ICON Public Limited (ICLR) 20-F 1 Mar 19 2018 FY Annual report (foreign) Financial data

Document and Entity Information

Document and Entity Information	12 Months Ended
	Dec. 31, 2018shares
Document And Entity Information [Abstract]
Document Type	20-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	ICLR
Entity Registrant Name	ICON PLC
Entity Central Index Key	1,060,955
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Well Known Seasoned Issuer	Yes
Entity Filler Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding	53,971,706

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 395,851	$ 282,859	$ 192,541	$ 103,911
Available for sale investments (Note 3a)	59,910	77,589	68,046
Accounts receivable, net (Note 17)	414,791	379,501
Unbilled revenue (Note 17)	362,926	268,509
Other receivables	40,459	33,798
Prepayments and other current assets	36,801	34,377
Income taxes receivable (Note 13)	19,445	24,385
Total current assets	1,330,183	1,101,018
Other Assets:
Property, plant and equipment, net (Note 6)	158,669	163,051
Goodwill (Note 4)	756,260	769,058	616,088
Other non-current assets	14,525	15,393
Non-current income taxes receivable (Note 13)	20,023	18,396
Non-current deferred tax asset (Note 13)	13,577	8,074
Investments in equity- long term (Note 3b)	6,963	0
Intangible assets (Note 5)	54,055	71,628
Total Assets	2,354,255	2,146,618
Current Liabilities:
Accounts payable	13,288	18,590
Payments on account (Note 17)	274,468	298,992
Other liabilities (Note 7)	317,143	233,503
Income taxes payable (Note 13)	5,724	14,973
Total current liabilities	610,623	566,058
Other Liabilities:
Non-current bank credit lines and loan facilities (Note 22)	349,264	348,888
Non-current other liabilities (Note 8)	13,446	17,111
Non-current government grants (Note 11)	877	966
Non-current income taxes payable (Note 13)	17,551	14,879
Non-current deferred tax liability (Note 13)	8,213	7,716
Commitments and contingencies (Note 15)	0	0
Total Liabilities	999,974	955,618
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, (Note 12), 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017.	4,658	4,664
Additional paid-in capital	529,642	481,337
Other undenominated capital (Note 12 (a))	983	912
Accumulated other comprehensive income (Note 21)	(69,328)	(38,713)
Retained earnings	888,326	742,800
Total Shareholders' Equity	1,354,281	1,191,000	$ 945,174	$ 763,096
Total Liabilities and Shareholders' Equity	$ 2,354,255	$ 2,146,618

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - € / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Ordinary shares, par value (in EUR per share)	€ 0.06	€ 0.06
Ordinary shares, shares authorized (in shares)	100,000,000	100,000,000
Ordinary shares, shares issued (in shares)	53,971,706	54,081,601
Ordinary shares, shares outstanding (in shares)	53,971,706	54,081,601

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Gross revenue	$ 2,595,777	$ 2,402,321	$ 2,364,956
Reimbursable expenses		(643,882)	(698,469)
Total revenue	2,595,777	1,758,439	1,666,487
Costs and expenses:
Selling, general and administrative	325,794	323,741	325,726
Depreciation and amortization	65,916	61,297	59,575
Restructuring (Note 14)	12,490	7,753	8,159
Total costs and expenses	2,222,420	1,420,101	1,354,793
Income from operations	373,357	338,338	311,694
Interest income	4,759	2,346	1,484
Interest expense	(13,502)	(12,627)	(13,006)
Income before income taxes expense	364,614	328,057	300,172
Income tax expense (Note 13)	(41,958)	(46,569)	(37,993)
Net income	$ 322,656	$ 281,488	$ 262,179
Net income per ordinary share:
Basic (dollars per share)	$ 5.96	$ 5.20	$ 4.75
Diluted (dollars per share)	$ 5.89	$ 5.13	$ 4.65
Weighted average number of ordinary shares outstanding:
Basic (in shares)	54,118,764	54,129,439	55,248,900
Diluted (in shares)	54,790,663	54,849,046	56,407,136
Direct Costs
Costs and expenses:
Direct costs	$ 1,818,220	$ 1,027,310	$ 961,333

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 322,656	$ 281,488	$ 262,179
Other comprehensive income, net of tax
Currency translation adjustment	(26,522)	33,966	(12,839)
Currency impact of long-term funding	(4,834)	13,730	(8,428)
Unrealized capital (loss)/gain– investments	(155)	(272)	11
Actuarial gain/(loss) on defined benefit pension plan	2,855	50	(2,485)
Amortization of interest rate hedge	(923)	(923)	(923)
Fair value of cash flow hedge	(1,036)	1,036	0
Total comprehensive income	$ 292,041	$ 329,075	$ 237,515

Consolidated Statements of Shar

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	Retained Earnings
Beginning balance (in shares) at Dec. 31, 2015		54,958,912
Beginning balance at Dec. 31, 2015	$ 763,096	$ 4,719	$ 383,355	$ 715	$ (61,636)	$ 435,943
Comprehensive Income:
Net income	262,179					262,179
Currency translation adjustment	(12,839)				(12,839)
Currency impact of long-term funding	(8,428)				(8,428)
Unrealized capital loss - investments	11				11
Actuarial gain/(loss) on defined benefit pension plan	(2,485)				(2,485)
Amortization of interest rate hedge	(923)				(923)
Fair value of cash flow hedge	0
Total comprehensive income	237,515
Exercise of share options (in shares)		393,240
Exercise of share options	10,139	$ 26	10,113
Issue of restricted share units / performance share units (in shares)		607,878
Issue of restricted share units / performance share units	41	$ 41
Share based compensation expense	40,343		40,343
Share issue costs	(17)		(17)
Repurchase of ordinary shares (in shares)		(1,429,187)
Repurchase of ordinary shares	(110,000)	$ (94)		94		(110,000)
Share repurchase costs	(275)					(275)
Excess income tax benefit on exercise of equity compensation	4,332		4,332
Ending balance (in shares) at Dec. 31, 2016		54,530,843
Ending balance at Dec. 31, 2016	945,174	$ 4,692	438,126	809	(86,300)	587,847
Comprehensive Income:
Net income	281,488					281,488
Currency translation adjustment	33,966				33,966
Currency impact of long-term funding	13,730				13,730
Unrealized capital loss - investments	(272)				(272)
Actuarial gain/(loss) on defined benefit pension plan	50				50
Amortization of interest rate hedge	(923)				(923)
Fair value of cash flow hedge	1,036				1,036
Total comprehensive income	329,075
Exercise of share options (in shares)		458,243
Exercise of share options	13,906	$ 31	13,875
Issue of restricted share units / performance share units (in shares)		681,742
Issue of restricted share units / performance share units	44	$ 44
Share based compensation expense	29,351		29,351
Share issue costs	(15)		(15)
Repurchase of ordinary shares (in shares)		(1,589,227)
Repurchase of ordinary shares	(133,106)	$ (103)		103		(133,106)
Share repurchase costs	$ (106)					(106)
Ending balance (in shares) at Dec. 31, 2017	54,081,601	54,081,601
Ending balance at Dec. 31, 2017	$ 1,191,000	$ 4,664	481,337	912	(38,713)	742,800
Comprehensive Income:
Cumulative effect adjustment from adoption of new accounting principal | Accounting Standards Update 2016-09	6,677					6,677
Net income	322,656					322,656
Currency translation adjustment	(26,522)				(26,522)
Currency impact of long-term funding	(4,834)				(4,834)
Unrealized capital loss - investments	(155)				(155)
Actuarial gain/(loss) on defined benefit pension plan	2,855				2,855
Amortization of interest rate hedge	(923)				(923)
Fair value of cash flow hedge	(1,036)				(1,036)
Total comprehensive income	292,041
Exercise of share options (in shares)		408,699
Exercise of share options	16,806	$ 29	16,777
Issue of restricted share units / performance share units (in shares)		489,568
Issue of restricted share units / performance share units	36	$ 36
Share based compensation expense	31,544		31,544
Share issue costs	(16)		(16)
Repurchase of ordinary shares (in shares)		(1,008,162)
Repurchase of ordinary shares	(128,960)	$ (71)		71		(128,960)
Share repurchase costs	$ (66)					(66)
Ending balance (in shares) at Dec. 31, 2018	53,971,706	53,971,706
Ending balance at Dec. 31, 2018	$ 1,354,281	$ 4,658	$ 529,642	$ 983	$ (69,328)	$ 888,326

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 322,656	$ 281,488	$ 262,179
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	70	228	151
Depreciation expense	50,565	43,436	42,125
Amortization of intangibles	15,351	17,861	17,450
Amortization of government grants	(47)	(44)	(44)
Interest on short term investments	(1,329)	(1,088)	(823)
Realized (gain)/loss on sale of short term investments	(56)	(112)	(50)
Gain on re-measurement of financial assets	(800)	0	0
Amortization of gain on interest rate hedge	(923)	(923)	(923)
Amortization of financing costs	812	556	566
Stock compensation expense	31,594	30,573	40,343
Deferred tax expense	1,652	10,729	1,545
Changes in assets and liabilities:
(Increase)/decrease in accounts receivable	(37,557)	57,747	2,526
Increase in unbilled revenue	(98,510)	(62,491)	(16,753)
Decrease/(increase) in other receivables	3,107	1,771	(1,829)
(Increase)/decrease in prepayments and other current assets	(3,237)	4,359	1,872
Decrease/(increase) in other non-current assets	856	(1,524)	(2,157)
Decrease in payments on account	(6,253)	(7,174)	(45,754)
Increase/(decrease) in other current liabilities	2,009	6,679	(44,713)
(Decrease)/increase in other non-current liabilities	(1,034)	(3,710)	3,008
Decrease in income taxes payable	(5,220)	(2,293)	(690)
(Decrease)/increase in accounts payable	(5,067)	7,014	1,175
Net cash provided by operating activities	268,639	383,082	259,204
Cash flows from investing activities:
Purchase of property, plant and equipment	(48,397)	(44,717)	(42,601)
Purchase of subsidiary undertakings	(1,645)	(144,131)	(54,209)
Cash acquired with subsidiary undertaking	0	19,649	3,168
Sale of available for sale investments	99,865	33,086	40,858
Purchase of available for sale investments	(80,956)	(41,701)	(22,030)
Purchase of investments in equity - long term	(6,163)	0	0
Net cash used in investing activities	(37,296)	(177,814)	(74,814)
Cash flows from financing activities:
Financing costs	(823)	0	0
Drawdown of credit lines and facilities	0	0	73,000
Repayment of credit lines and facilities	0	0	(73,000)
Proceeds from the exercise of equity compensation	16,842	13,950	10,180
Share issue costs	(16)	(15)	(17)
Excess tax benefit on exercise of equity compensation	0	0	6,402
Repurchase of ordinary shares	(128,960)	(133,106)	(110,000)
Share repurchase costs	(66)	(106)	(275)
Net cash used in financing activities	(113,023)	(119,277)	(93,710)
Effect of exchange rate movements on cash	(5,328)	4,327	(2,050)
Net increase in cash and cash equivalents	112,992	90,318	88,630
Cash and cash equivalents at beginning of year	282,859	192,541	103,911
Cash and cash equivalents at end of year	$ 395,851	$ 282,859	$ 192,541

