405	346,296
Income from operations	91,720	85,658
Interest income	823	555
Interest expense	(3,795)	(3,179)
Income before provision for income taxes	88,748	83,034
Provision for income taxes	(10,650)	(11,625)
Net income	78,098	$ 71,409
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Costs and expenses [Abstract]
Net income	937
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenues [Abstract]
Revenue	1,231
Reimbursable expenses	(158,753)
Revenue, Net	(157,522)
Costs and expenses [Abstract]
Direct costs
Direct Costs, Reimbursable Expenses	(158,753)
Direct Costs, Other	0
Selling, general and administrative expense	160
Depreciation and amortization	0
Total costs and expenses	(158,593)
Income from operations	1,071
Interest income	0
Interest expense	0
Income before provision for income taxes	1,071
Provision for income taxes	(134)
Net income	937
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenues [Abstract]
Revenue	621,356
Reimbursable expenses	(158,753)
Revenue, Net	462,603
Costs and expenses [Abstract]
Direct costs
Direct Costs, Reimbursable Expenses	0
Direct Costs, Other	271,839
Selling, general and administrative expense	81,075
Depreciation and amortization	16,898
Total costs and expenses	369,812
Income from operations	92,791
Interest income	823
Interest expense	(3,795)
Income before provision for income taxes	89,819
Provision for income taxes	(10,784)
Net income	$ 79,035

Impact of change in accountin60

Impact of change in accounting policies Statement of cash flows (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities [Abstract]
Net income	$ 78,098,000	$ 71,409,000
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	2,000	0
Depreciation expense	11,850,000	10,018,000
Amortization of intangibles	5,048,000	4,430,000
Amortization of government grants	(12,000)	(10,000)
Interest on short term investments	(333,000)	(251,000)
Stock compensation expense	9,392,000	8,809,000
Amortization of gain on interest rate hedge	(230,000)	(230,000)
Amortization of financing costs	390,000	140,000
Deferred taxes	(3,481,000)	(3,593,000)
Increase in accounts receivable	(12,979,000)	76,478,000
Increase in unbilled revenue	(65,904,000)	(32,944,000)
Increase in other receivables	(2,866,000)	(3,103,000)
Increase in prepayments and other current assets	(10,358,000)	(2,522,000)
Increase in other non-current assets	(179,000)	(526,000)
Decrease in payments on account	8,040,000	(6,115,000)
Increase in other current liabilities	27,523,000	22,568,000
Decrease in other non-current liabilities	(1,165,000)	1,082,000
Increase in income taxes payable	4,300,000	8,839,000
Decrease in accounts payable	(6,668,000)	4,848,000
Net cash provided by operating activities	40,468,000	159,327,000
Cash flows from investing activities [Abstract]
Purchase of property, plant and equipment	(8,304,000)	(8,298,000)
Purchase of subsidiary undertakings	(1,645,000)	0
Purchase of short term investments	(10,160,000)	(17,977,000)
Sale of short term investments	6,253,000	4,675,000
Net cash used in investing activities	(13,856,000)	(21,600,000)
Cash flows from financing activities [Abstract]
Financing costs	(823,000)	0
Proceeds from exercise of equity compensation	727,000	2,780,000
Share issue costs	(4,000)	(7,000)
Repurchase of ordinary shares	(38,208,000)	(96,404,000)
Share repurchase costs	(31,000)	(77,000)
Net cash used in financing activities	(38,339,000)	(93,708,000)
Effect of exchange rate movements on cash	869,000	739,000
Net decrease in cash and cash equivalents	(10,858,000)	44,758,000
Cash and cash equivalents at beginning of period	282,859,000	192,541,000
Cash and cash equivalents at end of period	272,001,000	$ 237,299,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Cash flows from operating activities [Abstract]
Net income	937,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Cash flows from operating activities [Abstract]
Net income	937,000
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	0
Depreciation expense	0
Amortization of intangibles	0
Amortization of government grants	0
Interest on short term investments	0
Stock compensation expense	0
Amortization of gain on interest rate hedge	0
Amortization of financing costs	0
Deferred taxes	6,574,000
Increase in accounts receivable	0
Increase in unbilled revenue	4,889,000
Increase in other receivables	12,268,000
Increase in prepayments and other current assets	0
Increase in other non-current assets	0
Decrease in payments on account	(25,100,000)
Increase in other current liabilities	0
Decrease in other non-current liabilities	0
Increase in income taxes payable	432,000
Decrease in accounts payable	0
Net cash provided by operating activities	0
Cash flows from investing activities [Abstract]
Purchase of property, plant and equipment	0
Purchase of subsidiary undertakings	0
Purchase of short term investments	0
Sale of short term investments	0
Net cash used in investing activities	0
Cash flows from financing activities [Abstract]
Proceeds from exercise of equity compensation	0
Share issue costs	0
Repurchase of ordinary shares	0
Share repurchase costs	0
Net cash used in financing activities	0
Effect of exchange rate movements on cash
Net decrease in cash and cash equivalents	0
Cash and cash equivalents at beginning of period
Cash and cash equivalents at end of period	0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Cash flows from financing activities [Abstract]
Financing costs	(823,000)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Cash flows from operating activities [Abstract]
Net income	79,035,000
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	2,000
Depreciation expense	11,850,000
Amortization of intangibles	5,048,000
Amortization of government grants	(12,000)
Interest on short term investments	(333,000)
Stock compensation expense	9,392,000
Amortization of gain on interest rate hedge	(230,000)
Amortization of financing costs	390,000
Deferred taxes	3,093,000
Increase in accounts receivable	(12,979,000)
Increase in unbilled revenue	(61,015,000)
Increase in other receivables	9,402,000
Increase in prepayments and other current assets	(10,358,000)
Increase in other non-current assets	(179,000)
Decrease in payments on account	(17,060,000)
Increase in other current liabilities	27,523,000
Decrease in other non-current liabilities	(1,165,000)
Increase in income taxes payable	4,732,000
Decrease in accounts payable	(6,668,000)
Net cash provided by operating activities	40,468,000
Cash flows from investing activities [Abstract]
Purchase of property, plant and equipment	(8,304,000)
Purchase of subsidiary undertakings	(1,645,000)
Purchase of short term investments	(10,160,000)
Sale of short term investments	6,253,000
Net cash used in investing activities	(13,856,000)
Cash flows from financing activities [Abstract]
Proceeds from exercise of equity compensation	727,000
Share issue costs	(4,000)
Repurchase of ordinary shares	(38,208,000)
Share repurchase costs	(31,000)
Net cash used in financing activities	(38,339,000)
Effect of exchange rate movements on cash	869,000
Net decrease in cash and cash equivalents	(10,858,000)
Cash and cash equivalents at beginning of period	282,859,000
Cash and cash equivalents at end of period	$ 272,001,000