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of business ICON plc and its subsidiaries ("the Company" or "ICON") is a clinical research organization ("CRO"), providing outsourced development services on a global basis to the pharmaceutical, biotechnology and medical device industries. We specialize in the strategic development, management and analysis of programs that support all stages of the clinical development process from compound selection to Phase I-IV clinical studies. Our vision is to be the Global CRO partner of choice in drug development by delivering best in class information, solutions and performance in clinical and outcomes research. We believe that we are one of a select group of CROs with the expertise and capability to conduct clinical trials in most major therapeutic areas on a global basis and have the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full service" solution. At December 31, 2018 we had approximately 13,670 employees, in 89 locations in 37 countries. During the year ended December 31, 2018 , we derived approximately 34.5% , 55.7% and 9.8% of our net revenue in the United States, Europe and Rest of World, respectively. We began operations in 1990 and have expanded our business predominately through internal growth, together with a number of strategic acquisitions to enhance our capabilities and expertise in certain areas of the clinical development process. We are incorporated in Ireland and our principal executive office is located at: South County Business Park, Leopardstown, Dublin 18, Republic of Ireland. The contact telephone number of this office is +353 1 2912000.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies The accounting policies noted below were applied in the preparation of the accompanying financial statements of the Company and are in conformity with accounting principles generally accepted in the United States. (a) Basis of consolidation The consolidated financial statements include the financial statements of the Company and all of its subsidiaries. All significant intercompany profits, transactions and account balances have been eliminated. The results of subsidiary undertakings acquired in the period are included in the Consolidated Statement of Operations from the date of acquisition. (b) Use of estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reported period. Actual results could differ from those estimates. The principal management estimates and judgments used in preparing the financial statements relate to revenue recognition and taxation. (c) Revenue recognition The Company primarily earns revenues by providing a number of different services to its customers. These services, which are integral elements of the clinical development process, include clinical trials management, consulting, contract staffing, and laboratory services. Contracts range in duration from a number of months to several years. ASC 606 - Revenue from Contracts with Customers (year-ended December 31, 2018) ICON adopted ASC 606 ' 'Revenue from Contracts with Customers ' 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. The comparative information has not been adjusted and therefore continues to be reported under ASC 605 ‘Revenue Recognition’ and therefore in accordance with previous accounting policies. See note 26 Impact of change in accounting policies for details of the impact of adoption of the new accounting policy. 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. Clinical trial service revenue 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 on a percentage of completion basis. We have concluded that under the new standard, a clinical trial service is a single performance obligation satisfied over time i.e. the full service obligation in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Promises offered to the customer are not distinct within the context of the contract. We have concluded that ICON is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support the clinical research project. The transaction price is determined by reference to the contract or change order value (total service revenue and pass-through/ reimbursable expenses) adjusted to reflect a realizable contract value. Revenue is recognized as the single performance obligation is satisfied. The progress towards completion for clinical service contracts is measured based on an input measure being total project costs (inclusive of third party costs) at each reporting period. Contracting services revenue On evaluation of the principles at (1) - (5) set-out above in respect of ASC 606, the Company has availed of the practical expedient which results in recognition of revenue on a right to invoice basis. Application of the practical expedient reflects the right to consideration from the customer in an amount that corresponds directly with the value to the customer of the performance completion to date. This reflects hours performed by contract staff. Consulting services revenue On application of the ASC 606 ' Revenue from Contracts with Customers ' principles at (1) - (5) set-out above, we have concluded that our consulting services contracts represent a single performance obligation satisfied over time. The transaction price is determined by reference to contract or change order value. Revenue is recognized as the performance obligation is satisfied. The progress towards completion for consulting contracts is measured based on total project inputs (time) at each reporting period. Laboratory services revenue Revenue is recognized when, or as, obligations under the terms of a contract are satisfied, which occurs when control of the products or services are transferred to the customer. Revenue for laboratory services is measured as the amount of consideration we expect to receive in exchange for transferring products or services. Where contracts with customers contain multiple performance obligations, the transaction price is allocated to each performance obligation based on the estimated relative selling price of the promised good or service. Service revenue is recognized over time as the services are delivered to the customer based on the extent of progress towards completion of the performance obligation. The determination of the methodology to measure progress requires judgment and is based on the nature of services provided. This requires an assessment of the transfer of value to the customer. The right to invoice measure of progress is generally related to rate per unit contracts, as the extent of progress towards completion is measured based on discrete service or time-based increments, such as samples tested or labor hours incurred. Revenue is recorded in the amount invoiced since that amounts corresponds to the value of the Company's performance and the transfer of value to the customer. ASC 605 - Revenue recognition (years ended December 31, 2017 and December 31, 2016) Revenue for services, as rendered, is recognized only after persuasive evidence of an arrangement exists, the sales price is fixed or determinable and collectability is reasonably assured. Clinical trials management revenue is recognized on a proportional performance method. Depending on the contractual terms revenue is either recognized on the percentage of completion method based on the relationship between hours incurred and the total estimated hours of the trial or on the unit of delivery method. Contract costs equate to the product of labor hours incurred and compensation rates. For the percentage of completion method, the input (effort expended) method has been used to measure progress towards completion as there is a direct relationship between input and productivity. Contract revenue is the product of the aggregated labor hours required to complete the specified contract tasks at the agreed contract rates. The Company regularly reviews the estimate of total contract time to ensure such estimates remain appropriate taking into account actual contract stage of completion, remaining time to complete and any identified changes to the contract scope. Remaining time to complete depends on the specific contract tasks, the complexity of the contract and can include geographical site selection and initiation, patient enrollment, patient testing and level of results analysis required. While the Company may routinely adjust time estimates, the Company's estimates and assumptions historically have been accurate in all material respects in the aggregate. Where revenue is recognized on the unit of delivery method, the basis applied is the number of units completed as a percentage of the total number of contractual units. Consulting revenue is recognized on a fee-for-service basis as each hour of the related service is performed. Contract staffing revenue is recognized on a fee-for-service basis, over the time the related service is performed, or in the case of permanent placement, once the candidate has been placed with the client. Laboratory service revenue is recognized on a fee-for-service basis. The Company accounts for laboratory service contracts as multiple element arrangements, with contractual elements comprising laboratory kits and laboratory testing, each of which can be sold separately. Sales prices for contractual elements are determined by reference to objective and reliable evidence of their sales price. Revenues for contractual elements are recognized on the basis of the number of deliverable units completed in the period. Informatics revenue is recognized on a fee-for-service basis. Informatics contracts are treated as multiple element arrangements, with contractual elements comprising license fee revenue, support fee revenue and revenue from software services, each of which can be sold separately. Sales prices for contractual elements are determined by reference to objective and reliable evidence of their sales price. License and support fee revenues are recognized rateably over the period of the related agreement. Revenue from software services is recognized using the percentage of completion method based on the relationship between hours incurred and the total estimated hours required to perform the service. Contracts generally contain provisions for renegotiation in the event of changes in the scope, nature, duration, or volume of services of the contract. Renegotiated amounts are recognized as revenue by revision to the total contract value arising as a result of an authorized customer change order. (d) Third party costs (Reimbursable expenses) Reimbursable expenses comprise investigator payments and certain other costs which are reimbursed by clients under terms specific to each contract to the investigators. Third party costs (Reimbursable expenses) and the related revenue were separately presented on the face of the Consolidated Statement of Operations for periods up to and including the year-ended December 31, 2017. See sections (c) and (e) for accounting policy in respect of the treatment of activity relating to reimbursable expenses on revenue (c) and costs (e) on adoption of ASC 606 ' Revenue from Contracts with Customers '. (e) Direct costs Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs. On adoption of ASC 606 'Revenue from Contracts with Customers', reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time i.e. the full service obligation is in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred. Direct costs for the year-ended December 31, 2018 are therefore inclusive of third party costs of $702.8 million . Investigator payment costs are recorded and reported reflecting investigator activity over the life of the contract. Investigator payments are made based on predetermined contractual arrangements. Payments may differ from the recording and reporting of the expense which is based on activity. (f) Advertising costs All costs associated with advertising and promotion are expensed as incurred. The advertising and promotion costs were $6,516,637 , $6,744,333 and $7,167,050 for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 respectively. (g) Foreign currencies and translation of subsidiaries The Company's financial statements are prepared in United States dollars. Transactions in currencies other than United States dollars are recorded at the rate ruling at the date of the transaction. Monetary assets and liabilities denominated in currencies other than United States dollars are translated into United States dollars at exchange rates prevailing at the Balance Sheet date. Adjustments resulting from these translations are charged or credited to income. Amounts charged or credited to the Consolidated Statement of Operations for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 were as follows: Year ended December 31, (in thousands) 2018 2017 2016 Amounts (credited)/ charged $ (3,876 ) $ 7,760 $ 2,094 The financial statements of subsidiaries with other functional currencies are translated at period end rates for the Consolidated Balance Sheet and average rates for the Consolidated Statement of Operations. Translation gains and losses arising are reported as a movement on accumulated other comprehensive income. Foreign currency transaction gains and losses are reported in other comprehensive income rather than through income where the foreign currency transaction is 'long-term investment' in nature i.e. settlement is not planned or anticipated in the foreseeable future. (h) Disclosure of fair value of financial instruments Cash, cash equivalents, unbilled revenue, other receivables, available for sale investments, prepayments and other current assets, accounts receivable, accounts payable, investigator payments, payments on account, accrued liabilities, accrued bonuses and income taxes payable have carrying amounts that approximate fair value due to the short term maturities of these instruments. Other liabilities' carrying amounts approximate fair value based on net present value of estimated future cash flows. Debt is measured at historical cost. Financial instruments are measured in the Consolidated Balance Sheet at fair value using a fair value hierarchy of valuation inputs. The hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. These levels are: Level 1: Inputs are based upon unadjusted quoted prices for identical instruments traded in active markets. Level 2: Inputs are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. The Group's Senior notes (private placement debt) is carried at $350.0 million (prior to related financing costs). The carrying value at December 31, 2018, closely approximates fair value. The Company classifies its investments in short term debt or equity investments as available for sale, as it does not actively trade such securities nor does it intend to hold them to maturity. The fair value of short term investments are represented by level 1 fair value measurements – quoted prices in active markets for identical assets. The unrealized movements in fair value are recognized in equity until disposal or sale, at which time, those unrealized movements from prior periods are recognized in Consolidated Statement of Operations. Losses other than temporary, which reduce the carrying amount below cost are recognized in Consolidated Statement of Operations. (i) Business combinations The cost of a business combination is measured as the aggregate of the fair values at the date of exchange of assets given, liabilities incurred or assumed and equity instruments issued in exchange for control. Where a business combination agreement provides for an adjustment to the cost of the acquisition which is contingent upon future events, the amount of the estimated adjustment is recognized at the acquisition date at the fair value of the contingent consideration. Any changes to this estimate outside the measurement period will depend on the classification of the contingent consideration. If the contingent consideration is classified as equity it shall not be re-measured and the settlement shall be accounted for within equity. If the contingent consideration is classified as a liability any adjustments will be accounted for through the Consolidated Statement of Operations or Other Comprehensive Income depending on whether the liability is considered a financial instrument. The assets, liabilities and contingent liabilities of businesses acquired are measured at their fair values at the date of acquisition. In the case of a business combination which is completed in stages, the fair values of the identifiable assets, liabilities and contingent liabilities are determined at the date of each exchange transaction. When the initial accounting for a business combination is determined provisionally, any subsequent adjustments to the provisional values allocated to the identifiable assets, liabilities and contingent liabilities are made within twelve months of the acquisition date and presented as adjustments to goodwill in the reporting period in which the adjustments are determined. (j) Goodwill and Impairment Goodwill represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill primarily comprises acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. Goodwill is stated net of any provision for impairment. The Company tests goodwill annually for any impairments or whenever events occur which may indicate impairment. The Company applied the provisions of ASU 2017-04 with effect from January 1, 2018. Under the amendment, the Company was required to perform its annual goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An impairment charge would be recognized for any amount by which the carrying amount exceeds the reporting unit's fair value up to the amount of existing goodwill. The amendment allows an entity to perform a qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. No impairment was recognized as a result of the impairment testing carried out for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 . (k) Intangible assets Intangible assets are amortized on a straight line basis over their estimated useful life. (l) Cash and cash equivalents Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value. (m) Investments in debt, equity and other Available for sale investments The Company classifies short-term investments as available for sale in accordance with the terms of FASB ASC 320, Investments – Debt and Equity Securities . Realized gains and losses are determined using specific identification. The investments are reported at fair value, with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of the investments are represented by accrued interest and unrealized gains/losses. Long term investments The Company classifies its interests in funds having considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2018 meet the definition of equity securities without readily determinable fair values. Effective from 1 January 2018, the Company concluded that the interests held at December 31, 2018 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. Any increases or decreases in fair value are recognized in net income in the period. These are therefore measured at Level 3 of the fair value hierarchy. (n) Accounts receivable, net Accounts receivable are recorded at fair value less an allowance for doubtful accounts. The allowance is an estimate based on historical collection experience, current economic and market conditions, and a review of the current status of each customer's trade accounts receivable. Account balances are written-off against the allowance when the Group determines that it is probable that the receivable will not be recovered. (o) Inventory Inventory is valued at the lower of cost and net realizable value and after provisions for obsolescence. The cost of inventories comprises the purchase price and attributable costs, less trade discounts. At December 31, 2018 the carrying value of inventory, included within prepayments and other current assets on the Consolidated Balance Sheet, was $2.3 million ( 2017 : $2.2 million ). (p) Property, plant and equipment Property, plant and equipment is stated at cost less accumulated depreciation. Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5 Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter. (q) Leased assets Costs in respect of operating leases are charged to the Consolidated Statement of Operations on a straight line basis over the lease term. Assets acquired under capital finance leases are included in the Consolidated Balance Sheet at the present value of the future minimum lease payments and are depreciated over the shorter of the lease term and their remaining useful lives. The corresponding liabilities are recorded in the Consolidated Balance Sheet and the interest element of the capital lease rental is charged to interest expense. (r) Income taxes The Company applies the asset and liability method of accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry-forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which these temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance to the amount more likely than not to be realized. The Company recognizes the effect of income tax positions only if those positions will more likely than not be sustained. Recognized income tax positions are measured at the largest amount of tax benefit that is greater than 50 percent likely of being realized upon settlement. Interest and penalties related to income taxes are included in income tax expense and classified with the related liability on the Consolidated Balance Sheet. The Company accounts for the impact of GILTI (“global intangible low-taxed income”) as a period item in the period it arises and has therefore not provided for deferred taxes in respect of this item. (s) Government grants Government grants received relating to capital expenditures are shown as deferred income and credited to income on a basis consistent with the depreciation policy of the relevant assets. Grants relating to categories of operating expenditures are credited to income in the period in which the expenditure to which they relate is charged. Under the grant agreements amounts received may become repayable in full should certain circumstances specified within the grant agreements occur, including downsizing by the Company, disposing of the related assets, ceasing to carry on its business or the appointment of a receiver over any of its assets. The Company has not recognized any loss contingency having assessed as remote the likelihood of these events arising. (t) Research and development credits Research and development credits are available to the Company under the tax laws in certain jurisdictions, based on qualifying research and development spend as defined under those tax laws. Research and development credits are generally recognized as a reduction of income tax expense. However, certain tax jurisdictions provide refundable credits that are not wholly dependent on the Company's ongoing income tax status or income tax position. In these circumstances the benefit of these credits is not recorded as a reduction to income tax expense, but rather as a reduction of operating expenditure. (u) Pension costs The Company contributes to defined contribution plans covering all eligible employees. The Company contributes to these plans based upon various fixed percentages of employee compensation and such contributions are expensed as incurred. The Company operates, through two subsidiaries, a defined benefit plan for certain of its United Kingdom and Swiss employees. The Company accounts for the costs of these plans in accordance with ASC 715-30. These plans are presented in accordance with the requirements of FASB ASC 715-60 Defined Benefit Plans – Other Post retirement . (v) Net income per ordinary share Basic net income per ordinary share has been computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed by adjusting the weighted average number of ordinary shares outstanding during the period for all potentially dilutive ordinary shares outstanding during the period and adjusting net income for any changes in income or loss that would result from the conversion of such potential ordinary shares. There is no difference in net income used for basic and diluted net income per ordinary share. The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows: Year Ended December 31, 2018 2017 2016 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 54,118,764 54,129,439 55,248,900 Effect of dilutive share options outstanding 671,899 719,607 1,158,236 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 54,790,663 54,849,046 56,407,136 (w) Share-based compensation The Company accounts for its share options, restricted share units ("RSUs") and performance share units ("PSUs") in accordance with the provisions of FASB ASC 718, Compensation – Stock Compensation. Share-based compensation expense for equity-settled awards made to employees and Directors is measured and recognized based on estimated grant date fair values. These equity-settled awards include employee share options, RSUs and PSUs. Share-based compensation expense for share options awarded to employees and Directors is estimated at the grant date based on each option's fair value as calculated using the Black-Scholes option-pricing model. Share-based compensation for RSUs and PSUs awarded to employees and Directors is calculated based on the market value of the Company's shares on the date of award of the RSUs and PSUs. The value of awards expected to vest is recognized as an expense over the requisite service periods. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates. Estimating the grant date fair value of share options as of the grant date using an option-pricing model, such as the Black-Scholes model, is affected by the Company's share price as well as assumptions regarding a number of complex variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards. Liability classified awards are measured at the fair value of the award on the grant date and remeasured at each reporting period at fair value until the award is settled. (x) Impairment of long-lived assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured at the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount of the asset or fair value less selling costs. (y) Derivative financial instruments We enter into transactions in the normal course of business using various financial instruments in order to hedge against exposure to fluctuating exchange and interest rates. We use derivative financial instruments to reduce exposure to fluctuations in interest rates. A derivative is a financial instrument or other contract whose value changes in response to some underlying variable, which has an initial net investment smaller than would be required for other instruments that have a similar response to the variable and that will be settled at a future date. We do not enter into derivative financial instruments for trading or speculative purposes. We did not hold any interest rate swap contracts or forward currency contracts at December 31, 2018 or December 31, 2017 . We use derivative financial instruments to reduce exposure to fluctuations in foreign exchange rates. During the years-ended December 31, 2017 and December 31, 2018 we entered into forward currency contracts in respect of identified exposure arising from euro payments. All contracts expired during the year ended December 31, 2018 . Our accounting policies for derivative financial instruments are based on whether they meet the criteria for designation as cash flow or fair value hedges. A designated hedge of the exposure to variability in the future cash flows of an asset or a liability, or of a forecasted transaction, is referred to as a cash flow hedge. A designated hedge of the exposure to changes in fair value of an asset or a liability is referred to as a fair value hedge. The criterion for designating a derivative as a hedge includes the assessment of the instrument's effectiveness in risk reduction, matching of the derivative instrument to its underlying transaction and the probability that the underlying transaction will occur. For derivatives with cash flow hedge accounting designation, we report the gain or loss from the effective portion of the hedge as a component of Other Comprehensive Income and reclassify it into earnings in the same period or periods in which the hedged transaction affects earnings and within the same Statement of Operations line item as the impact of the hedged transaction. For derivatives with fair value hedge accounting designation, we recognize gains or losses from the change in fair value of these derivatives, as well as the offsetting change in the fair value of the underlying hedged item, in earnings. Fair value gains and losses arising on derivative financial instruments not qualifying for hedge accounting are reported in our Consolidated Statement of Operations. (z) Financing costs and gain on interest rate hedge The interest rate in respect of the Senior Notes is fixed at 3.64% for the five year term of the agreement. Th

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments (a) Available for sale investments December 31, 2018 December 31, 2017 (in thousands) At start of year $77,589 $68,046 Purchases 80,956 41,701 Sales and maturities (99,865 ) (33,086 ) Interest on short term investments 1,329 1,088 Realized gain on sale of short term investments 56 112 Unrealized capital loss – investments (155 ) (272 ) At end of year $59,910 $77,589 The Company classifies its investment in short term investments as available for sale. Short term investments comprise highly liquid investments with maturities of greater than three months and minimum "A-" rated fixed and floating rate securities. Short term investments at December 31, 2018 have an average maturity of 1.22 years compared to 1.58 years at December 31, 2017 . The investments are reported at fair value with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of investments are represented by accrued interest and unrealized gains/losses. The fair value of short term investments are represented by level 1 fair value measurements – quoted prices in active markets for identical assets. The following table represents our available for sale short term investments by major security type as of December 31, 2018 : Maturity by period Cost Total Unrealized gains / (losses) Fair Value Total Less than 1 year 1 to 5 years (U.S.$ in millions) US government debt securities 14.91 (0.07 ) 14.84 5.96 8.88 Corporate securities 44.92 (1.19 ) 43.73 11.48 32.25 Term deposits 1.34 — 1.34 0.70 0.64 Total (U.S.$ in millions) $61.17 ($1.26 ) $59.91 $18.14 $41.77 The contractual maturity of certain investments in the portfolio is greater than 12 months; however, classification as short-term investments reflects the Company practice and intention in respect of these investments. The company recognizes the unrealized losses in fair value in equity as these unrealized losses on short term investments have been considered as temporary. (b) Investments in equity - long term The Company entered into subscription agreements with a number of funds. Capital totaling $6.2 million had been advanced under the terms of the subscription agreements at December 31, 2018. The Company determined that the interests in the funds meet the definition of equity securities without readily determinable fair values. Effective from 1 January 2018, the Company concluded that the interests held at December 31, 2018 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. An increase in fair value of $0.8 million was recognized in net income during the period bringing the carrying value of the subscriptions to $6.963 million at December 31, 2018. The Company had committed to future investments of $21.3 million in respect of these funds.

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill December 31, 2018 December 31, 2017 (in thousands) Opening goodwill $ 769,058 $ 616,088 Current year acquisitions — 129,222 Prior period acquisition (note 4 (a)) 1,048 1,393 Foreign exchange movement (13,846 ) 22,355 Closing goodwill $ 756,260 $ 769,058 The Company has made a number of strategic acquisitions since inception to enhance its capabilities and experience in certain areas of the clinical development process. Goodwill arising on acquisition represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill primarily comprises of the acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. The Company acquired Mapi Développement SAS ('Mapi') during the year-ended December 31, 2017 resulting in the recognition of goodwill of $130.3 million (note 4 (a)). The Company tests goodwill annually for impairment or whenever events occur which may indicate impairment. The results of the Company's goodwill impairment testing assessed at September 30, 2018 during the year ended December 31, 2018 provided no evidence of impairment and indicated the existence of sufficient headroom such that a reasonably possible change to the key assumptions used would be unlikely to result in an impairment of the related goodwill. (a) 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 table following summarizes the Company’s assessment of the fair values of the assets acquired and liabilities assumed: July 27, 2017 (in thousands) Cash 19,649 Property, plant and equipment 4,872 Goodwill* 130,270 Order book 13,012 Customer list 18,392 Accounts receivable 15,874 Unbilled revenue 6,984 Prepayments and other current assets 2,587 Other receivables 1,430 Income taxes receivable 4,262 Accounts payable (2,994 ) Payments on account (31,445 ) Other liabilities (24,952 ) 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. In finalizing the goodwill on acquisition of Mapi in the twelve month period from acquisition, fair value adjustments were made which resulted in increases in other liabilities ( $3.9 million ), plant and equipment ( $1.7 million ) and accounts receivable ( $1.7 million ) and income taxes receivable ( $1.5 million ) and decreases in unbilled revenue ( $4.8 million ), prepayments and other current assets ( $1.9 million ) and other receivables ( $1.0 million ) and in payment on account ( $2.6 million ) and non-current deferred tax liability ( $9.1 million ). Customer list and order backlog assets were also finalized. The proforma effect of the Mapi acquisition if completed on January 1, 2016 would have resulted in net revenue, net income and earnings per share for the fiscal years ending December 31, 2017 and December 31, 2016 as follows: Year Ended 2017 2016 (in thousands) Net revenue $ 1,811,018 $ 1,750,643 Net income $ 284,903 $ 263,101 Basic earnings per share $ 5.26 $ 4.76 Diluted earnings per share $ 5.19 $ 4.66 (b) Acquisition of ICON Government & Public Health Solutions (formerly Clinical Research Management (ClinicalRM)) On September 15, 2016, a subsidiary of the Company, ICON US Holdings Inc. acquired ICON Government & Public Health Solutions (''GPHS'') (formerly Clinical Research Management (ClinicalRM)) which resulted in net cash outflows of $52.4 million (including certain payments made on behalf of GPHS totaling $9.2 million ). GPHS 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. GPHS 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 $6.0 million . The evaluation of the performance and forecast performance of GPHS 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 $6.0 million being recorded within selling, general & administrative expenses in the Consolidated Statement of Operations during the year ended December 31, 2017 . The acquisition of GPHS 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 GPHS 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. The proforma effect of the GPHS acquisition if completed on January 1, 2015 would have resulted in net revenue, net income and earnings per share for the fiscal years ended December 31, 2016 and December 31, 2015 as follows: Year Ended December 31, 2016 2015 (in thousands) Net revenue $ 1,713,245 $ 1,639,085 Net income $ 266,148 $ 244,167 Basic earnings per share $ 4.82 $ 4.16 Diluted earnings per share $ 4.72 $ 4.05 (c) Acquisition of PMG On December 4, 2015, a subsidiary of the Company, ICON Clinical Research LLC. acquired PMG for cash consideration of $65.4 million , including certain payments on behalf of PMG totaling $10.1 million . PMG is an integrated network of 52 clinical research sites in North Carolina, South Carolina, Tennessee, Illinois and Iowa. The site network includes wholly owned facilities and dedicated clinical research sites. PMG conducts clinical trials in all major therapeutic areas and has particular expertise in vaccine, gastroenterology, cardiovascular, neurology and endocrinology studies. It has a proprietary database of clinical trial participants. It also has access to in excess of 2 million active patients via electronic medical records through its partnerships with health care institutions and community physical practices.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets December 31, 2018 December 31, 2017 Cost (in thousands) Customer relationships acquired $ 109,622 $ 91,230 Technology asset acquired 11,169 11,169 Order backlog 31,220 18,208 Trade names/ brands acquired 2,766 2,766 Volunteer list acquired 1,325 1,325 Non-compete arrangements 489 489 Mapi intangible asset — 32,305 Foreign exchange movement (5,085 ) (2,389 ) Total cost 151,506 155,103 Accumulated amortization (100,249 ) (84,898 ) Foreign exchange movement 2,798 1,423 Net book value $ 54,055 $ 71,628 On July 27, 2017, a subsidiary of the Company, ICON Clinical Research Limited acquired Mapi Group. Mapi is a leading patient-centered health outcomes research and commercialization company. The acquisition of Mapi strengthens ICON’s existing commercialization and outcomes research business adding significant commercialization presence, analytics, real world evidence generation and strategic regulatory services. The value of certain customer relationships and order backlog identified of $18.4 million and $13.0 million respectively were recognized on acquisition and are being amortized over approximately over 8 years and 8.5 years , the estimated period of benefit. In total, $5.8 million has been amortized in the period since the date of acquisition. On September 15, 2016, a subsidiary of the Company, ICON US Holdings Inc., acquired ICON Government & Public Health Solutions (''GPHS''), a full-service CRO specializing in preclinical through Phase IV support of clinical research and clinical trial services for biologics, drugs and devices. The organization helps customers progress their products to market faster with a wide array of research, regulatory and sponsor services within the U.S. and around the globe. GPHS provide full service and functional research solutions to a broad range of US government agencies. The value of certain customer relationship, order backlog and brand assets identified of $4.0 million , $1.7 million and $1.4 million respectively are being amortized over approximately 7 years , 2 years and 5 years respectively, the estimated period of benefit. In total, $4.4 million has been amortized in the period since the date of acquisition. On December 4, 2015, a subsidiary of the Company, ICON Clinical Research LLC, acquired PMG, an integrated network of 52 clinical research sites in North Carolina, South Carolina, Tennessee, Illinois and Iowa. The site network includes wholly owned facilities and dedicated clinical research sites. PMG conducts clinical trials in all major therapeutic areas and has particular expertise in vaccine, gastroenterology, cardiovascular, neurology and endocrinology studies. The value of certain customer relationship and order backlog assets identified of $6.9 million and $3.0 million respectively are being amortized over approximately 7 years and 2 years respectively, the estimated period of benefit. In total, $6.0 million has been amortized in the period since the date of acquisition. The order backlog is fully amortized at December 31, 2018 . On February 27, 2015, a subsidiary of the Company, ICON Holdings Unlimited Company (formerly ICON Holdings), acquired MediMedia Pharma Solutions. Headquartered in Yardley, Pennsylvania, MediMedia Pharma Solutions includes MediMedia Managed Markets and Complete Healthcare Communications. MediMedia Managed Markets is a leading provider of strategic payer-validated market access solutions. Complete Healthcare Communications is one of the leading medical and scientific communication agencies working with medical affairs, commercial and brand development teams within life science companies. The value of certain customer relationships and order backlog identified of $22.8 million and $2.5 million respectively are being amortized over approximately 7 years and 1 year , the estimated period of benefit. $15.0 million has been amortized in the period since the date of acquisition. The order backlog is fully amortized at December 31, 2018 . On May 7, 2014, a subsidiary of the Company, ICON US Holdings Inc., acquired Aptiv Solutions, Inc. ("Aptiv"), a global biopharmaceutical and medical device development services company and leader in adaptive clinical trials. Aptiv offers full-service clinical trial consulting and regulatory support for drugs, medical devices and diagnostics with a specific focus on strategy to increase product development efficiency and productivity. The value of certain customer relationships and order backlog identified of $21.4 million and $7.9 million respectively are being amortized over approximately 7 years and 3 years , the estimated period of benefit. In total, $22.2 million has been amortized in the period since the date of acquisition. The order backlog is fully amortized at December 31, 2018 . On February 15, 2013, subsidiaries of the Company, ICON Clinical Research LLC (formerly ICON Clinical Research, Inc.) and ICON Clinical Research (U.K.) Limited, acquired the Clinical Trial Services division of Cross Country Healthcare, Inc. Cross Country Healthcare's Clinical Trial Services division includes US resourcing providers, ClinForce and Assent Consulting, whose services include contract staffing, permanent placement and functional service provision ("FSP"). The value of certain customer relationships and order backlog identified of $3.3 million and $0.6 million respectively are being amortized over approximately 3 years and 1 year , the estimated period of benefit. The full $3.9 million has been amortized in the period since the date of acquisition. On February 28, 2012, a subsidiary of the Company, ICON Clinical Research LLC (formerly ICON Clinical Research, Inc.), acquired PriceSpective, a strategy consulting company. The value of certain customer relationships identified of $10.2 million is being amortized over approximately 10 years , the estimated period of benefit. The value of order backlog and certain non-compete arrangements identified of $0.4 million and $0.4 million respectively are being amortized over approximately 0.8 years and 3 years , the estimated period of benefit. In total, $7.8 million has been amortized in the period since the date of acquisition. On February 15, 2012, a subsidiary of the Company, ICON Clinical Research Limited, acquired BeijingWits Medical, a Chinese CRO. The value of certain customer relationships and order backlog identified of $1.8 million and $0.4 million respectively are being amortized over approximately 10 years and 4 years , the estimated period of benefit. The value of certain non-compete arrangements identified of $0.01 million are being amortized over approximately 5 years , the estimated period of benefit. In total, $1.7 million has been amortized in the period since the date of acquisition. On July 14, 2011, a subsidiary of the Company, ICON Clinical Research Limited, acquired Firecrest Clinical Limited, a provider of technology solutions that boost investigator site performance and study management. The value of certain technology assets and customer relationships identified of $11.2 million and $5.2 million respectively are being amortized over approximately 7.5 years , the estimated period of benefit. The value of the Firecrest trade name and order backlog identified of $1.4 million and $1.2 million respectively are being amortized over approximately 4.5 years and 1.2 years , the estimated period of benefit. In total, $16.4 million has been amortized in the period since the date of acquisition. On January 14, 2011, a subsidiary of the Company, ICON Clinical Research (U.K.) Limited, acquired Oxford Outcomes Limited, an international health outcomes consultancy business. The value of certain customer relationships and order backlog identified of $6.6 million and $0.6 million respectively were amortized over approximately 6.5 years and 2 years , the estimated period of benefit. The intangible assets identified have been fully amortized at December 31, 2018 . On November 14, 2008, subsidiaries of the Company, ICON Holdings Clinical Research International Limited and ICON Clinical Research LLC (formerly ICON Clinical Research, Inc.), acquired Prevalere Life Sciences, a US provider of bioanalytical and immunoassay laboratory services. The value of certain customer relationships identified of $7.4 million is being amortized over periods ranging from approximately 7 to 11 years , the estimated period of the benefit. In total, $7.2 million has been amortized in the period since the date of acquisition. On February 11, 2008, a subsidiary of the Company, ICON Clinical Research LLC (formerly ICON Clinical Research, Inc.), acquired Healthcare Discoveries, a US provider of Phase I clinical trial services. The value of certain client relationships identified of $1.6 million was amortized over periods ranging from approximately 2 to 9 years , the estimated periods of benefit. The value of certain volunteer lists identified of $1.3 million was amortized over approximately 6 years , the estimated period of benefit. The intangible assets identified have been fully amortized at December 31, 2018 . Future intangible asset amortization expense for the years ended December 31, 2019 to December 31, 2023 is as follows: Year Ended December 31,(in thousands) 2019 $ 13,183 2020 12,895 2021 10,857 2022 6,069 2023 4,221 $ 47,225

Property, Plant and Equipment,

Property, Plant and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, Net	Property, Plant and Equipment, net December 31, 2018 December 31, 2017 (in thousands) Cost Land $ 3,476 $ 3,464 Building 86,621 88,411 Computer equipment and software 399,192 358,874 Office furniture and fixtures 83,215 78,372 Laboratory equipment 36,092 34,918 Leasehold improvements 25,827 24,097 Motor vehicles 144 42 634,567 588,178 Less accumulated depreciation and asset write offs (475,898 ) (425,127 ) Property, plant and equipment (net) $ 158,669 $ 163,051

Other Liabilities

Other Liabilities	12 Months Ended
	Dec. 31, 2018
Other Liabilities Disclosure [Abstract]
Other Liabilities	Other Liabilities December 31, 2018 December 31, 2017 (in thousands) Personnel related liabilities $ 171,866 $ 168,964 Facility related liabilities 14,012 13,061 General trade and overhead liabilities* 118,845 41,789 Other liabilities 4,289 4,628 Short term government grants (note 11) 42 35 Restructuring (note 14) 8,089 5,026 $ 317,143 $ 233,503 *includes amounts due to third parties in respect of reimbursable expenses of $85.6 million at December 31, 2018.

Non-Current Other Liabilities

Non-Current Other Liabilities	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Non-Current Other Liabilities	Non-Current Other Liabilities December 31, 2018 December 31, 2017 (in thousands) Defined benefit pension obligations, net (note 9) $ 3,320 $ 6,061 Other non-current liabilities 10,126 11,050 $ 13,446 $ 17,111

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Employee Benefits	Employee Benefits Certain Company employees are eligible to participate in a defined contribution plan (the "Plan"). Participants in the Plan may elect to defer a portion of their pre-tax earnings into a pension plan, which is run by an independent party. The Company matches participant's contributions typically at 8% of the participant's annual compensation. Contributions to the plan are recorded as an expense in the selling, general and administrative line in the Consolidated Statement of Operations. Contributions for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 were $25,241,000 , $20,355,000 and $20,952,000 respectively. The Company's United States operations maintain a retirement plan (the "U.S. Plan") that qualifies as a deferred salary arrangement under Section 401(k) of the Internal Revenue Code. Participants in the U.S. Plan may elect to defer a portion of their pre-tax earnings, up to the Internal Revenue Service annual contribution limit. The Company matches participant's contributions up to 3% and matches 50% of participant's contributions thereafter to a maximum Company contribution of 4.5% of the participant's annual compensation. Contributions to this U.S. Plan are recorded, in the year contributed, as an expense in the Consolidated Statement of Operations. Contributions for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 were $15,532,000 , $14,946,000 and $15,223,000 respectively. ICON Development Solutions Limited pension plan One of the Company's subsidiaries, ICON Development Solutions Limited, operates a defined benefit pension plan in the United Kingdom for its employees. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2018 , December 31, 2017 and December 31, 2016 , consist of units held in independently administered funds. The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60, Defined Benefit Plans – Other Postretirement . The plan has been closed to new entrants with effect from July 1, 2003. Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (30,045 ) $ (37,759 ) Fair value of plan assets 27,297 32,423 Funded status $ (2,748 ) $ (5,336 ) Non-current other liabilities (note 8) $ (2,748 ) $ (5,336 ) Change in benefit obligation December 31, 2018 December 31, 2017 (in thousands) Benefit obligation at beginning of year $ 37,759 $ 32,906 Service cost 124 112 Interest cost 895 929 Plan participants' contributions 24 22 Expenses — (8 ) Benefits paid (3,049 ) (68 ) Actuarial (gain)/loss (3,844 ) 658 Foreign currency exchange rate changes (1,864 ) 3,208 Benefit obligation at end of year $ 30,045 $ 37,759 Change in plan assets December 31, 2018 December 31, 2017 (in thousands) Fair value of plan assets at beginning of year $ 32,423 $ 24,876 Actual return on plan assets (584 ) 979 Employer contributions 153 4,008 Plan participants' contributions 24 22 Benefits paid (3,049 ) (68 ) Foreign currency exchange rate changes (1,670 ) 2,606 Fair value of plan assets at end of year $ 27,297 $ 32,423 The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company. The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit cost: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Service cost $ 124 $ 112 $ 75 Interest cost 895 929 1,017 Expected return on plan assets (624 ) (586 ) (646 ) Amortization of net loss 248 250 — Expenses — (8 ) 8 Net periodic benefit cost $ 643 $ 697 $ 454 The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : December 31, 2018 December 31, 2017 December 31, 2016 Discount rate 2.5 % 2.7 % 4.0 % Rate of compensation increase 3.7 % 3.9 % 3.7 % Expected rate of return on plan assets 2.0 % 2.1 % 3.0 % Other comprehensive income December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Actuarial (gain)/loss - benefit obligation $ (3,844 ) $ 658 $ 10,057 Actuarial loss/(gain) – plan assets 1,208 (393 ) (5,215 ) Actuarial gain recognized in net periodic benefit cost (248 ) (250 ) — Total $ (2,884 ) $ 15 $ 4,842 The estimated net loss and prior service cost for the defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year are $0.1 million and $Nil respectively. Amounts recognized in accumulated other comprehensive income that have not yet been recognized as components of net periodic benefit cost are as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Net actuarial loss $ 4,254 $ 7,138 $ 7,123 Total $ 4,254 $ 7,138 $ 7,123 Benefit Obligation The following assumptions were used in determining the benefit obligation at December 31, 2018 and December 31, 2017 : December 31, 2018 December 31, 2017 Discount rate 2.9 % 2.5 % Rate of compensation increase 3.7 % 3.7 % The discount rate is determined by reference to UK long dated government and corporate bond yields at the Balance Sheet date. This is represented by the iboxx corporate bond over 15 year index plus 10 basis points. Plan Assets The assets of the scheme are invested with Legal and General and are held in a combination of: the Active Corporate Bond over 10 Year fund, Gilt and Index Linked Gilt funds. The overall investment strategy is that approximately 75% of investments are in government bonds (both fixed interest and index linked), approximately 25% of investments are held in corporate bonds. There is no self-investment in employer related assets. The expected long-term rate of return on assets at December 31, 2018 of 2.1% was calculated as the value of the fund after application of a market value reduction factor. The expected long term rates of return on different asset classes are as follows: Asset Category Expected long-term return per annum Corporate Bonds 2.9 % Gilts 1.8 % Cash 2.9 % The long-term expected return on corporate bonds and gilts (fixed interest and index linked) is determined by reference to bond yields and gilt yields at the Balance Sheet date. The underlying asset split of the fund is shown below. Asset Category December 31, 2018 December 31, 2017 Corporate Bonds 25 % 22 % Gilts 71 % 65 % Cash 4 % 13 % 100 % 100 % Applying the above expected long term rates of return to the asset distribution at December 31, 2018 , gives rise to an expected overall rate of return of scheme assets of approximately 2.1% per annum. Plan Asset Fair Value Measurements Quoted Prices in Active Markets for Identical Assets Level 1 (in thousands) December 31, 2018 December 31, 2017 Cash $ 1,029 $ 4,086 Fixed Income Securities Legal and General Active Corporate Bond – Over 10 Year 6,688 7,188 Legal and General Gilt Funds 7,136 7,611 Legal and General Index Linked Gilt Funds 12,444 13,538 $ 27,297 $ 32,423 Cash Flows The Company expects to contribute $0.2 million to the pension fund in the year ending December 31, 2019 . The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2019 295 2020 311 2021 383 2022 412 2023 403 Years 2024 - 2028 $ 3,451 The expected cash flows are estimated figures based on the members expected to retire over the next 10 years assuming no early retirements plus an additional amount based on recent average withdrawal experience. At the present time it is not clear whether annuities will be purchased when members reach retirement or whether pensions will be paid each month out of scheme assets. The cash flows above have been estimated on the assumption that pensions will be paid monthly out of scheme assets. If annuities are purchased, then the expected benefit payments will be significantly different from those shown above. Aptiv Solutions pension plan On May 7, 2014 the Company acquired 100% of the common stock of Aptiv Solutions ("Aptiv"). The acquisition of Aptiv was accounted for as a business combination in accordance with FASB ASC 805 Business Combinations . The Company has a defined benefit plan covering its employees in Switzerland as mandated by the Swiss government. Benefits are based on the employee's years of service and compensation. Benefits are paid directly by the Company when they become due, in conformity with the funding requirements of applicable government regulations. The plan is managed externally and the related pension costs and liabilities are assessed in accordance with the advice of a professionally qualified actuary. Plan assets at December 31, 2018 and December 31, 2017 consist of units held in independently administered funds. The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60, Defined Benefit Plans – Other Postretirement . Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (5,279 ) $ (5,927 ) Fair value of plan assets 4,707 5,202 Funded status $ (572 ) $ (725 ) Non-current other liabilities (note 8) $ (572 ) $ (725 ) Change in benefit obligation December 31, 2018 December 31, 2017 (in thousands) Benefit obligation at beginning of year $ 5,927 $ 6,928 Service cost 138 243 Interest cost 47 54 Plan participants' contributions 83 120 Settlement (409 ) (1,019 ) Prior service cost (8 ) — Transferred (benefits paid)/balances (77 ) (76 ) Actuarial gain (372 ) (626 ) Foreign currency exchange rate changes (50 ) 303 Benefit obligation at end of year $ 5,279 $ 5,927 Change in plan assets December 31, December 31, 2018 2017 (in thousands) Fair value of plan assets at beginning of year $ 5,202 $ 6,006 Expected return on plan assets 41 47 Actual return on plan assets (240 ) (296 ) Scheme contributions 109 157 Plan participants' contributions 83 120 Transferred (benefits paid)/balances (77 ) (76 ) Settlement (409 ) (1,019 ) Foreign currency exchange rate changes (2 ) 263 Fair value of plan assets at end of year $ 4,707 $ 5,202 The fair values of the assets above do not include any of the Company's own financial instruments, property occupied by, or other assets used by, the Company. December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Service cost $ 138 $ 243 $ 352 Interest cost 47 54 82 Expected return on plan assets (41 ) (47 ) (48 ) Amortization of net (gain)/loss (69 ) (43 ) 22 Amortization of prior service credit (8 ) (8 ) (8 ) Settlement (93 ) (214 ) (136 ) Curtailment — — — Net periodic benefit credit $ (26 ) $ (15 ) $ 264 The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : December 31, 2018 December 31, 2017 December 31, 2016 Discount rate 0.80 % 0.75 % 0.95 % Rate of compensation increase 2.00 % 2.00 % 2.00 % Expected rate of return on plan assets 0.80 % 0.75 % 0.95 % Other comprehensive income December 31, 2018 December 31, 2017 December 31, 2016 Actuarial gain - benefit obligation $ (372 ) $ (626 ) $ (1,157 ) Actuarial loss/(gain) – plan assets 240 296 (1,233 ) Prior service credit recognized in net periodic benefit cost 93 215 136 Actuarial gain/(loss) recognized in net periodic benefit cost 69 43 (22 ) Amortization of net prior service credit 8 8 8 Net prior service cost occurring during the year (9 ) (1 ) (89 ) Total $ 29 $ (65 ) $ (2,357 ) The estimated net gain and prior service credit for the defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year are $95,000 and $9,000 respectively. Amounts recognized in accumulated other comprehensive income that have not yet been recognized as components of net periodic benefit cost are as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Net actuarial gain $ (1,254 ) $ (1,283 ) $ (1,218 ) Total $ (1,254 ) $ (1,283 ) $ (1,218 ) Benefit Obligation The following assumptions were used in determining the benefit obligation at December 31, 2018 and December 31, 2017 : December 31, 2018 December 31, 2017 Discount rate 0.80 % 0.80 % Rate of compensation increase 2.00 % 2.00 % The discount rate is determined by reference to Swiss corporate bond yields at the Balance Sheet date. Plan Assets The pension plan is an insured arrangement with Swiss Life. The assets are an insurance contract whose value depends on the amount saved by employees and the interest granted by Swiss Life. The value of assets does not depend on the performance of any underlying assets. There is no self-investment in employer related assets. Cash Flows The Company expects to contribute $0.1 million to its pension fund in the year ending December 31, 2019 . The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2019 678 2020 212 2021 209 2022 204 2023 199 Years 2023 - 2028 $ 908 The expected cash flows are estimated figures based on the members expected to retire over the next 10 years assuming no early retirements plus an additional amount based on recent average withdrawal experience. At the present time it is not clear whether annuities will be purchased when members reach retirement or whether pensions will be paid each month out of scheme assets. The cash flows above have been estimated on the assumption that pensions will be paid monthly out of scheme assets. If annuities are purchased, then the expected benefit payments will be significantly different from those shown above.

Equity Incentive Schemes and St

Equity Incentive Schemes and Stock Compensation Charges	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity Incentive Schemes and Stock Compensation Charges	Equity Incentive Schemes and Stock Compensation Charges Share Options On July 21, 2008 the Company adopted the Employee Share Option Plan 2008 (the "2008 Employee Plan") pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any employee, or any Director holding a salaried office or employment with the Company or a Subsidiary for the purchase of ordinary shares. On the same date, the Company also adopted the Consultants Share Option Plan 2008 (the "2008 Consultants Plan"), pursuant to which the Compensation and Organization Committee of the Company's Board of Directors may grant options to any consultant, adviser or non-executive Director retained by the Company or any Subsidiary for the purchase of ordinary shares. On February 14, 2017 both the 2008 Employee Plan and the 2008 Consultants Plan (together the "2008 Option Plans") were amended and restated in order to increase the number of options that can be issued under the 2008 Consultants Plan from 400,000 to 1 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 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. Prior to 2018, share options typically vest over a period of five years from date of grant and expire eight years from date of grant. Share options granted to non-executive directors during 2018 vest over 12 months and expire eight years from the date of grant. The maximum contractual term of options outstanding at December 31, 2018 is eight years . The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : Options Granted Under Plans Weighted Average Exercise Price Weighted Average Grant Date Fair Value Outstanding at December 31, 2015 1,626,582 $ 34.87 $ 11.94 Granted 256,191 $ 69.61 $ 20.10 Exercised (393,240 ) $ 25.79 $ 9.84 Cancelled (23,089 ) $ 29.74 $ 11.19 Outstanding at December 31, 2016 1,466,444 $ 43.45 $ 13.94 Granted 219,113 $ 85.98 $ 25.06 Exercised (458,243 ) $ 30.35 $ 10.72 Cancelled (55,921 ) $ 54.35 $ 16.76 Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 167,557 $ 118.90 $ 36.84 Exercised (408,699 ) $ 41.12 $ 13.55 Cancelled (9,505 ) $ 32.35 $ 11.39 Outstanding at December 31, 2018 920,746 $ 74.32 $ 22.39 Vested and exercisable at December 31, 2018 397,923 $ 56.10 $ 17.11 The weighted average remaining contractual life of options outstanding and options exercisable at December 31, 2018 , was 5.01 years and 3.83 years respectively ( 2017 : 4.86 years and 3.44 years respectively). 241,149 options are expected to vest during the year ended December 31, 2019 ( 255,198 options were expected to vest during the year ended December 31, 2018 ). The intrinsic value of options exercised during the year ended December 31, 2018 amounted to $38.2 million . The intrinsic value of options outstanding and options exercisable at December 31, 2018 amounted to $48.6 million and $28.3 million respectively. Intrinsic value is calculated based on the market value versus strike price of the Company's shares at the date of exercise. Non-vested shares outstanding as at December 31, 2018 are as follows: Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Fair Value Non-vested outstanding at December 31, 2017 694,727 $ 68.06 $ 20.03 Granted 167,557 118.90 36.84 Vested (336,756 ) 62.21 18.51 Forfeited (2,705 ) 56.58 16.84 Non-vested outstanding at December 31, 2018 522,823 $ 88.18 $ 26.41 Outstanding and exercisable share options: The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2018 : Options Outstanding Options Exercisable Range Exercise Price Number of Shares Weighted Average Remaining Contractual Life Weighted Average Exercise Price Number of Shares Weighted Average Exercise Price $ 20.28 18,190 0.16 18,190 $ 22.30 52,811 1.32 52,811 $ 23.66 1,161 1.57 1,161 $ 26.71 450 1.69 450 $ 32.37 20,848 2.33 20,848 $ 36.22 2,141 2.46 2,141 $ 37.90 920 2.93 920 $ 40.83 40,502 3.39 25,360 $ 47.03 22,685 3.17 14,020 $ 48.67 50,896 3.21 34,906 $ 51.35 2,030 3.60 1,224 $ 65.60 66,752 5.38 24,543 $ 66.47 4,267 4.39 1,572 $ 66.97 1,249 4.45 — $ 68.39 135,400 4.18 79,069 $ 71.95 127,905 5.17 74,473 $ 83.47 127,136 6.17 22,461 $ 90.03 77,846 6.38 23,774 $ 115.11 107,794 7.17 — $ 125.74 59,763 7.38 — $20.28 - $125.74 920,746 5.01 $ 74.32 397,923 $ 56.10 Options outstanding include both vested and unvested options as at December 31, 2018 . Options exercisable represent options which have vested at December 31, 2018 . From the date of grant, substantially all options vest over a five year period at 20% per annum. Fair value of Stock Options Assumptions The weighted average fair value of options granted during the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 was calculated using the Black-Scholes option pricing model. The weighted average fair values and assumptions were as follows: Year Ended December 31, 2018 December 31, 2017 December 31, 2016 Weighted average fair value $ 36.84 $ 25.06 $ 20.10 Assumptions: Expected volatility 30 % 29 % 30 % Dividend yield — % — % — % Risk-free interest rate 2.76 % 1.93 % 1.39 % Expected life 5.0 years 5.0 years 5.0 years Expected volatility is based on the historical volatility of our common stock over a period equal to the expected term of the options; the expected life represents the weighted average period of time that options granted are expected to be outstanding given consideration to vesting schedules and our historical experience of past vesting and termination patterns. The risk-free rate is based on the U.S. government zero-coupon bonds yield curve in effect at time of the grant for periods corresponding with the expected life of the option. Restricted Share Units and Performance Share Units On 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 year ended December 31, 2018 : PSU Outstanding Number of Shares PSU Weighted Average Fair Value PSU Weighted Average Remaining Contractual Life RSU Outstanding Number of Shares RSU Weighted Average 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 71,906 $ 116.02 160,113 $ 123.42 Shares vested (215,826 ) $ 68.28 (276,495 ) $ 67.99 Forfeited (116,053 ) $ 70.89 (64,911 ) $ 78.92 Outstanding at December 31, 2018 251,053 $ 85.95 0.96 534,677 $ 89.50 1.22 The fair value of RSUs vested for the year ended December 31, 2018 totaled $18.8 million ( 2017 : $16.6 million ). The fair value of PSUs vested for the year ended December 31, 2018 totaled $14.7 million ( 2017 : $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, 147,630 PSUs (net of forfeitures) have been granted. Depending on the actual amount of EPS from 2015 to 2021 , up to an additional 103,423 PSUs may also be granted. Non-cash stock compensation expense Income from operations for the year ended December 31, 2018 is stated after charging $31.6 million in respect of non-cash stock compensation expense. Non-cash stock compensation expense has been allocated as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Direct costs $ 17,408 $ 18,020 $ 21,903 Selling, general and administrative $ 14,186 $ 12,553 $ 18,440 Total compensation costs $ 31,594 $ 30,573 $ 40,343 Total non-cash stock compensation expense not yet recognized at December 31, 2018 amounted to $43.1 million . The weighted average period over which this is expected to be recognized is 2.19 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 has applied the updated standard prospectively in the twelve months of the year ended December 31, 2017. The income tax expense for the year ended December 31, 2018 reflects a net income tax benefit of $12.7 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2018 was $3.6 million . The income tax expense for the year ended December 31, 2017 reflects a net income tax benefit of $9.3 million in connection with stock compensation (including excess tax benefits) and the total tax benefit in connection with stock options exercised during 2017 was $3.2 million . The income tax expense for the year ended December 31, 2016 reflects a net income tax benefit of $3.5 million in connection with stock compensation (excluding excess tax benefits) and the cash tax benefit realized in connection with stock options exercised during 2016 was $3.4 million .