Impact of change in accountin61

Impact of change in accounting policies Statement of shareholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Ordinary shares, shares issued	53,793,839		54,081,601
Beginning balance	$ 1,191,000
Net income	78,098	$ 71,409
Currency translation adjustment	10,661
Currency impact of long term funding	1,685
Tax on currency impact of long term funding	(4)
Unrealized capital gain – investments	(288)
Amortization of interest rate hedge	(230)
Fair value of cash flow hedge	402
Total comprehensive income	90,332
Exercise of share options	724
Issue of restricted share units	3
Non-cash stock compensation expense	(4)
Share issuance costs	9,283
Share repurchase program	38,208
Share repurchase costs	31
Ending balance	1,204,995
Common Stock [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Ordinary shares, shares issued			54,081,601
Beginning balance	$ 4,664
Exercise of share options (in shares)	21,574
Exercise of share options	$ 2
Issue of restricted share units (in shares)	35,776
Issue of restricted share units	$ 3
Share repurchase program (in shares)	(345,112)
Share repurchase program	$ 26
Additional Paid-in Capital
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	481,337
Exercise of share options	722
Non-cash stock compensation expense	(4)
Share issuance costs	9,283
Other Undenominated Capital
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	912
Share repurchase program	(26)
AOCI Attributable to Parent [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	(38,713)
Currency translation adjustment	10,635
Currency impact of long term funding	1,685
Tax on currency impact of long term funding	(4)
Unrealized capital gain – investments	(288)
Amortization of interest rate hedge	(230)
Fair value of cash flow hedge	402
Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Beginning balance	742,800
Net income	78,098
Currency translation adjustment	26
Total comprehensive income	78,124
Share repurchase program	38,208
Share repurchase costs	31
Ending balance	734,581
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	937
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	937
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total comprehensive income	91,269
Ending balance	$ 1,254,036
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Common Stock [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Ordinary shares, shares issued	53,793,839
Ending balance	$ 4,643
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Additional Paid-in Capital
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Ending balance	491,338
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other Undenominated Capital
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Ending balance	938
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | AOCI Attributable to Parent [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total comprehensive income	12,208
Ending balance	(26,505)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Retained Earnings [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total comprehensive income	79,061
Ending balance	$ 783,622