Government Grants

Government Grants	12 Months Ended
	Dec. 31, 2018
Deferred Revenue Disclosure [Abstract]
Government Grants	Government Grants December 31, 2018 December 31, 2017 (in thousands) Received $ 3,539 $ 3,539 Less accumulated amortization (2,792 ) (2,745 ) Foreign exchange translation adjustment 172 207 Total government grants 919 1,001 Less current portion (42 ) (35 ) Non-current government grants $ 877 $ 966 Capital grants received may be refundable in full if certain events occur. Such events, as set out in the related grant agreements, include sale of the related asset, liquidation of the Company or failure to comply with other conditions of the grant agreements. No loss contingency has been recognized as the likelihood of such events arising has been assessed as remote. Government grants amortized to the profit and loss account amounted to $47,000 for the year ended December 31, 2018 . A net charge of $44,000 was recorded in respect of government grants during the year ended December 31, 2017 . As at December 31, 2018 the Company had $3.2 million in restricted retained earnings, pursuant to the terms of grant agreements.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Share Capital	Share Capital Holders of ordinary shares will be entitled to receive such dividends as may be recommended by the Board of Directors of the Company and approved by the shareholders and/or such interim dividends as the Board of Directors of the Company may decide. On liquidation or a winding up of the Company, the par value of the ordinary shares will be repaid out of the assets available for distribution among the holders of the ordinary shares of the Company. Holders of ordinary shares have no conversion or redemption rights. On a show of hands, every holder of an ordinary share present in person or proxy at a general meeting of shareholders shall have one vote, for each ordinary share held with no individual having more than one vote. During the year ended December 31, 2018 , 408,699 options were exercised by employees at an average exercise price of $41.12 per share for total proceeds of $16.8 million . During the year ended December 31, 2018 , 276,495 ordinary shares were issued in respect of certain RSUs and 215,826 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2017 , 458,243 options were exercised by employees at an average exercise price of $30.35 per share for total proceeds of $13.9 million . During the year ended December 31, 2017 , 361,102 ordinary shares were issued in respect of certain RSUs and 320,640 ordinary shares were issued in respect of PSUs previously awarded by the Company. During the year ended December 31, 2016 , 393,240 options were exercised by employees at an average exercise price of $25.79 per share for total proceeds of $10.1 million . During the year ended December 31, 2016 , 296,386 ordinary shares were issued in respect of certain RSUs and 311,492 ordinary shares were issued in respect of PSUs previously awarded by the Company. (a) Share Repurchase Program On October 3, 2016 the Company commenced a previously announced share buyback program of up to $400 million . During the year ended December 31, 2018 , the Company redeemed a total of 1,008,162 ordinary shares under this program for total consideration of $129.0 million . At December 31, 2018 a total of 4,026,576 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $372.1 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 a other undenominated capital fund as required under Irish Company Law. On July 31, 2015 the Company commenced a buyback program of up to $400 million under which the Company could acquire its outstanding ordinary shares (by way of redemption), in accordance with Irish law, the United States securities laws and the Company's constitutional documents through open market share acquisitions. A total of 5,316,062 ordinary shares were redeemed by the Company under this buyback program for a total consideration of $400 million . 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 a other undenominated capital reserve as required under Irish Company Law. The share buyback program was completed in December 31, 2015, with a total of 6,198,481 ordinary shares redeemed during the year ended December 31, 2015 for total consideration of $457.9 million (including the programe for $400 million ). Under the repurchase program, a broker purchased the Company's shares from time to time on the open market or in privately negotiated transactions in accordance with agreed terms and limitations. The program was designed to allow share repurchases during periods when the Company would ordinarily not be permitted to do so because it may be in possession of material non-public or price-sensitive information or due to applicable insider trading laws or self-imposed trading blackout periods. The Company's instructions to the broker were irrevocable and the trading decisions in respect of the repurchase program were made independently of and uninfluenced by the Company. The Company confirms that on entering the share repurchase plans it had no material non-public, price-sensitive or inside information regarding the Company or its securities. Furthermore, the Company will not enter into additional plans whilst in possession of such information. The timing and actual number of shares acquired by way of the redemption will be dependent on market conditions, legal and regulatory requirements and the other terms and limitations contained in the program. In addition, acquisitions under the program may be suspended or discontinued in certain circumstances in accordance with the agreed terms. Therefore, there can be no assurance as to the timing or number of shares that may be acquired under the program.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company's United States and Irish based subsidiaries file tax returns in the United States and Ireland respectively. Other foreign subsidiaries are taxed separately under the laws of their respective countries. The components of income before income tax expense are as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 243,988 $ 218,306 $ 201,221 United States 27,499 28,426 11,466 Other 93,127 81,325 87,485 Income before provision for income taxes $ 364,614 $ 328,057 $ 300,172 The components of provision for income taxes are as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Provision for income taxes: Current tax expense: Ireland $ 28,042 $ 20,084 $ 22,931 United States 2,885 5,792 7,768 Other 9,379 9,964 5,749 Total current tax expense 40,306 35,840 36,448 Deferred tax expense/(benefit): Ireland 1,054 261 1,284 United States 875 8,980 613 Other (277 ) 1,488 (352 ) Total deferred tax expense 1,652 10,729 1,545 Provision for income taxes 41,958 46,569 37,993 Impact on shareholders equity and other comprehensive income of the tax consequence of : Excess tax benefit on stock compensation — — (4,332 ) Currency impact on long term funding 119 973 (396 ) Fair value of cash flow hedge (148 ) 148 — Total $ 41,929 $ 47,690 $ 33,265 Ireland's statutory income tax rate is 12.5% . The Company's consolidated reported provision for income taxes differed from the amount that would result from applying the Irish statutory rate as set forth below: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Taxes at Irish statutory rate of 12.5% (2017:12.5%; 2016:12.5%) $ 45,577 $ 41,007 $ 37,522 Foreign and other income taxed at higher rates 7,649 6,324 4,642 Research & development tax incentives (1,243 ) (830 ) (907 ) Movement in valuation allowance 5,667 1,329 1,208 Effects of change in tax rates (147 ) 925 576 Increase/(decrease) in unrecognized tax benefits (5,423 ) 933 (1,521 ) Impact of stock compensation (8,301 ) (9,917 ) (4,121 ) Impact of deemed repatriation under US Tax Reform — 7,694 — Other (1,821 ) (896 ) 594 Provision for income taxes $ 41,958 $ 46,569 $ 37,993 In 2017, the provision for income taxes included non-recurring items related to US Tax Reform (H.R.1). The income tax expense recognized in respect of deemed repatriation of historic earnings of non-U.S. subsidiaries owned by our U.S. subsidiaries was $7.7 million . The income tax expense recognized in respect of the change in the US federal income tax rate from 35% to 21% was $0.5 million (included in “Effects of change in tax rates” above). The tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below: December 31, 2018 December 31, 2017 (in thousands) Deferred tax liabilities: Property, plant and equipment $ 981 $ 1,139 Goodwill 25,149 22,655 Other intangible assets 9,397 11,801 Other 5,703 4,139 Total deferred tax liabilities recognized 41,230 39,734 Deferred tax assets: Operating loss and tax credits carry-forwards 29,995 24,962 Property, plant and equipment 4,893 4,062 Accrued expenses and payments on account 24,599 24,433 Stock compensation 6,490 5,786 Deferred compensation 2,197 2,548 Deferred revenue 5,681 — Other 2 740 Total deferred tax assets 73,857 62,531 Valuation allowance for deferred tax assets (27,263 ) (22,439 ) Deferred tax assets recognized 46,594 40,092 Overall net deferred tax asset $ 5,364 $ 358 At December 31, 2018 Ireland subsidiaries had tax credit carry-forwards for income tax purposes that may be carried forward indefinitely, available for offset against future tax liabilities, if any, of $4.0 million ( 2017 : $4.5 million ). At December 31, 2018 U.S. subsidiaries had U.S. federal and state net operating loss ("NOL") carry-forwards of approximately $15.1 million and $32.6 million , respectively. These NOLs are available for offset against future taxable income and expire between 2022 and 2037. Of the $15.1 million U.S. federal NOLs, approximately $2.2 million is available for offset against future U.S. federal taxable income. The subsidiary's ability to use the remaining U.S. federal and state NOL carry-forwards is limited on an annual basis due to changes of ownership in 2000, 2010, 2014 and 2017, as defined by Section 382 of the Internal Revenue Code of 1986, as amended. Of the U.S. federal NOLs, $12.9 million are limited by Section 382. Of the $12.9 million of losses, the amounts are available as follows: $4.9 million for the years 2019 – 2020, $7.5 million in 2021-2025, $0.5 million for the years 2026 – 2035. As at December 31, 2018 , U.S subsidiaries also had excess disallowed interest carry-forwards of $21.9 million . These carry-forwards are available for offset against future taxable income in the event that the U.S subsidiaries have excess capacity for interest deductions in future years. At December 31, 2018 other than those in the U.S. and Ireland, we had operating loss carry-forwards for income tax purposes that may be carried forward indefinitely, available to offset against future taxable income, if any, of approximately $72.9 million ( 2017 : $77.2 million ). In addition at December 31, 2018 those subsidiaries had tax credit carry-forwards for income tax purposes that may be carried forward indefinitely, available to offset against future tax liabilities, if any, of $4.9 million ( 2017 : $4.8 million ). At December 31, 2018 those subsidiaries also had additional operating loss carry forwards of $4.7 million which are due to expire between 2019 and 2025 (2017: $4.7 million ) and operating carry-forwards of $3.2 million which are due to expire between 2026 and 2035 (2017: $0 million ). The expected expiry dates of these losses are as follows: Federal NOL's State NOL's (in thousands) 2021-2034 14,323 12,158 2035-2037 766 20,464 $ 15,089 $ 32,622 In addition, US subsidiaries have alternative minimum tax credit carry-forwards of approximately $0.4 million that are available to reduce future U.S. federal regular income taxes through 2020. Any remaining alternative minimum tax credits will be fully refundable in 2021. We also have minimum tax credit carry-forwards of approximately $0.3 million that are available to offset future U.S. federal income taxes. These credits will be fully used or refunded before 2022. The valuation allowance at December 31, 2018 was approximately $27.3 million . The valuation allowance for deferred tax assets as of December 31, 2017 and December 31, 2016 was $22.4 million and $20.3 million respectively. The net change in the total valuation allowance was an increase of $4.8 million during 2018 and an increase of $2.1 million during 2017 . Of the total increase of $4.8 million in 2018 , $5.6 million resulted in current year income tax expense and $0.8 million was recognized in Other Comprehensive Income. Of the total increase of $2.1 million in 2017 , $0.5 million resulted in a current year income tax expense, and $1.6 million was recognized in Other Comprehensive Income. The valuation allowances at December 31, 2018 and December 31, 2017 were primarily related to operating losses and tax credits carried forward that, in the judgment of management, are not more likely than not to be realized. In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers the scheduled reversal of deferred tax liabilities and projected future taxable income in making this assessment. In respect of deferred tax assets not subject to a valuation allowance, management considers that it is more likely than not that these deferred tax assets will be realized on the basis that there will be sufficient reversals of deferred tax liabilities and taxable income in future periods. During 2017 , there were no movements in the valuation allowance that had a material impact on the effective tax rate. During 2018 , the Company recognized a valuation allowance of $6.2 million in respect of disallowed interest carry-forwards generated during the year as management does not consider it more likely than not that the Company will have sufficient capacity with which to utilize these losses. The Company has recognized a deferred tax liability of $4.9 million ( 2017 : $3.1 million ) for investments in foreign subsidiaries where the Company does not consider the earnings to be indefinitely reinvested. For the deferred tax liability not recognized in respect of temporary differences related to investments in foreign subsidiaries which are consider to be indefinitely reinvested, it is not practicable to calculate the exact unrecognized deferred tax liability, however it is not expected to be material as Ireland allows a tax credit in respect of distributions from foreign subsidiaries at the statutory tax rate in the jurisdiction of the subsidiary so that no material tax liability would be expected to arise in the event these earnings were ever remitted. In addition, withholding taxes applicable to remittances from foreign subsidiaries would not be expected to be material given Ireland’s tax treaty network and the EU parent subsidiary directive. A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Unrecognized tax benefits at start of year $ 23,720 $ 26,620 $ 28,166 Increase related to prior year tax positions 2,084 — 1,151 Decrease related to prior year tax positions (2,915 ) (3,050 ) (2,483 ) Increase related to current year tax positions 3,065 4,765 1,104 Settlements (182 ) (2,523 ) (837 ) Lapse of statute of limitations (4,339 ) (2,092 ) (481 ) Unrecognized tax benefits at end of year $ 21,433 $ 23,720 $ 26,620 The relevant statute of limitations for unrecognized tax benefits totaling $1.3 million could potentially expire during 2019. Included in the balance of total unrecognized tax benefits at December 31, 2018 were potential benefits of $21.4 million , which if recognized, would affect the effective rate on income tax from continuing operations. The balance of total unrecognized tax benefits at December 31, 2017 and December 31, 2016 included potential benefits which, if recognized, would affect the effective rate of income tax from continuing operations of $23.7 million and $26.6 million respectively. Interest and penalties recognized as a net benefit during the year ended December 31, 2018 amounted to $1.3 million ( 2017 : net expense of $0.9 million , 2016 : net expense of $0.1 million ) and are included within the provision for income taxes. Total accrued interest and penalties as of December 31, 2018 and December 31, 2017 were $1.1 million and $2.4 million respectively and are included in closing income taxes payable at those dates. Our major tax jurisdictions are the United States and Ireland. We may potentially be subjected to tax audits in both our major jurisdictions. In the United States tax periods open to audit include the years ended December 31, 2015 , December 31, 2016 , December 31, 2017 and December 31, 2018 . In Ireland, tax periods open to audit include the years ended December 31, 2014 , December 31, 2015 , December 31, 2016 , December 31, 2017 and December 31, 2018 . During such audits, local tax authorities may challenge the positions taken by us in our tax returns.

Restructuring Charges

Restructuring Charges	12 Months Ended
	Dec. 31, 2018
Restructuring and Related Activities [Abstract]
Restructuring Charges	Restructuring charges Restructuring and other items recognized during the year ended December 31, 2018 comprise: Year Ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Restructuring charges $ 12,490 7,753 $ 8,159 Net charge $ 12,490 7,753 $ 8,159 Restructuring Charges A restructuring charge of $12.5 million was recognized during the year ended December 31, 2018 , under a restructuring plan adopted following a review of operations. The restructuring plan reflected resource rationalization across the business to improve resource utilizations, resulting in a charge of $9.7 million and office consolidation resulting in the recognition of an onerous lease obligation of $2.8 million . Workforce reductions Onerous Lease Total (in thousands) Total provision recognized $ 9,684 $ 2,806 $ 12,490 Utilized (5,399 ) (672 ) $ (6,071 ) Foreign exchange — — — Provision at December 31, 2018 $ 4,285 $ 2,134 $ 6,419 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 twelve months ended December 31, 2018 . Workforce (in thousands) Restructuring charges $ 7,753 Utilized (4,656 ) Provision at December 31, 2017 $ 3,097 Utilized (1,015 ) Provision at December 31, 2018 $ 2,082 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 charge of $6.2 million and office consolidation resulting in the recognition of an onerous lease of $2.0 million during the twelve months ended December 31, 2016. No additional charge was recorded during the twelve months ended December 31, 2017 and December 31, 2018 . 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 — (317 ) (317 ) Provision at December 31, 2018 $ — $ — $ — At December 31, 2018 $7.2 million is included within other liabilities and $1.3 million within non-current other liabilities.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Litigation The Company is not party to any litigation or other legal proceedings that the Company believes could reasonably be expected to have a material adverse effect on the Company's business, results of operations and financial condition. Operating Leases The Company has several non-cancelable operating leases, primarily for facilities, that expire over the next 13 years . These leases generally contain renewal options and require the Company to pay all executory costs such as maintenance and insurance. The Company recognized $40.3 million , $44.0 million and $44.0 million in rental expense, including rates, for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 respectively. Future minimum rental commitments for operating leases with non-cancelable terms in excess of one year are as follows: Minimum rental payments (in thousands) 2019 32,634 2020 26,839 2021 21,306 2022 15,781 2023 10,829 Thereafter 19,194 Total $ 126,583

Disaggregation of Revenue

Disaggregation of Revenue	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	Disaggregation of Revenue Revenue disaggregated by customer profile is as follows: YTD December 31, 2018 December 31, 2017 (in thousands) Top client $ 352,335 $ 503,875 Clients 2-5 671,723 505,818 Clients 6-10 385,741 348,841 Clients 11-25 461,351 421,759 Other 724,627 622,028 Total $2,595,777 $2,402,321 Revenue for the year ended December 31, 2018 reflects revenue as reported on adoption of ASC 606. Our customers have similar profiles and economic characteristics, and therefore have similar degrees of risk and growth opportunities. Trade accounts receivable, unbilled services (contract assets) and payments on account (contract liabilities) Trade accounts receivables and unbilled revenue are as follows: December 31, 2018 December 31, 2017 (in thousands) Billed services (accounts receivable) $ 423,680 $ 388,431 Unbilled services (unbilled revenue) 362,926 268,509 Trade accounts receivable and unbilled revenue 786,606 656,940 Allowance for doubtful accounts (note 18) (8,889 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 777,717 $ 648,010 Unbilled services and payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 362,926 $ 268,509 $ 94,417 35.2 % Unearned revenue (payments on account) (274,468 ) (298,992 ) 24,524 (8.2 )% Net balance $ 88,458 $ (30,483 ) $ 118,941 390.2 % Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected. These assets are recorded as unbilled receivables and therefore contract assets rather than accounts receivables when receipt of the consideration is conditional on something other than the passage of time. Liabilities are recorded for amounts that are collected in advance of the satisfaction of performance obligations. Unbilled services/revenue balances arise where invoicing or billing is based on the timing of agreed milestones related to service contracts for clinical research. Contractual billing arrangements in respect of certain reimbursable expenses (principally investigators) require billing by the investigator to the Company prior to billing by the Company to the customer. The most significant impact of application of ASC 606 is the measurement of a clinical trial service as a single performance obligation recognized over time. We 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 a clinical trial. The progress towards completion for clinical service contracts is measured based on total project costs (including reimbursable costs). Reimbursable expenses are included within direct costs on adoption of ASC 606 and are recorded based on activity undertaken by the third party. Amounts owed to investigators and others in respect of reimbursable expenses at December 31, 2018 were $85.6 million ( see note 7 Other liabilities ). Payments on account decreased by $24.5 million over the same period resulting in an increase of $118.9 million in the net balance of unbilled services and payments on account between December 31, 2017 and December 31, 2018 . These fluctuations are primarily due to timing of payments and invoicing related to the Group's clinical trial management contracts. Billings and payments are established by contractual provisions including predetermined payment schedules which may or may not correspond to the timing of the transfer of control of the Company's services under the contract. Unbilled services arise from long-term contract when a cost-based input method of revenue recognition is applied and revenue recognized exceeds the amount billed to the customer. The bad debt expense recognized on the Group's receivables and unbilled services was $0.7 million for the twelve months ended December 31, 2018 . As of December 31, 2018 approximately $5.3 billion of revenue is expected to be recognized in the future in respect of unsatisfied performance obligations. The Company expects to recognize revenue on approximately 40% of the unrealized performance obligation over the next 12 months, with the remainder recognized thereafter over the duration of the customer contracts.

Trade Accounts Receivable, Unbi

Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities)	Disaggregation of Revenue Revenue disaggregated by customer profile is as follows: YTD December 31, 2018 December 31, 2017 (in thousands) Top client $ 352,335 $ 503,875 Clients 2-5 671,723 505,818 Clients 6-10 385,741 348,841 Clients 11-25 461,351 421,759 Other 724,627 622,028 Total $2,595,777 $2,402,321 Revenue for the year ended December 31, 2018 reflects revenue as reported on adoption of ASC 606. Our customers have similar profiles and economic characteristics, and therefore have similar degrees of risk and growth opportunities. Trade accounts receivable, unbilled services (contract assets) and payments on account (contract liabilities) Trade accounts receivables and unbilled revenue are as follows: December 31, 2018 December 31, 2017 (in thousands) Billed services (accounts receivable) $ 423,680 $ 388,431 Unbilled services (unbilled revenue) 362,926 268,509 Trade accounts receivable and unbilled revenue 786,606 656,940 Allowance for doubtful accounts (note 18) (8,889 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 777,717 $ 648,010 Unbilled services and payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 362,926 $ 268,509 $ 94,417 35.2 % Unearned revenue (payments on account) (274,468 ) (298,992 ) 24,524 (8.2 )% Net balance $ 88,458 $ (30,483 ) $ 118,941 390.2 % Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected. These assets are recorded as unbilled receivables and therefore contract assets rather than accounts receivables when receipt of the consideration is conditional on something other than the passage of time. Liabilities are recorded for amounts that are collected in advance of the satisfaction of performance obligations. Unbilled services/revenue balances arise where invoicing or billing is based on the timing of agreed milestones related to service contracts for clinical research. Contractual billing arrangements in respect of certain reimbursable expenses (principally investigators) require billing by the investigator to the Company prior to billing by the Company to the customer. The most significant impact of application of ASC 606 is the measurement of a clinical trial service as a single performance obligation recognized over time. We 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 a clinical trial. The progress towards completion for clinical service contracts is measured based on total project costs (including reimbursable costs). Reimbursable expenses are included within direct costs on adoption of ASC 606 and are recorded based on activity undertaken by the third party. Amounts owed to investigators and others in respect of reimbursable expenses at December 31, 2018 were $85.6 million ( see note 7 Other liabilities ). Payments on account decreased by $24.5 million over the same period resulting in an increase of $118.9 million in the net balance of unbilled services and payments on account between December 31, 2017 and December 31, 2018 . These fluctuations are primarily due to timing of payments and invoicing related to the Group's clinical trial management contracts. Billings and payments are established by contractual provisions including predetermined payment schedules which may or may not correspond to the timing of the transfer of control of the Company's services under the contract. Unbilled services arise from long-term contract when a cost-based input method of revenue recognition is applied and revenue recognized exceeds the amount billed to the customer. The bad debt expense recognized on the Group's receivables and unbilled services was $0.7 million for the twelve months ended December 31, 2018 . As of December 31, 2018 approximately $5.3 billion of revenue is expected to be recognized in the future in respect of unsatisfied performance obligations. The Company expects to recognize revenue on approximately 40% of the unrealized performance obligation over the next 12 months, with the remainder recognized thereafter over the duration of the customer contracts.

Provision for Doubtful Debts

Provision for Doubtful Debts	12 Months Ended
	Dec. 31, 2018
Accounts Receivable, Net [Abstract]
Provision for Doubtful Debts	Provision for Doubtful Debts The Company does business with most major international pharmaceutical companies. Provision for doubtful debts at December 31, 2018 comprises: December 31, 2018 December 31, 2017 (in thousands) Opening provision $ 8,930 $ 9,450 Amounts used during the year (995 ) (2,733 ) Amounts provided during the year 3,083 5,116 Amounts released during the year (2,355 ) (3,106 ) Foreign exchange 226 203 Closing provision $ 8,889 $ 8,930

Business Segment and Geographic

Business Segment and Geographical Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Business Segment and Geographical Information	Business Segment and Geographical Information The Company is a clinical research organization ("CRO"), providing outsourced development services on a global basis to the pharmaceutical, biotechnology and medical device industries. It specializes in the strategic development, management and analysis of programs that support all stages of the clinical development process - from compound selection to Phase I-IV clinical studies. The Company has the expertise and capability to conduct clinical trials in most major therapeutic areas on a global basis and has the operational flexibility to provide development services on a stand-alone basis or as part of an integrated "full service" solution. The Company has expanded predominately through internal growth, together with a number of strategic acquisitions to enhance its expertise and capabilities in certain areas of the clinical development process. The Company determines and presents operating segments based on the information that is internally provided to the chief operating decision maker, together the ('CODM') in accordance with FASB ASC 280-10 Disclosure about Segments of an Enterprise 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 in accordance with the requirements of ASC 280-10 Disclosures about Segments of an Enterprises and Related Information . The Company operates as one business segment, which is the provision of outsourced development services on a global basis to the pharmaceutical, biotechnology and medical devices industries. Revenues are allocated to individual entities based on where the work is performed in accordance with the Company's global transfer pricing model. Revenues and income from operations in Ireland are a function of this transfer pricing model. Given ICON Clinical Research Limited's ("ICON Ireland") role in the development and management of the Group, it's ownership of key intellectual property, customer relationships, its key role in the mitigation of risks faced by the Group, plus the responsibility for maintaining the Group's global network, ICON Ireland acts as the group entrepreneur and enters into the majority of the Company's customer contracts. As such, ICON Ireland remunerates most of the other operating entities ("cost plus service providers") in the ICON Group on the basis of a guaranteed cost plus mark up for the services they perform in each of their local territories. The cost plus mark up for each ICON entity is established to ensure that each of ICON Ireland and the ICON entities in the various geographical areas that are involved in the conduct of services for customers, earn an appropriate arms-length return having regard to the assets owned, risks borne and functions performed by each entity from these intercompany transactions. The cost plus mark-up policy is reviewed annually to ensure that it is market appropriate. Under this method, the residual operating profits (or losses) of the Group, once the cost plus service providers have been paid their respective intercompany service fee, are retained by ICON Ireland. The geographic split of revenue disclosed for each region outside Ireland is the cost plus revenue attributable to these entities. The revenues disclosed as relating to Ireland are the net revenues after deducting the cost plus revenues attributable to the activities performed outside Ireland. The Company's areas of operation outside of Ireland include the United States, United Kingdom, Belgium, France, Germany, Italy, Spain, The Netherlands, Sweden, Turkey, Poland, Czech Republic, Latvia, Russia, Ukraine, Hungary, Israel, Romania, Canada, Mexico, Brazil, Colombia, Argentina, Chile, Peru, India, China (including Hong Kong), South Korea, Japan, Thailand, Taiwan, Singapore, The Philippines, Australia, New Zealand, and South Africa. There have been no changes to the basis of segmentation or the measurement basis for the segment results since the prior year. Reportable segment information at December 31, 2018 and December 31, 2017 and for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 is as follows: a) The distribution of revenue by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 Restated* Restated* (in thousands) Ireland $ 1,066,200 $ 944,130 $ 995,365 Rest of Europe 379,883 355,552 319,935 U.S. 894,978 881,829 866,901 Other 254,716 220,810 182,755 Total $ 2,595,777 $ 2,402,321 $ 2,364,956 * 2017 and 2016 restated to reflect gross revenue. b) The distribution of income from operations, including restructuring and other items, by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 257,089 $ 232,032 $ 216,149 Rest of Europe 36,280 26,493 34,200 U.S. 58,561 58,322 41,348 Other 21,427 21,491 19,997 Total $ 373,357 $ 338,338 $ 311,694 c) The distribution of income from operations, excluding restructuring and other items, by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 269,196 $ 240,115 $ 218,334 Rest of Europe 36,904 26,351 36,509 U.S. 58,340 58,164 44,590 Other 21,407 21,461 20,420 Total $ 385,847 $ 346,091 $ 319,853 d) The distribution of property, plant and equipment, net, by geographical area was as follows: December 31, 2018 December 31, 2017 (in thousands) Ireland $ 106,206 $ 111,329 Rest of Europe 9,807 9,026 U.S. 25,535 27,797 Other 17,121 14,899 Total $ 158,669 $ 163,051 e) The distribution of depreciation and amortization by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 34,721 $ 26,277 $ 25,766 Rest of Europe 5,331 6,857 6,914 U.S. 21,605 24,246 23,462 Other 4,259 3,917 3,433 Total $ 65,916 $ 61,297 $ 59,575 f) The distribution of total assets by geographical area was as follows: Year Ended December 31, 2018 December 31, 2017 (in thousands) Ireland $ 1,073,411 $ 880,378 Rest of Europe 514,010 504,418 U.S. 646,512 650,681 Other 120,322 111,141 Total $ 2,354,255 $ 2,146,618 g) The distribution of capital expenditures by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 30,942 $ 24,468 $ 27,670 Rest of Europe 2,590 2,819 2,851 U.S. 9,311 11,027 8,432 Other 5,554 6,403 3,648 Total $ 48,397 $ 44,717 $ 42,601 h) The following table sets forth the clients which represented 10% or more of the Company's net revenue in each of the periods set out below. Year ended December 31, 2018 December 31, 2017 December 31, 2016 Client A 14 % 21 % 26 % i) The distribution of interest income by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 2,620 $ 1,084 $ 407 Rest of Europe 1,750 1,222 1,040 U.S. 17 16 2 Other 372 24 35 Total $ 4,759 $ 2,346 $ 1,484 j) The distribution of the income tax charge by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 29,096 $ 20,345 $ 24,215 Rest of Europe (434 ) 1,921 5,528 U.S. 3,761 14,772 8,381 Other 9,535 9,531 (131 ) Total $ 41,958 $ 46,569 $ 37,993

Supplemental Disclosure of Cash

Supplemental Disclosure of Cash Flow Information	12 Months Ended
	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Disclosure of Cash Flow Information	Supplemental Disclosure of Cash Flow Information Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Cash paid for interest $ 13,060 $ 13,094 $ 13,615 Cash paid for income taxes $ 18,558 $ 12,305 $ 10,205

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income Year ended December 31, 2018 December 31, 2017 (in thousands) Currency translation adjustments $ (62,710 ) $ (36,188 ) Currency impact on long term funding (5,016 ) (182 ) Actuarial loss on defined benefit pension plan (note 9) (3,000 ) (5,855 ) Unrealized capital loss – investments (note 3) (450 ) (295 ) Realized gain on interest rate hedge 4,658 4,658 Amortization of interest rate hedge (2,810 ) (1,887 ) Fair value of cash flow hedge — 1,036 Total $ (69,328 ) $ (38,713 )

Long-Term Debt - Senior Notes

Long-Term Debt - Senior Notes	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Long-Term Debt - Senior Notes	Long-Term Debt – Senior Notes In December 2015 the Company issued $350 million in the private placement market which is due for repayment in 2020. The interest rate in respect of the Senior Notes is fixed at 3.64% for the five year term of the agreement. The associated interest cost is recognized in interest expense in the period since drawdown in December 2015. In October 2015 , the Company entered into an interest rate hedge in respect of the planned issuance of the Senior Notes in December 2015. The interest rate hedge matured in November 2015 when the interest rate on the Senior Notes was fixed. The interest rate hedge was effective in accordance with Financial Accounting Standards Board ("FASB") ASC 815, " Derivatives and Hedging ". The cash proceeds ( $4.6 million ), representing the realized gain on the interest rate hedge, were received on maturity in November 2015 and are recorded within Other Comprehensive Income. The realized gain is being amortized to the income statement, net against interest payable, over the period of the Senior Notes. The Senior Notes agreement also includes certain financial covenants that require compliance with a consolidated leverage ratio, a minimum EBIT to consolidated net interest charge ratio and a maximum amount of priority debt, each of which are defined in the Note Purchase and Guarantee Agreement. The Company was in compliance with these covenants at December 31, 2018 . The Senior Notes have not been, and will not be, registered under the Securities Act of 1933, as amended, and may not be offered or sold in the United States absent registration or an applicable exemption from registration requirements.

Impact of New Accounting Pronou

Impact of New Accounting Pronouncements	12 Months Ended
	Dec. 31, 2018
Accounting Changes and Error Corrections [Abstract]
Impact of New Accounting Pronouncements	Impact of New Accounting Pronouncements Impact of new accounting pronouncements adopted during fiscal year-ended December 31, 2018 ASC 606 'Revenue from Contracts with Customers' In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which provides that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The updated standard replaces most existing revenue recognition guidance in U.S. GAAP when it becomes effective and permits the use of either the retrospective or cumulative effect transition method. To achieve the core principle of the new standard, an entity should apply the following steps: (1) identify the contract(s) with a customer; (2) identify the performance obligations 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 a performance obligation. Subsequent to issuing ASU 2014-09, the FASB issued the following amendments concerning clarification of ASU 2014-09. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606), Principal versus Agent Considerations (Reporting Revenue Gross versus Net) (“ASU 2016-08”) , which further clarifies the implementation guidance on principal versus agent considerations. The new guidance requires either a retrospective or a modified retrospective approach to adoption. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606), Identifying Performance Obligations and Licensing (“ASU 2016-10”) , which clarifies the identification of performance obligations and the licensing implementation guidance, while retaining the related principles for those areas. In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients (“ASU 2016-12”), which provides clarification on assessing the collectability criterion, presentation of sales taxes, measurement date for non-cash consideration and completed contracts at transition. The updated standard was effective for ICON in the first quarter of the year ended December 31, 2018. ICON elected to adopt the updated standard using the cumulative effect transition method. Under this transition method, ICON applied the new standard as of the date of initial application (i.e. January 1, 2018), without restatement of comparative period amounts. ICON recorded the cumulative effect of initially applying the new standard (to revenue and cost) as an adjustment to the opening balance of equity at the date of initial application. The most significant impact relates to our assessment of measurement of performance and percentage of completion in respect of our clinical trials service revenue. ICON applied the requirements of the new standard to those contracts not completed at the date of initial application. See note 26 Impact of New Accounting Policy for the impact of the adoption of the new accounting standard. In May 2017, the FASB issued ASU 2017-10 ' Service Concession Arrangements (Topic 853): Determining the Customer of the Operation Services' which clarifies that the customer in a service concession arrangement is always the grantor. This ASU is effective at the same time as ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (the new revenue standard). • If an entity had early adopted the new revenue standard before this ASU was issued (May 16, 2017), the entity may adopt this ASU on its effective date with certain specific transition provisions. • If an entity early adopts the new revenue standard after this ASU was issued, the entity must adopt this ASU at the same time as the new revenue standard with certain specific transition provisions. • An entity may elect to early adopt this ASU before the adoption of the new revenue standard with certain specific transition provisions. The adoption of the ASU did not have a significant impact on the financial statements. In January 2017, the FASB issued ASU 2017-01 ' Business combinations - Clarifying the definition of a business ' to provide a new framework for determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. The ASU may be early adopted. The adoption of ASU 2017-01 did not have a significant impact on the financial statements. In March 2017, the FASB issued ASU 2017-07 ' Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost' which requires companies to present the service cost component of net benefit cost in the same line items in which they report compensation cost. Companies will present all other components of net benefit cost outside operating income, if this subtotal is presented. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2017-07 did not have a significant impact on the financial statements. In January 2016, the FASB issued ASU 2016-01 'Financial Instruments- Overall (Subtopic 825-10): Recognition and Measurement of the Financial Assets and Financial Liabilities' which will significantly change the income statement impact of equity investments and the recognition of changes in fair value of financial liabilities when the fair value option is elected. The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-01did not have a significant impact on the financial statements. In March 2016, the FASB issued ASU 2016-04 , 'Recognition of Breakage for Certain Prepaid Stored-Value Products' , which allows entities to recognize breakage on prepaid stored-value products consistent with how breakage is recognized under the new revenue standard. The exception applies to prepaid stored-value products in physical or digital form, with stored monetary values that are redeemable for goods and services, including those that can be redeemed for cash (e.g. prepaid gift cards issued on a specific payment network and redeemable at network-accepting merchant locations, prepaid telecommunication cards, and traveler's checks). The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-04 did not have a significant impact on the financial statements. In August 2016, the FASB issued ASU 2016-15, ' Classification of Certain Cash Receipts and Cash Payments' , which addresses eight classification issues related to the statement of cash flows: • Debt prepayment or debt extinguishment costs; • Settlement of zero-coupon bonds; • Contingent consideration payments made after a business combination; • Proceeds from the settlement of insurance claims; • Proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; • Distributions received from equity method investees; • Beneficial interests in securitization transactions; and • Separately identifiable cash flows and application of the predominance principle. The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-15 did not have a significant impact on the financial statements. Any contingent consideration payment arrangements arising on business combinations effected in the future will be reviewed for cash flow statement classification in line with ASU 2016-15. There were no contingent consideration payment arrangements during the year ended December 31, 2018. In October 2016, the FASB issued ASU 2016-16, ' Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory', which requires entities to recognize at the transaction date the income tax consequences of intercompany asset transfers other than inventory. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. For all other entities, the ASU is effective for annual periods in fiscal years beginning after December 15, 2018, and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-16 did not have a significant impact on the financial statements. In November 2016, the FASB issued ASU 2016-18, ' Statement of Cash Flows (Topic 230): Restricted Cash', which requires companies to include cash and cash equivalents that have restrictions on withdrawal or use in total cash and cash equivalents on the statement of cash flows. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. For all other entities, the ASU is effective for annual periods in fiscal years beginning after December 15, 2018, and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-18 did not have a material impact on the financial statements. Financial statement effects of tax reform H.R.1 was enacted on December 22, 2017. The effective date of the law for most provisions is January 1, 2018 however the effects were required to be recognized in December 2017 financial statements. In response, the SEC staff issued SAB 118, which allows registrants to record provisional amounts during a 'measurement period'. See Note 13 for assessment of impact of H.R.1 on the December 31, 2017 financial statements. Impact of new accounting pronouncements which will be adopted during fiscal year-ended December 31, 2019 In February 2016, the FASB issued ASU 2016-02, ' Leases ', requiring lessees to recognize a right-of-use asset and a lease liability on the Consolidated Balance Sheet for all leases with the exception of short-term leases. For lessees, leases will continue to be classified as either operating or finance leases in the income statement. Lessor accounting is similar to the current model but updated to align with certain changes to the lessee model. Lessors will continue to classify leases as operating, direct financing or sales-type leases. The effective date of the new standard for public companies is for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted. The new standard must be adopted using a modified retrospective transition and requires application of the new guidance at the beginning of the earliest comparative period presented. The updated standard is effective for us beginning in the first quarter of the year-ended December 31, 2019. We are currently evaluating the effect that the updated standard will have on our consolidated financial statements and related disclosures. See Note 15 Commitments and Contingencies for details of operating leases held during year-ended December 31, 2018. A lease liability and right-of-use asset will be recorded on the Consolidated Balance Sheet at December 31, 2019. We estimate the impact of the adoption of ASC 842 will be the recognition of a right-of use asset and of lease liabilities in the range of $100 million to $110 million . Under the new standard we expect an insignificant change in net result due to the replacement of operating leases expenses with amortization of the lease asset and the interest expense. In July 2018, the FASB issued ASU 2018-11, which creates a new, optional transition method for implementing ASU 2016-02 and a lessor practical expedient for separating lease and non-lease components. This ASU is effective: • For entities that have not early adopted ASU 2016-02, with the effective date of ASU 2016-02. • For entities that have early adopted ASU 2016-02, upon issuance. However, it can only be adopted by entities either at (1) the beginning of the company’s first reporting period after issuance or (2) the entity’s mandatory ASC 842 effective date. The Company has elected to adopt this transition method for implementing ASU 2016-02 and the related practical expedient. In July 2018, the FASB issued ASU 2018-10, which clarifies and corrects errors in ASC 842. The effective date and transition requirements in ASU 2018-10 are the same as the effective date and transition requirements of ASU 2016-02. In January 2018, the FASB issued ASU 2018-01, which clarifies that land easements are in the scope of ASU 2016-02, Leases, and provides transition relief. The effective date and transition requirements in ASU 2018-01 are the same as the effective date and transition requirements of ASU 2016-02. In August 2017, the FASB issued ASU 2017-12 ' Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities ' which changes the recognition and presentation requirements of hedge accounting, including: • Eliminating the requirement to separately measure and report hedge ineffectiveness; and • Presenting all items that affect earnings in the same income statement line item as the hedged item. The ASU also provides new alternatives for: • Applying hedge accounting to additional hedging strategies; • Measuring the hedged item in fair value hedges of interest rate risk; • Reducing the cost and complexity of applying hedge accounting by easing the requirements for effectiveness testing, hedge documentation and application of the critical terms match method; and • Reducing the risk of material error correction if a company applies the shortcut method inappropriately. This ASU is effective for public business entities, for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted any time after the issuance of the ASU, including in an interim period. If adopted at other than the beginning of a fiscal year, cumulative effect adjustments are reflected as of the beginning of the fiscal year. The adoption of the ASU is not expected to have a significant impact on the financial statements. In February 2017, the FASB issued ASU 2017-05 ' Other Income-Gains and Losses from the Derecognition of Nonfinancial Assets (Subtopic 610-20): Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets'. In February 2017, the FASB issued ASU 2017-05, which clarifies the guidance in Subtopic 610-20 on accounting for derecognition of a nonfinancial asset. The ASU also defines in-substance nonfinancial assets and includes guidance on partial sales of non-financial assets. The adoption of ASU 2017-05 is not expected to have an impact on the financial statements. In July 2017, the FASB issued ASU 2017-11 ' Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception ' under which down round features will not cause certain equity-linked financial instruments to be accounted for as derivatives. A company that presents EPS information will reflect the effect of a down round feature of free-standing equity-linked financial instruments in EPS only if it is triggered. The ASU is effective for public business entities, for annual and interim periods in fiscal years beginning after December 15, 2018. The adoption of the ASU is not expected to have an impact on the financial statements. In June 2016, the FASB issued ASU 2016-13, ' Measurement of Credit Losses on Financial Instruments' , which significantly changes the way entities recognize impairment of many financial assets by requiring immediate recognition of estimated credit losses expected to occur over their remaining life. The ASU is effective for public business entities that are SEC filers for interim and annual periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-13 is not expected to have a significant impact on the financial statements. In July 2018, the FASB issued ASU 2018-09, which clarifies and corrects unintended application of guidance, and makes improvements to several Codification Topics. The changes are part of an ongoing FASB project to make non-substantive technical corrections, clarifications, and improvements that are not expected to have a significant effect on accounting practice or create a significant administrative cost to most entities. Most of the amendments are effective immediately. Some of the amendments are effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2018. • All other entities for annual periods in fiscal years beginning after December 15, 2019, and interim periods in fiscal years beginning after December 15, 2020. Other amendments, which affect recently issued ASUs that are not yet effective, are effective with the original ASU. The impact of adoption of ASU 2018-19 is not expected to be significant. In June 2018, the FASB issued ASU 2018-08, which clarifies and improves current guidance about whether a transfer of assets (or the reduction, settlement, or cancellation of liabilities) is a contribution or an exchange transaction. For transactions in which an entity is either a public business entity or an NFP that has issued, or is a conduit bond obligor for securities that are traded, listed, or quoted on an exchange or an over-the-counter market and serves as a resource recipient, the entity should apply the amendments in this ASU on contributions received to annual and interim periods in fiscal years beginning after June 15, 2018. All other entities should apply the amendments for transactions in which the entity serves as the resource recipient to annual periods in fiscal years beginning after December 15, 2018 and interim periods in fiscal years beginning after December 15, 2019. For transactions in which an entity is either a public business entity or an NFP that has issued, or is a conduit bond obligor for securities that are traded, listed, or quoted on an exchange or an over-the-counter market and serves as a resource provider, the entity should apply the amendments in this ASU on contributions made to annual and interim periods in fiscal years beginning after December 15, 2018. All other entities should apply the amendments for transactions in which the entity serves as the resource provider for annual periods in fiscal years beginning after December 15, 2019 and interim periods in fiscal years beginning after December 15, 2020. Early adoption of the amendments is permitted. The adoption of ASU 2018-08 is not expected to have a significant impact on the financial statements. In February 2018, the FASB issued ASU 2018-02, which allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted, including adoption in any interim period, (1) for public business entities for reporting periods for which financial statements have not yet been issued and (2) for all other entities for reporting periods for which financial statements have not yet been made available for issuance. The adoption of ASU 2018-02 is not expected to have a significant impact on the financial statements. In December 2017, the FASB issued ASU 2017-15. The ASU eliminates Topic 995, which includes an exemption to the recognition of deferred taxes on certain statutory reserve deposits that were, but are no longer, tax deferred. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted for all entities, including adoption in an interim period. The adoption of ASU 2017-15 is not expected to have a significant impact on the financial statements. Impact of other new accounting pronouncements In January 2017, the FASB issued ASU 2017-04 ' Intangibles - Goodwill and Other: Simplifying the test for goodwill impairment ' which requires an entity to no longer perform a hypothetical purchase price allocation to measure goodwill impairment. Instead, impairment will be measured using the difference between the carrying amount and the fair value of the reporting unit. The ASU is effective for public businesses, that are SEC filers, for annual and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2017-04 is not expected to have a significant impact on the financial statements. In August 2018, the FASB issued ASU 2018-15, which clarifies that implementation costs incurred by customers in cloud computing arrangements are deferred if they would be capitalized by customers in software licensing arrangements under the internal-use software guidance. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2019. • Early adoption is permitted. Our evaluation of the impact of adoption of ASU 2018-15 is on-going. In August 2018, the FASB issued ASU 2018-14, which aims to improve the overall usefulness of disclosures to financial statement users and reduce unnecessary costs to companies when preparing defined benefit plan disclosures. This ASU is effective for: • Public business entities for financial statements issued for fiscal years ending after December 15, 2020. • Retrospective adoption is required and early adoption is permitted. Our evaluation of the disclosure impact of adoption of ASU 2018-14 is on-going. In August 2018, the FASB issued ASU 2018-13, which aims to improve the overall usefulness of disclosures to financial statement users and reduce unnecessary costs to companies when preparing fair value measurement disclosures. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2019. Retrospective adoption is required except for the following changes, which are required to be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption: • Changes in unrealized gains and losses included in other comprehensive income for Level 3 instruments; • The range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements; and • The narrative description of measurement uncertainty. Early adoption is permitted. An entity may early adopt any eliminated or modified disclosure requirements and delay adoption of the additional disclosure requirements until their effective date. In such cases, users should refer to the Codification for disclosure requirements. Our evaluation of the disclosure impact of adoption of ASU 2018-13 is on-going. In August 2018, the FASB issued ASU 2018-12, which changes how insurance entities recognize, measure, present and disclose long-duration contracts. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2020. • Early adoption is permitted. The adoption of ASU 2018-12 is not expected to impact on the financial statements. In June 2018, the FASB issued ASU 2018-07, which more closely aligns the accounting for employee and nonemployee share-based payments. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2018. • The adoption of ASU 2018-07 is not expected to have a significant impact on the financial statements.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Parties	Related Parties Subsidiaries of the Company earned revenue of $633,000 (2017: $743,000 ) from DS Biopharma Limited (formerly Dignity Sciences Limited) during the year. Dr. John Climax is Chief Executive Officer and both Dr. John Climax and Dr. Ronan Lambe are Directors and shareholders of DS Biopharma Limited. $338,000 was recorded as due from DS Biopharma Limited at December 31, 2018. The contract terms were agreed on an arm’s length basis. During the year ended December 31, 2017, personal expenses totaling $178,000 were settled by the Company on behalf of Mr. Ciaran Murray. Payment was received in advance from Mr. Murray in respect of these expenses. The Company transferred ownership of an asset at fair value ( $77,000 ) to Mr. Ciaran Murray effective November 1, 2017. Payment was received in full in January 2018. On July 22, 2016, Mr. Thomas Lynch retired as a Director of the Company, having previously resigned as Chairman of the Company in March 2016. A charge of €231,750 was recorded during 2018 in respect of consultancy services provided by a company controlled by Mr. Lynch. $ 64,000 was recorded as due to Mr. Lynch under the terms of the agreement at December 31, 2018.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 8, 2019 the Company announced a share buyback program of up to 1 million shares to be executed opportunistically during 2019 depending on cash commitments. All ordinary shares that are redeemed under the buyback program will be cancelled in accordance with the constitutional documents of the Company and the nominal value of these shares transferred to an undenoninated capital fund as required under Irish Company law. On January 25, 2019 a subsidiary of the Company, ICON Laboratory Services Inc. acquired 100% of the equity share capital of MolecularMD Corp. The initial consideration on acquisition is $42 million . The Company has evaluated subsequent events from the balance sheet date through March 1, 2019, the date at which the consolidated financial statements were available to be issued. The Company has determined that there are no items , other than the item described above, to disclose.

Impact of Change in Accounting

Impact of Change in Accounting Policies	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Impact of change in accounting policies	mpact of change in 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 twelve months ended December 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. 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 service obligation in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Promises offered to the customer are not distinct within the context of the contract. We have concluded that ICON is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support the clinical research project. The transaction price is determined by reference to the contract or change order value (total service revenue and pass-through/ reimbursable expenses) adjusted to reflect a realizable contract value. Revenue is recognized as the single performance obligation is satisfied. The progress towards completion for clinical service contracts is measured based on an input measure being total project costs (inclusive of third party costs) at each reporting period. Incremental costs of obtaining a contract were also considered on adoption of ASC 606. Commission costs of $12 million were recognized as an asset on the Consolidated Balance Sheet in respect of those contracts that exceed one year. Where commission costs relate to contracts that are less than one year, the practical expedient is applied as the amortization period of the asset which would arise on deferral would be one year or less. A deferred tax asset of $6.9 million was recognized in respect of the timing differences arising from cumulative impact of non tax adjustments recorded on adoption of ASC 606. These are expected to reverse in future periods. The tables on the pages following summarize the impact of adopting ASC 606 on the consolidated financial statements for the twelve months ended December 31, 2018 . ICON plc CONDENSED CONSOLIDATED BALANCE SHEETS AS AT DECEMBER 31, 2018 December 31, 2018 As reported Adjustments Balance without adoption of Topic 606 ASSETS (in thousands) Current Assets: Cash and cash equivalents $ 395,851 $ — $ 395,851 Available for sale investments 59,910 — 59,910 Accounts receivable, net 414,791 — 414,791 Unbilled revenue 362,926 46,642 409,568 Other receivables 40,459 (12,580 ) 27,879 Prepayments and other current assets 36,801 — 36,801 Income taxes receivable 19,445 — 19,445 Total current assets 1,330,183 34,062 1,364,245 Other Assets: Property, plant and equipment, net 158,669 — 158,669 Goodwill 756,260 — 756,260 Other non-current assets 14,525 — 14,525 Non-current income taxes receivable 20,023 — 20,023 Non-current deferred tax asset 13,577 (5,680 ) 7,897 Investments in equity- long term 6,963 — 6,963 Intangible assets 54,055 — 54,055 Total Assets $ 2,354,255 $ 28,382 $ 2,382,637 LIABILITIES AND SHAREHOLDERS’ EQUITY Current Liabilities: Accounts payable $ 13,288 $ — $ 13,288 Payments on account 274,468 59,242 333,710 Other liabilities 317,143 (84,342 ) 232,801 Income taxes payable 5,724 1,669 7,393 Total current liabilities 610,623 (23,431 ) 587,192 Other Liabilities: Non-current bank credit lines and loan facilities 349,264 — 349,264 Non-current other liabilities 13,446 — 13,446 Non-current government grants 877 — 877 Non-current income taxes payable 17,551 — 17,551 Non-current deferred tax liability 8,213 — 8,213 Commitments and contingencies — — — Total Liabilities 999,974 (23,431 ) 976,543 Shareholders' Equity: Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017. 4,658 — 4,658 Additional paid‑in capital 529,642 — 529,642 Other undenominated capital 983 — 983 Accumulated other comprehensive income (69,328 ) — (69,328 ) Retained earnings 888,326 51,813 940,139 Total Shareholders' Equity 1,354,281 51,813 1,406,094 Total Liabilities and Shareholders' Equity $ 2,354,255 $ 28,382 $ 2,382,637 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS FOR THE TWELVE MONTHS ENDED DECEMBER 31, 2018 YTD December 31, 2018 December 31, 2018 December 31, 2018 As reported Adjustments Balance without adoption of Topic 606 (in thousands except share and per share data) Revenue: Revenue $ 2,595,777 $ 4,657 $ 2,600,434 Reimbursable expenses — (702,812 ) $ (702,812 ) (698,155 ) 1,897,622 Costs and expenses: Direct costs 1,818,220 (702,812 ) 1,115,408 Selling, general and administrative expense 325,794 472 326,266 Depreciation and amortization 65,916 — 65,916 Restructuring 12,490 — 12,490 Total costs and expenses 2,222,420 (702,340 ) 1,520,080 Income from operations 373,357 4,185 377,542 Interest income 4,759 — 4,759 Interest expense (13,502 ) — (13,502 ) Income before income tax expense 364,614 4,185 368,799 Income tax expense (Note 13) (41,958 ) (476 ) (42,434 ) Net income $ 322,656 $ 3,709 $ 326,365 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME 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 — — — — — 322,656 322,656 Impact of change in accounting policy — — — — — 3,709 3,709 Currency translation adjustment — — — — (26,522 ) — (26,522 ) Currency impact of long-term funding — — — — (4,834 ) — (4,834 ) Unrealized capital loss - investments — — — — (155 ) — (155 ) Actuarial gain on defined benefit pension plan — — — — 2,855 — 2,855 Amortization of interest rate hedge — — — — (923 ) — (923 ) Fair value of cash flow hedge — — — — (1,036 ) — (1,036 ) Total comprehensive income — — — — — — 295,750 Exercise of share options 408,699 29 16,777 — — — 16,806 Issue of restricted share units / performance share units 489,568 36 — — — — 36 Share based compensation expense — — 31,544 — — — 31,544 Share issue costs — — (16 ) — — — (16 ) Repurchase of ordinary shares (1,008,162 ) (71 ) — 71 — (128,960 ) (128,960 ) Share repurchase costs — — — — — (66 ) (66 ) Balance at December 31, 2018 53,971,706 4,658 529,642 983 (69,328 ) 940,139 1,406,094 There is no impact of adoption of ASC 606 on the CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME. An adjustment to retained earnings as at January 1, 2018 arises on adoption of ASC 606 and this is presented in the primary statement reflecting ASC 606 adoption. ICON plc CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS FOR THE TWELVE MONTHS ENDED DECEMBER 31, 2018 As Reported Adjustments Balance without adoption of Topic 606 (in thousands) Cash flows from operating activities: Net income 322,656 3,709 326,365 Adjustments to reconcile net income to net cash provided by operating activities: Loss on disposal of property, plant and equipment 70 — 70 Depreciation expense 50,565 — 50,565 Amortization of intangibles 15,351 — 15,351 Amortization of government grants (47 ) — (47 ) Interest on short term investments (1,329 ) — (1,329 ) Realized (gain)/loss on sale of short term investments (56 ) — (56 ) Gain on re-measurement of financial assets (800 ) — (800 ) Amortization of gain on interest rate hedge (923 ) — (923 ) Amortization of financing costs 812 — 812 Stock compensation expense 31,594 — 31,594 Deferred tax expense 1,652 5,680 7,332 Changes in assets and liabilities: — — Increase in accounts receivable (37,557 ) — (37,557 ) Increase in unbilled revenue (98,510 ) 1,462 (97,048 ) Decrease in other receivables 3,107 12,580 15,687 Increase in prepayments and other current assets (3,237 ) — (3,237 ) Decrease in other non-current assets 856 — 856 Decrease in payments on account (6,253 ) 59,242 52,989 Increase in other current liabilities 2,009 (84,342 ) (82,333 ) Decrease in other non-current liabilities (1,034 ) — (1,034 ) Decrease in income taxes payable (5,220 ) 1,669 (3,551 ) Decrease in accounts payable (5,067 ) — (5,067 ) Net cash provided by operating activities 268,639 — 268,639 Cash flows from investing activities: — — Purchase of property, plant and equipment (48,397 ) — (48,397 ) Purchase of subsidiary undertakings (1,645 ) — (1,645 ) Sale of available for sale investments 99,865 — 99,865 Purchase of available for sale investments (80,956 ) — (80,956 ) Purchase of investments in equity - long term (6,163 ) — (6,163 ) Net cash used in investing activities (37,296 ) — (37,296 ) Cash flows from financing activities: Financing costs (823 ) — (823 ) Proceeds from the exercise of equity compensation 16,842 — 16,842 Share issue costs (16 ) — (16 ) Repurchase of ordinary shares (128,960 ) — (128,960 ) Share repurchase costs (66 ) — (66 ) Net cash used in financing activities (113,023 ) — (113,023 ) Effect of exchange rate movements on cash (5,328 ) — (5,328 ) Net increase in cash and cash equivalents 112,992 — 112,992 Cash and cash equivalents at beginning of year 282,859 — 282,859 Cash and cash equivalents at end of year $ 395,851 $ — $ 395,851

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Consolidation	Basis of consolidation The consolidated financial statements include the financial statements of the Company and all of its subsidiaries. All significant intercompany profits, transactions and account balances have been eliminated. The results of subsidiary undertakings acquired in the period are included in the Consolidated Statement of Operations from the date of acquisition.
Use of Estimates	Use of estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reported period. Actual results could differ from those estimates. The principal management estimates and judgments used in preparing the financial statements relate to revenue recognition and taxation.
Revenue Recognition	Revenue recognition The Company primarily earns revenues by providing a number of different services to its customers. These services, which are integral elements of the clinical development process, include clinical trials management, consulting, contract staffing, and laboratory services. Contracts range in duration from a number of months to several years. ASC 606 - Revenue from Contracts with Customers (year-ended December 31, 2018) ICON adopted ASC 606 ' 'Revenue from Contracts with Customers ' 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. The comparative information has not been adjusted and therefore continues to be reported under ASC 605 ‘Revenue Recognition’ and therefore in accordance with previous accounting policies. See note 26 Impact of change in accounting policies for details of the impact of adoption of the new accounting policy. 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. Clinical trial service revenue 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 on a percentage of completion basis. We have concluded that under the new standard, a clinical trial service is a single performance obligation satisfied over time i.e. the full service obligation in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Promises offered to the customer are not distinct within the context of the contract. We have concluded that ICON is the contract principal in respect of both direct services and in the use of third parties (principally investigator services) that support the clinical research project. The transaction price is determined by reference to the contract or change order value (total service revenue and pass-through/ reimbursable expenses) adjusted to reflect a realizable contract value. Revenue is recognized as the single performance obligation is satisfied. The progress towards completion for clinical service contracts is measured based on an input measure being total project costs (inclusive of third party costs) at each reporting period. Contracting services revenue On evaluation of the principles at (1) - (5) set-out above in respect of ASC 606, the Company has availed of the practical expedient which results in recognition of revenue on a right to invoice basis. Application of the practical expedient reflects the right to consideration from the customer in an amount that corresponds directly with the value to the customer of the performance completion to date. This reflects hours performed by contract staff. Consulting services revenue On application of the ASC 606 ' Revenue from Contracts with Customers ' principles at (1) - (5) set-out above, we have concluded that our consulting services contracts represent a single performance obligation satisfied over time. The transaction price is determined by reference to contract or change order value. Revenue is recognized as the performance obligation is satisfied. The progress towards completion for consulting contracts is measured based on total project inputs (time) at each reporting period. Laboratory services revenue Revenue is recognized when, or as, obligations under the terms of a contract are satisfied, which occurs when control of the products or services are transferred to the customer. Revenue for laboratory services is measured as the amount of consideration we expect to receive in exchange for transferring products or services. Where contracts with customers contain multiple performance obligations, the transaction price is allocated to each performance obligation based on the estimated relative selling price of the promised good or service. Service revenue is recognized over time as the services are delivered to the customer based on the extent of progress towards completion of the performance obligation. The determination of the methodology to measure progress requires judgment and is based on the nature of services provided. This requires an assessment of the transfer of value to the customer. The right to invoice measure of progress is generally related to rate per unit contracts, as the extent of progress towards completion is measured based on discrete service or time-based increments, such as samples tested or labor hours incurred. Revenue is recorded in the amount invoiced since that amounts corresponds to the value of the Company's performance and the transfer of value to the customer. ASC 605 - Revenue recognition (years ended December 31, 2017 and December 31, 2016) Revenue for services, as rendered, is recognized only after persuasive evidence of an arrangement exists, the sales price is fixed or determinable and collectability is reasonably assured. Clinical trials management revenue is recognized on a proportional performance method. Depending on the contractual terms revenue is either recognized on the percentage of completion method based on the relationship between hours incurred and the total estimated hours of the trial or on the unit of delivery method. Contract costs equate to the product of labor hours incurred and compensation rates. For the percentage of completion method, the input (effort expended) method has been used to measure progress towards completion as there is a direct relationship between input and productivity. Contract revenue is the product of the aggregated labor hours required to complete the specified contract tasks at the agreed contract rates. The Company regularly reviews the estimate of total contract time to ensure such estimates remain appropriate taking into account actual contract stage of completion, remaining time to complete and any identified changes to the contract scope. Remaining time to complete depends on the specific contract tasks, the complexity of the contract and can include geographical site selection and initiation, patient enrollment, patient testing and level of results analysis required. While the Company may routinely adjust time estimates, the Company's estimates and assumptions historically have been accurate in all material respects in the aggregate. Where revenue is recognized on the unit of delivery method, the basis applied is the number of units completed as a percentage of the total number of contractual units. Consulting revenue is recognized on a fee-for-service basis as each hour of the related service is performed. Contract staffing revenue is recognized on a fee-for-service basis, over the time the related service is performed, or in the case of permanent placement, once the candidate has been placed with the client. Laboratory service revenue is recognized on a fee-for-service basis. The Company accounts for laboratory service contracts as multiple element arrangements, with contractual elements comprising laboratory kits and laboratory testing, each of which can be sold separately. Sales prices for contractual elements are determined by reference to objective and reliable evidence of their sales price. Revenues for contractual elements are recognized on the basis of the number of deliverable units completed in the period. Informatics revenue is recognized on a fee-for-service basis. Informatics contracts are treated as multiple element arrangements, with contractual elements comprising license fee revenue, support fee revenue and revenue from software services, each of which can be sold separately. Sales prices for contractual elements are determined by reference to objective and reliable evidence of their sales price. License and support fee revenues are recognized rateably over the period of the related agreement. Revenue from software services is recognized using the percentage of completion method based on the relationship between hours incurred and the total estimated hours required to perform the service. Contracts generally contain provisions for renegotiation in the event of changes in the scope, nature, duration, or volume of services of the contract. Renegotiated amounts are recognized as revenue by revision to the total contract value arising as a result of an authorized customer change order. (d) Third party costs (Reimbursable expenses) Reimbursable expenses comprise investigator payments and certain other costs which are reimbursed by clients under terms specific to each contract to the investigators. Third party costs (Reimbursable expenses) and the related revenue were separately presented on the face of the Consolidated Statement of Operations for periods up to and including the year-ended December 31, 2017. See sections (c) and (e) for accounting policy in respect of the treatment of activity relating to reimbursable expenses on revenue (c) and costs (e) on adoption of ASC 606 ' Revenue from Contracts with Customers '. (e) Direct costs Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs. On adoption of ASC 606 'Revenue from Contracts with Customers', reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time i.e. the full service obligation is in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred.
Direct Costs	Direct costs Direct costs consist of compensation, associated employee benefits and share-based payments for project-related employees and other direct project-related costs. On adoption of ASC 606 'Revenue from Contracts with Customers', reimbursable expenses are presented within direct costs. This presentation is to align the presentation of costs with our assessment that our clinical trial service is a single performance obligation satisfied over time i.e. the full service obligation is in respect of a clinical trial (including those services performed by investigators and other parties) is considered a single performance obligation. Reimbursable expenses are recorded once the activity which forms the basis for the cost has occurred. Direct costs for the year-ended December 31, 2018 are therefore inclusive of third party costs of $702.8 million . Investigator payment costs are recorded and reported reflecting investigator activity over the life of the contract. Investigator payments are made based on predetermined contractual arrangements. Payments may differ from the recording and reporting of the expense which is based on activity.
Advertising Costs	Advertising costs All costs associated with advertising and promotion are expensed as incurred.
Foreign Currencies and Translation of Subsidiaries	The financial statements of subsidiaries with other functional currencies are translated at period end rates for the Consolidated Balance Sheet and average rates for the Consolidated Statement of Operations. Translation gains and losses arising are reported as a movement on accumulated other comprehensive income. Foreign currency transaction gains and losses are reported in other comprehensive income rather than through income where the foreign currency transaction is 'long-term investment' in nature i.e. settlement is not planned or anticipated in the foreseeable future. Foreign currencies and translation of subsidiaries The Company's financial statements are prepared in United States dollars. Transactions in currencies other than United States dollars are recorded at the rate ruling at the date of the transaction. Monetary assets and liabilities denominated in currencies other than United States dollars are translated into United States dollars at exchange rates prevailing at the Balance Sheet date. Adjustments resulting from these translations are charged or credited to income.
Disclosure of Fair Value of Financial Instruments	Disclosure of fair value of financial instruments Cash, cash equivalents, unbilled revenue, other receivables, available for sale investments, prepayments and other current assets, accounts receivable, accounts payable, investigator payments, payments on account, accrued liabilities, accrued bonuses and income taxes payable have carrying amounts that approximate fair value due to the short term maturities of these instruments. Other liabilities' carrying amounts approximate fair value based on net present value of estimated future cash flows. Debt is measured at historical cost. Financial instruments are measured in the Consolidated Balance Sheet at fair value using a fair value hierarchy of valuation inputs. The hierarchy prioritizes the inputs into three levels based on the extent to which inputs used in measuring fair value are observable in the market. Each fair value measurement is reported in one of three levels, which is determined by the lowest level input that is significant to the fair value measurement in its entirety. These levels are: Level 1: Inputs are based upon unadjusted quoted prices for identical instruments traded in active markets. Level 2: Inputs are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant assumptions are observable in the market or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Inputs are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. The Group's Senior notes (private placement debt) is carried at $350.0 million (prior to related financing costs). The carrying value at December 31, 2018, closely approximates fair value. The Company classifies its investments in short term debt or equity investments as available for sale, as it does not actively trade such securities nor does it intend to hold them to maturity. The fair value of short term investments are represented by level 1 fair value measurements – quoted prices in active markets for identical assets. The unrealized movements in fair value are recognized in equity until disposal or sale, at which time, those unrealized movements from prior periods are recognized in Consolidated Statement of Operations. Losses other than temporary, which reduce the carrying amount below cost are recognized in Consolidated Statement of Operations.
Business Combinations	Business combinations The cost of a business combination is measured as the aggregate of the fair values at the date of exchange of assets given, liabilities incurred or assumed and equity instruments issued in exchange for control. Where a business combination agreement provides for an adjustment to the cost of the acquisition which is contingent upon future events, the amount of the estimated adjustment is recognized at the acquisition date at the fair value of the contingent consideration. Any changes to this estimate outside the measurement period will depend on the classification of the contingent consideration. If the contingent consideration is classified as equity it shall not be re-measured and the settlement shall be accounted for within equity. If the contingent consideration is classified as a liability any adjustments will be accounted for through the Consolidated Statement of Operations or Other Comprehensive Income depending on whether the liability is considered a financial instrument. The assets, liabilities and contingent liabilities of businesses acquired are measured at their fair values at the date of acquisition. In the case of a business combination which is completed in stages, the fair values of the identifiable assets, liabilities and contingent liabilities are determined at the date of each exchange transaction. When the initial accounting for a business combination is determined provisionally, any subsequent adjustments to the provisional values allocated to the identifiable assets, liabilities and contingent liabilities are made within twelve months of the acquisition date and presented as adjustments to goodwill in the reporting period in which the adjustments are determined.
Goodwill and Impairment	Goodwill and Impairment Goodwill represents the excess of the cost of acquired entities over the net amounts assigned to assets acquired and liabilities assumed. Goodwill primarily comprises acquired workforce in place which does not qualify for recognition as an asset apart from goodwill. Goodwill is stated net of any provision for impairment. The Company tests goodwill annually for any impairments or whenever events occur which may indicate impairment. The Company applied the provisions of ASU 2017-04 with effect from January 1, 2018. Under the amendment, the Company was required to perform its annual goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An impairment charge would be recognized for any amount by which the carrying amount exceeds the reporting unit's fair value up to the amount of existing goodwill. The amendment allows an entity to perform a qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary.
Intangible Assets	Intangible assets Intangible assets are amortized on a straight line basis over their estimated useful life.
Cash and Cash Equivalents	Cash and cash equivalents Cash and cash equivalents include cash and highly liquid investments with initial maturities of three months or less and are stated at cost, which approximates market value.
Investment in Debt, Equity and Other	Investments in debt, equity and other Available for sale investments The Company classifies short-term investments as available for sale in accordance with the terms of FASB ASC 320, Investments – Debt and Equity Securities . Realized gains and losses are determined using specific identification. The investments are reported at fair value, with unrealized gains or losses reported in a separate component of shareholders' equity. Any differences between the cost and fair value of the investments are represented by accrued interest and unrealized gains/losses. Long term investments The Company classifies its interests in funds having considered the nature of its investment, the extent of influence over operating and financial decisions and the availability of readily determinable fair values. The Company determined that the interests in funds at December 31, 2018 meet the definition of equity securities without readily determinable fair values. Effective from 1 January 2018, the Company concluded that the interests held at December 31, 2018 qualify for the NAV practical expedient in ASC 820 ' Fair value measurements and disclosure s'. Any increases or decreases in fair value are recognized in net income in the period.
Accounts Receivable, net	Accounts receivable, net Accounts receivable are recorded at fair value less an allowance for doubtful accounts. The allowance is an estimate based on historical collection experience, current economic and market conditions, and a review of the current status of each customer's trade accounts receivable. Account balances are written-off against the allowance when the Group determines that it is probable that the receivable will not be recovered.
Inventory	Inventory Inventory is valued at the lower of cost and net realizable value and after provisions for obsolescence. The cost of inventories comprises the purchase price and attributable costs, less trade discounts.
Property, Plant and Equipment	Property, plant and equipment Property, plant and equipment is stated at cost less accumulated depreciation. Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5 Leasehold improvements are amortized using the straight line method over the estimated useful life of the asset or the lease term, whichever is shorter.
Leased Assets	Leased assets Costs in respect of operating leases are charged to the Consolidated Statement of Operations on a straight line basis over the lease term. Assets acquired under capital finance leases are included in the Consolidated Balance Sheet at the present value of the future minimum lease payments and are depreciated over the shorter of the lease term and their remaining useful lives. The corresponding liabilities are recorded in the Consolidated Balance Sheet and the interest element of the capital lease rental is charged to interest expense.
Income Taxes	Income taxes The Company applies the asset and liability method of accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry-forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which these temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance to the amount more likely than not to be realized. The Company recognizes the effect of income tax positions only if those positions will more likely than not be sustained. Recognized income tax positions are measured at the largest amount of tax benefit that is greater than 50 percent likely of being realized upon settlement. Interest and penalties related to income taxes are included in income tax expense and classified with the related liability on the Consolidated Balance Sheet. The Company accounts for the impact of GILTI (“global intangible low-taxed income”) as a period item in the period it arises and has therefore not provided for deferred taxes in respect of this item.
Government Grants	Government grants Government grants received relating to capital expenditures are shown as deferred income and credited to income on a basis consistent with the depreciation policy of the relevant assets. Grants relating to categories of operating expenditures are credited to income in the period in which the expenditure to which they relate is charged. Under the grant agreements amounts received may become repayable in full should certain circumstances specified within the grant agreements occur, including downsizing by the Company, disposing of the related assets, ceasing to carry on its business or the appointment of a receiver over any of its assets. The Company has not recognized any loss contingency having assessed as remote the likelihood of these events arising.
Research and Development Credits	Research and development credits Research and development credits are available to the Company under the tax laws in certain jurisdictions, based on qualifying research and development spend as defined under those tax laws. Research and development credits are generally recognized as a reduction of income tax expense. However, certain tax jurisdictions provide refundable credits that are not wholly dependent on the Company's ongoing income tax status or income tax position. In these circumstances the benefit of these credits is not recorded as a reduction to income tax expense, but rather as a reduction of operating expenditure.
Pension Costs	Pension costs The Company contributes to defined contribution plans covering all eligible employees. The Company contributes to these plans based upon various fixed percentages of employee compensation and such contributions are expensed as incurred. The Company operates, through two subsidiaries, a defined benefit plan for certain of its United Kingdom and Swiss employees. The Company accounts for the costs of these plans in accordance with ASC 715-30. These plans are presented in accordance with the requirements of FASB ASC 715-60 Defined Benefit Plans – Other Post retirement .
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.
Share-Based Compensation	Share-based compensation The Company accounts for its share options, restricted share units ("RSUs") and performance share units ("PSUs") in accordance with the provisions of FASB ASC 718, Compensation – Stock Compensation. Share-based compensation expense for equity-settled awards made to employees and Directors is measured and recognized based on estimated grant date fair values. These equity-settled awards include employee share options, RSUs and PSUs. Share-based compensation expense for share options awarded to employees and Directors is estimated at the grant date based on each option's fair value as calculated using the Black-Scholes option-pricing model. Share-based compensation for RSUs and PSUs awarded to employees and Directors is calculated based on the market value of the Company's shares on the date of award of the RSUs and PSUs. The value of awards expected to vest is recognized as an expense over the requisite service periods. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates. Estimating the grant date fair value of share options as of the grant date using an option-pricing model, such as the Black-Scholes model, is affected by the Company's share price as well as assumptions regarding a number of complex variables. These variables include, but are not limited to, the expected share price volatility over the term of the awards, risk-free interest rates and the expected term of the awards.
Impairment of Long-Lived Assets	Impairment of long-lived assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured at the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of are reported at the lower of the carrying amount of the asset or fair value less selling costs.
Derivative Financial Instruments	Derivative financial instruments We enter into transactions in the normal course of business using various financial instruments in order to hedge against exposure to fluctuating exchange and interest rates. We use derivative financial instruments to reduce exposure to fluctuations in interest rates. A derivative is a financial instrument or other contract whose value changes in response to some underlying variable, which has an initial net investment smaller than would be required for other instruments that have a similar response to the variable and that will be settled at a future date. We do not enter into derivative financial instruments for trading or speculative purposes. We did not hold any interest rate swap contracts or forward currency contracts at December 31, 2018 or December 31, 2017 . We use derivative financial instruments to reduce exposure to fluctuations in foreign exchange rates. During the years-ended December 31, 2017 and December 31, 2018 we entered into forward currency contracts in respect of identified exposure arising from euro payments. All contracts expired during the year ended December 31, 2018 . Our accounting policies for derivative financial instruments are based on whether they meet the criteria for designation as cash flow or fair value hedges. A designated hedge of the exposure to variability in the future cash flows of an asset or a liability, or of a forecasted transaction, is referred to as a cash flow hedge. A designated hedge of the exposure to changes in fair value of an asset or a liability is referred to as a fair value hedge. The criterion for designating a derivative as a hedge includes the assessment of the instrument's effectiveness in risk reduction, matching of the derivative instrument to its underlying transaction and the probability that the underlying transaction will occur. For derivatives with cash flow hedge accounting designation, we report the gain or loss from the effective portion of the hedge as a component of Other Comprehensive Income and reclassify it into earnings in the same period or periods in which the hedged transaction affects earnings and within the same Statement of Operations line item as the impact of the hedged transaction. For derivatives with fair value hedge accounting designation, we recognize gains or losses from the change in fair value of these derivatives, as well as the offsetting change in the fair value of the underlying hedged item, in earnings. Fair value gains and losses arising on derivative financial instruments not qualifying for hedge accounting are reported in our Consolidated Statement of Operations.
Financing Costs and Gain on Interest Rate Hedge	Financing costs and gain on interest rate hedge The interest rate in respect of the Senior Notes is fixed at 3.64% for the five year term of the agreement. The associated interest cost is recognized in interest expense in the period since drawdown in December 2015. Cash proceeds ( $4.6 million ) received in November 2015 in respect of the realized hedge gain are amortized to the Consolidated Statement of Operations, net against interest payable, over the period of the Senior Notes. Deferred financing costs (including issue costs relating to the Senior Notes) are reported at cost less accumulated amortization and the related amortization expense is included in interest expense, in our Consolidated Statement of Operations.
Reclassifications	Reclassifications Certain amounts in the consolidated financial statements have been reclassified where necessary to conform to the current year presentation.
Impact of New Accounting Pronouncements	Impact of New Accounting Pronouncements Impact of new accounting pronouncements adopted during fiscal year-ended December 31, 2018 ASC 606 'Revenue from Contracts with Customers' In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which provides that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The updated standard replaces most existing revenue recognition guidance in U.S. GAAP when it becomes effective and permits the use of either the retrospective or cumulative effect transition method. To achieve the core principle of the new standard, an entity should apply the following steps: (1) identify the contract(s) with a customer; (2) identify the performance obligations 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 a performance obligation. Subsequent to issuing ASU 2014-09, the FASB issued the following amendments concerning clarification of ASU 2014-09. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606), Principal versus Agent Considerations (Reporting Revenue Gross versus Net) (“ASU 2016-08”) , which further clarifies the implementation guidance on principal versus agent considerations. The new guidance requires either a retrospective or a modified retrospective approach to adoption. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606), Identifying Performance Obligations and Licensing (“ASU 2016-10”) , which clarifies the identification of performance obligations and the licensing implementation guidance, while retaining the related principles for those areas. In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients (“ASU 2016-12”), which provides clarification on assessing the collectability criterion, presentation of sales taxes, measurement date for non-cash consideration and completed contracts at transition. The updated standard was effective for ICON in the first quarter of the year ended December 31, 2018. ICON elected to adopt the updated standard using the cumulative effect transition method. Under this transition method, ICON applied the new standard as of the date of initial application (i.e. January 1, 2018), without restatement of comparative period amounts. ICON recorded the cumulative effect of initially applying the new standard (to revenue and cost) as an adjustment to the opening balance of equity at the date of initial application. The most significant impact relates to our assessment of measurement of performance and percentage of completion in respect of our clinical trials service revenue. ICON applied the requirements of the new standard to those contracts not completed at the date of initial application. See note 26 Impact of New Accounting Policy for the impact of the adoption of the new accounting standard. In May 2017, the FASB issued ASU 2017-10 ' Service Concession Arrangements (Topic 853): Determining the Customer of the Operation Services' which clarifies that the customer in a service concession arrangement is always the grantor. This ASU is effective at the same time as ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (the new revenue standard). • If an entity had early adopted the new revenue standard before this ASU was issued (May 16, 2017), the entity may adopt this ASU on its effective date with certain specific transition provisions. • If an entity early adopts the new revenue standard after this ASU was issued, the entity must adopt this ASU at the same time as the new revenue standard with certain specific transition provisions. • An entity may elect to early adopt this ASU before the adoption of the new revenue standard with certain specific transition provisions. The adoption of the ASU did not have a significant impact on the financial statements. In January 2017, the FASB issued ASU 2017-01 ' Business combinations - Clarifying the definition of a business ' to provide a new framework for determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. The ASU may be early adopted. The adoption of ASU 2017-01 did not have a significant impact on the financial statements. In March 2017, the FASB issued ASU 2017-07 ' Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost' which requires companies to present the service cost component of net benefit cost in the same line items in which they report compensation cost. Companies will present all other components of net benefit cost outside operating income, if this subtotal is presented. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2017-07 did not have a significant impact on the financial statements. In January 2016, the FASB issued ASU 2016-01 'Financial Instruments- Overall (Subtopic 825-10): Recognition and Measurement of the Financial Assets and Financial Liabilities' which will significantly change the income statement impact of equity investments and the recognition of changes in fair value of financial liabilities when the fair value option is elected. The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-01did not have a significant impact on the financial statements. In March 2016, the FASB issued ASU 2016-04 , 'Recognition of Breakage for Certain Prepaid Stored-Value Products' , which allows entities to recognize breakage on prepaid stored-value products consistent with how breakage is recognized under the new revenue standard. The exception applies to prepaid stored-value products in physical or digital form, with stored monetary values that are redeemable for goods and services, including those that can be redeemed for cash (e.g. prepaid gift cards issued on a specific payment network and redeemable at network-accepting merchant locations, prepaid telecommunication cards, and traveler's checks). The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-04 did not have a significant impact on the financial statements. In August 2016, the FASB issued ASU 2016-15, ' Classification of Certain Cash Receipts and Cash Payments' , which addresses eight classification issues related to the statement of cash flows: • Debt prepayment or debt extinguishment costs; • Settlement of zero-coupon bonds; • Contingent consideration payments made after a business combination; • Proceeds from the settlement of insurance claims; • Proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; • Distributions received from equity method investees; • Beneficial interests in securitization transactions; and • Separately identifiable cash flows and application of the predominance principle. The ASU was effective for public business entities for interim and annual periods in fiscal years beginning after December 15, 2017. The adoption of ASU 2016-15 did not have a significant impact on the financial statements. Any contingent consideration payment arrangements arising on business combinations effected in the future will be reviewed for cash flow statement classification in line with ASU 2016-15. There were no contingent consideration payment arrangements during the year ended December 31, 2018. In October 2016, the FASB issued ASU 2016-16, ' Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory', which requires entities to recognize at the transaction date the income tax consequences of intercompany asset transfers other than inventory. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. For all other entities, the ASU is effective for annual periods in fiscal years beginning after December 15, 2018, and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-16 did not have a significant impact on the financial statements. In November 2016, the FASB issued ASU 2016-18, ' Statement of Cash Flows (Topic 230): Restricted Cash', which requires companies to include cash and cash equivalents that have restrictions on withdrawal or use in total cash and cash equivalents on the statement of cash flows. This ASU was effective for public business entities for annual and interim periods in fiscal years beginning after December 15, 2017. For all other entities, the ASU is effective for annual periods in fiscal years beginning after December 15, 2018, and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-18 did not have a material impact on the financial statements. Financial statement effects of tax reform H.R.1 was enacted on December 22, 2017. The effective date of the law for most provisions is January 1, 2018 however the effects were required to be recognized in December 2017 financial statements. In response, the SEC staff issued SAB 118, which allows registrants to record provisional amounts during a 'measurement period'. See Note 13 for assessment of impact of H.R.1 on the December 31, 2017 financial statements. Impact of new accounting pronouncements which will be adopted during fiscal year-ended December 31, 2019 In February 2016, the FASB issued ASU 2016-02, ' Leases ', requiring lessees to recognize a right-of-use asset and a lease liability on the Consolidated Balance Sheet for all leases with the exception of short-term leases. For lessees, leases will continue to be classified as either operating or finance leases in the income statement. Lessor accounting is similar to the current model but updated to align with certain changes to the lessee model. Lessors will continue to classify leases as operating, direct financing or sales-type leases. The effective date of the new standard for public companies is for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted. The new standard must be adopted using a modified retrospective transition and requires application of the new guidance at the beginning of the earliest comparative period presented. The updated standard is effective for us beginning in the first quarter of the year-ended December 31, 2019. We are currently evaluating the effect that the updated standard will have on our consolidated financial statements and related disclosures. See Note 15 Commitments and Contingencies for details of operating leases held during year-ended December 31, 2018. A lease liability and right-of-use asset will be recorded on the Consolidated Balance Sheet at December 31, 2019. We estimate the impact of the adoption of ASC 842 will be the recognition of a right-of use asset and of lease liabilities in the range of $100 million to $110 million . Under the new standard we expect an insignificant change in net result due to the replacement of operating leases expenses with amortization of the lease asset and the interest expense. In July 2018, the FASB issued ASU 2018-11, which creates a new, optional transition method for implementing ASU 2016-02 and a lessor practical expedient for separating lease and non-lease components. This ASU is effective: • For entities that have not early adopted ASU 2016-02, with the effective date of ASU 2016-02. • For entities that have early adopted ASU 2016-02, upon issuance. However, it can only be adopted by entities either at (1) the beginning of the company’s first reporting period after issuance or (2) the entity’s mandatory ASC 842 effective date. The Company has elected to adopt this transition method for implementing ASU 2016-02 and the related practical expedient. In July 2018, the FASB issued ASU 2018-10, which clarifies and corrects errors in ASC 842. The effective date and transition requirements in ASU 2018-10 are the same as the effective date and transition requirements of ASU 2016-02. In January 2018, the FASB issued ASU 2018-01, which clarifies that land easements are in the scope of ASU 2016-02, Leases, and provides transition relief. The effective date and transition requirements in ASU 2018-01 are the same as the effective date and transition requirements of ASU 2016-02. In August 2017, the FASB issued ASU 2017-12 ' Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities ' which changes the recognition and presentation requirements of hedge accounting, including: • Eliminating the requirement to separately measure and report hedge ineffectiveness; and • Presenting all items that affect earnings in the same income statement line item as the hedged item. The ASU also provides new alternatives for: • Applying hedge accounting to additional hedging strategies; • Measuring the hedged item in fair value hedges of interest rate risk; • Reducing the cost and complexity of applying hedge accounting by easing the requirements for effectiveness testing, hedge documentation and application of the critical terms match method; and • Reducing the risk of material error correction if a company applies the shortcut method inappropriately. This ASU is effective for public business entities, for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted any time after the issuance of the ASU, including in an interim period. If adopted at other than the beginning of a fiscal year, cumulative effect adjustments are reflected as of the beginning of the fiscal year. The adoption of the ASU is not expected to have a significant impact on the financial statements. In February 2017, the FASB issued ASU 2017-05 ' Other Income-Gains and Losses from the Derecognition of Nonfinancial Assets (Subtopic 610-20): Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets'. In February 2017, the FASB issued ASU 2017-05, which clarifies the guidance in Subtopic 610-20 on accounting for derecognition of a nonfinancial asset. The ASU also defines in-substance nonfinancial assets and includes guidance on partial sales of non-financial assets. The adoption of ASU 2017-05 is not expected to have an impact on the financial statements. In July 2017, the FASB issued ASU 2017-11 ' Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception ' under which down round features will not cause certain equity-linked financial instruments to be accounted for as derivatives. A company that presents EPS information will reflect the effect of a down round feature of free-standing equity-linked financial instruments in EPS only if it is triggered. The ASU is effective for public business entities, for annual and interim periods in fiscal years beginning after December 15, 2018. The adoption of the ASU is not expected to have an impact on the financial statements. In June 2016, the FASB issued ASU 2016-13, ' Measurement of Credit Losses on Financial Instruments' , which significantly changes the way entities recognize impairment of many financial assets by requiring immediate recognition of estimated credit losses expected to occur over their remaining life. The ASU is effective for public business entities that are SEC filers for interim and annual periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2016-13 is not expected to have a significant impact on the financial statements. In July 2018, the FASB issued ASU 2018-09, which clarifies and corrects unintended application of guidance, and makes improvements to several Codification Topics. The changes are part of an ongoing FASB project to make non-substantive technical corrections, clarifications, and improvements that are not expected to have a significant effect on accounting practice or create a significant administrative cost to most entities. Most of the amendments are effective immediately. Some of the amendments are effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2018. • All other entities for annual periods in fiscal years beginning after December 15, 2019, and interim periods in fiscal years beginning after December 15, 2020. Other amendments, which affect recently issued ASUs that are not yet effective, are effective with the original ASU. The impact of adoption of ASU 2018-19 is not expected to be significant. In June 2018, the FASB issued ASU 2018-08, which clarifies and improves current guidance about whether a transfer of assets (or the reduction, settlement, or cancellation of liabilities) is a contribution or an exchange transaction. For transactions in which an entity is either a public business entity or an NFP that has issued, or is a conduit bond obligor for securities that are traded, listed, or quoted on an exchange or an over-the-counter market and serves as a resource recipient, the entity should apply the amendments in this ASU on contributions received to annual and interim periods in fiscal years beginning after June 15, 2018. All other entities should apply the amendments for transactions in which the entity serves as the resource recipient to annual periods in fiscal years beginning after December 15, 2018 and interim periods in fiscal years beginning after December 15, 2019. For transactions in which an entity is either a public business entity or an NFP that has issued, or is a conduit bond obligor for securities that are traded, listed, or quoted on an exchange or an over-the-counter market and serves as a resource provider, the entity should apply the amendments in this ASU on contributions made to annual and interim periods in fiscal years beginning after December 15, 2018. All other entities should apply the amendments for transactions in which the entity serves as the resource provider for annual periods in fiscal years beginning after December 15, 2019 and interim periods in fiscal years beginning after December 15, 2020. Early adoption of the amendments is permitted. The adoption of ASU 2018-08 is not expected to have a significant impact on the financial statements. In February 2018, the FASB issued ASU 2018-02, which allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted, including adoption in any interim period, (1) for public business entities for reporting periods for which financial statements have not yet been issued and (2) for all other entities for reporting periods for which financial statements have not yet been made available for issuance. The adoption of ASU 2018-02 is not expected to have a significant impact on the financial statements. In December 2017, the FASB issued ASU 2017-15. The ASU eliminates Topic 995, which includes an exemption to the recognition of deferred taxes on certain statutory reserve deposits that were, but are no longer, tax deferred. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2018. Early adoption is permitted for all entities, including adoption in an interim period. The adoption of ASU 2017-15 is not expected to have a significant impact on the financial statements. Impact of other new accounting pronouncements In January 2017, the FASB issued ASU 2017-04 ' Intangibles - Goodwill and Other: Simplifying the test for goodwill impairment ' which requires an entity to no longer perform a hypothetical purchase price allocation to measure goodwill impairment. Instead, impairment will be measured using the difference between the carrying amount and the fair value of the reporting unit. The ASU is effective for public businesses, that are SEC filers, for annual and interim periods in fiscal years beginning after December 15, 2019. The adoption of ASU 2017-04 is not expected to have a significant impact on the financial statements. In August 2018, the FASB issued ASU 2018-15, which clarifies that implementation costs incurred by customers in cloud computing arrangements are deferred if they would be capitalized by customers in software licensing arrangements under the internal-use software guidance. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2019. • Early adoption is permitted. Our evaluation of the impact of adoption of ASU 2018-15 is on-going. In August 2018, the FASB issued ASU 2018-14, which aims to improve the overall usefulness of disclosures to financial statement users and reduce unnecessary costs to companies when preparing defined benefit plan disclosures. This ASU is effective for: • Public business entities for financial statements issued for fiscal years ending after December 15, 2020. • Retrospective adoption is required and early adoption is permitted. Our evaluation of the disclosure impact of adoption of ASU 2018-14 is on-going. In August 2018, the FASB issued ASU 2018-13, which aims to improve the overall usefulness of disclosures to financial statement users and reduce unnecessary costs to companies when preparing fair value measurement disclosures. This ASU is effective for all entities for annual and interim periods in fiscal years beginning after December 15, 2019. Retrospective adoption is required except for the following changes, which are required to be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption: • Changes in unrealized gains and losses included in other comprehensive income for Level 3 instruments; • The range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements; and • The narrative description of measurement uncertainty. Early adoption is permitted. An entity may early adopt any eliminated or modified disclosure requirements and delay adoption of the additional disclosure requirements until their effective date. In such cases, users should refer to the Codification for disclosure requirements. Our evaluation of the disclosure impact of adoption of ASU 2018-13 is on-going. In August 2018, the FASB issued ASU 2018-12, which changes how insurance entities recognize, measure, present and disclose long-duration contracts. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2020. • Early adoption is permitted. The adoption of ASU 2018-12 is not expected to impact on the financial statements. In June 2018, the FASB issued ASU 2018-07, which more closely aligns the accounting for employee and nonemployee share-based payments. This ASU is effective for: • Public business entities for annual and interim periods in fiscal years beginning after December 15, 2018. • The adoption of ASU 2018-07 is not expected to have a significant impact on the financial statements.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Adjustments Resulting From Foreign Currency Translations	Amounts charged or credited to the Consolidated Statement of Operations for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 were as follows: Year ended December 31, (in thousands) 2018 2017 2016 Amounts (credited)/ charged $ (3,876 ) $ 7,760 $ 2,094
Estimated Useful Lives of Assets	Depreciation of property, plant and equipment is computed using the straight line method based on the estimated useful lives of the assets as listed below: Years Building 40 Computer equipment and software 2-8 Office furniture and fixtures 8 Laboratory equipment 5 Motor vehicles 5
Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share	The reconciliation of the number of shares used in the computation of basic and diluted net income per ordinary share is as follows: Year Ended December 31, 2018 2017 2016 Weighted average number of ordinary shares outstanding for basic net income per ordinary share 54,118,764 54,129,439 55,248,900 Effect of dilutive share options outstanding 671,899 719,607 1,158,236 Weighted average number of ordinary shares outstanding for diluted net income per ordinary share 54,790,663 54,849,046 56,407,136

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Cost and Fair Value of Investments	December 31, 2018 December 31, 2017 (in thousands) At start of year $77,589 $68,046 Purchases 80,956 41,701 Sales and maturities (99,865 ) (33,086 ) Interest on short term investments 1,329 1,088 Realized gain on sale of short term investments 56 112 Unrealized capital loss – investments (155 ) (272 ) At end of year $59,910 $77,589
Available For Sale Short Term Investments by Major Security Type	The following table represents our available for sale short term investments by major security type as of December 31, 2018 : Maturity by period Cost Total Unrealized gains / (losses) Fair Value Total Less than 1 year 1 to 5 years (U.S.$ in millions) US government debt securities 14.91 (0.07 ) 14.84 5.96 8.88 Corporate securities 44.92 (1.19 ) 43.73 11.48 32.25 Term deposits 1.34 — 1.34 0.70 0.64 Total (U.S.$ in millions) $61.17 ($1.26 ) $59.91 $18.14 $41.77

Goodwill (Tables)

Goodwill (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	December 31, 2018 December 31, 2017 (in thousands) Opening goodwill $ 769,058 $ 616,088 Current year acquisitions — 129,222 Prior period acquisition (note 4 (a)) 1,048 1,393 Foreign exchange movement (13,846 ) 22,355 Closing goodwill $ 756,260 $ 769,058
Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed	The table following summarizes the Company’s assessment of the fair values of the assets acquired and liabilities assumed: July 27, 2017 (in thousands) Cash 19,649 Property, plant and equipment 4,872 Goodwill* 130,270 Order book 13,012 Customer list 18,392 Accounts receivable 15,874 Unbilled revenue 6,984 Prepayments and other current assets 2,587 Other receivables 1,430 Income taxes receivable 4,262 Accounts payable (2,994 ) Payments on account (31,445 ) Other liabilities (24,952 ) 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. In finalizing the goodwill on acquisition of Mapi in the twelve month period from acquisition, fair value adjustments were made which resulted in increases in other liabilities ( $3.9 million ), plant and equipment ( $1.7 million ) and accounts receivable ( $1.7 million ) and income taxes receivable ( $1.5 million ) and decreases in unbilled revenue ( $4.8 million ), prepayments and other current assets ( $1.9 million ) and other receivables ( $1.0 million ) and in payment on account ( $2.6 million ) and non-current deferred tax liability ( $9.1 million ). Customer list and order backlog assets were also finalized. 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 GPHS 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.
Summary of Proforma Effect in Net Revenue Net Income and Earnings Per Share	The proforma effect of the Mapi acquisition if completed on January 1, 2016 would have resulted in net revenue, net income and earnings per share for the fiscal years ending December 31, 2017 and December 31, 2016 as follows: Year Ended 2017 2016 (in thousands) Net revenue $ 1,811,018 $ 1,750,643 Net income $ 284,903 $ 263,101 Basic earnings per share $ 5.26 $ 4.76 Diluted earnings per share $ 5.19 $ 4.66 The proforma effect of the GPHS acquisition if completed on January 1, 2015 would have resulted in net revenue, net income and earnings per share for the fiscal years ended December 31, 2016 and December 31, 2015 as follows: Year Ended December 31, 2016 2015 (in thousands) Net revenue $ 1,713,245 $ 1,639,085 Net income $ 266,148 $ 244,167 Basic earnings per share $ 4.82 $ 4.16 Diluted earnings per share $ 4.72 $ 4.05

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets	December 31, 2018 December 31, 2017 Cost (in thousands) Customer relationships acquired $ 109,622 $ 91,230 Technology asset acquired 11,169 11,169 Order backlog 31,220 18,208 Trade names/ brands acquired 2,766 2,766 Volunteer list acquired 1,325 1,325 Non-compete arrangements 489 489 Mapi intangible asset — 32,305 Foreign exchange movement (5,085 ) (2,389 ) Total cost 151,506 155,103 Accumulated amortization (100,249 ) (84,898 ) Foreign exchange movement 2,798 1,423 Net book value $ 54,055 $ 71,628
Schedule of Future Intangible Asset Amortization Expense	Future intangible asset amortization expense for the years ended December 31, 2019 to December 31, 2023 is as follows: Year Ended December 31,(in thousands) 2019 $ 13,183 2020 12,895 2021 10,857 2022 6,069 2023 4,221 $ 47,225

Property, Plant and Equipment_2

Property, Plant and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property Plant and Equipment Net	December 31, 2018 December 31, 2017 (in thousands) Cost Land $ 3,476 $ 3,464 Building 86,621 88,411 Computer equipment and software 399,192 358,874 Office furniture and fixtures 83,215 78,372 Laboratory equipment 36,092 34,918 Leasehold improvements 25,827 24,097 Motor vehicles 144 42 634,567 588,178 Less accumulated depreciation and asset write offs (475,898 ) (425,127 ) Property, plant and equipment (net) $ 158,669 $ 163,051

Other Liabilities (Tables)

Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Other Liabilities Disclosure [Abstract]
Schedule of Other Liabilities	December 31, 2018 December 31, 2017 (in thousands) Personnel related liabilities $ 171,866 $ 168,964 Facility related liabilities 14,012 13,061 General trade and overhead liabilities* 118,845 41,789 Other liabilities 4,289 4,628 Short term government grants (note 11) 42 35 Restructuring (note 14) 8,089 5,026 $ 317,143 $ 233,503 *includes amounts due to third parties in respect of reimbursable expenses of $85.6 million at December 31, 2018.

Non-Current Other Liabilities (

Non-Current Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Non-Current Other Liabilities	December 31, 2018 December 31, 2017 (in thousands) Defined benefit pension obligations, net (note 9) $ 3,320 $ 6,061 Other non-current liabilities 10,126 11,050 $ 13,446 $ 17,111

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Schedule of Pension Costs	The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60, Defined Benefit Plans – Other Postretirement . Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (5,279 ) $ (5,927 ) Fair value of plan assets 4,707 5,202 Funded status $ (572 ) $ (725 ) Non-current other liabilities (note 8) $ (572 ) $ (725 ) Change in benefit obligation December 31, 2018 December 31, 2017 (in thousands) Benefit obligation at beginning of year $ 5,927 $ 6,928 Service cost 138 243 Interest cost 47 54 Plan participants' contributions 83 120 Settlement (409 ) (1,019 ) Prior service cost (8 ) — Transferred (benefits paid)/balances (77 ) (76 ) Actuarial gain (372 ) (626 ) Foreign currency exchange rate changes (50 ) 303 Benefit obligation at end of year $ 5,279 $ 5,927 Change in plan assets December 31, December 31, 2018 2017 (in thousands) Fair value of plan assets at beginning of year $ 5,202 $ 6,006 Expected return on plan assets 41 47 Actual return on plan assets (240 ) (296 ) Scheme contributions 109 157 Plan participants' contributions 83 120 Transferred (benefits paid)/balances (77 ) (76 ) Settlement (409 ) (1,019 ) Foreign currency exchange rate changes (2 ) 263 Fair value of plan assets at end of year $ 4,707 $ 5,202 The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60, Defined Benefit Plans – Other Postretirement . The plan has been closed to new entrants with effect from July 1, 2003. Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (30,045 ) $ (37,759 ) Fair value of plan assets 27,297 32,423 Funded status $ (2,748 ) $ (5,336 ) Non-current other liabilities (note 8) $ (2,748 ) $ (5,336 ) Change in benefit obligation December 31, 2018 December 31, 2017 (in thousands) Benefit obligation at beginning of year $ 37,759 $ 32,906 Service cost 124 112 Interest cost 895 929 Plan participants' contributions 24 22 Expenses — (8 ) Benefits paid (3,049 ) (68 ) Actuarial (gain)/loss (3,844 ) 658 Foreign currency exchange rate changes (1,864 ) 3,208 Benefit obligation at end of year $ 30,045 $ 37,759 Change in plan assets December 31, 2018 December 31, 2017 (in thousands) Fair value of plan assets at beginning of year $ 32,423 $ 24,876 Actual return on plan assets (584 ) 979 Employer contributions 153 4,008 Plan participants' contributions 24 22 Benefits paid (3,049 ) (68 ) Foreign currency exchange rate changes (1,670 ) 2,606 Fair value of plan assets at end of year $ 27,297 $ 32,423
Schedule of Funded Status	The pension costs of this plan are presented in the following tables in accordance with the requirements of ASC 715-60, Defined Benefit Plans – Other Postretirement . The plan has been closed to new entrants with effect from July 1, 2003. Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (30,045 ) $ (37,759 ) Fair value of plan assets 27,297 32,423 Funded status $ (2,748 ) $ (5,336 ) Non-current other liabilities (note 8) $ (2,748 ) $ (5,336 ) Funded status December 31, 2018 December 31, 2017 (in thousands) Projected benefit obligation $ (5,279 ) $ (5,927 ) Fair value of plan assets 4,707 5,202 Funded status $ (572 ) $ (725 ) Non-current other liabilities (note 8) $ (572 ) $ (725 )
Schedule of Components of Net Periodic Benefit Cost	December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Service cost $ 138 $ 243 $ 352 Interest cost 47 54 82 Expected return on plan assets (41 ) (47 ) (48 ) Amortization of net (gain)/loss (69 ) (43 ) 22 Amortization of prior service credit (8 ) (8 ) (8 ) Settlement (93 ) (214 ) (136 ) Curtailment — — — Net periodic benefit credit $ (26 ) $ (15 ) $ 264 The following amounts were recorded in the Consolidated Statement of Operations as components of the net periodic benefit cost: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Service cost $ 124 $ 112 $ 75 Interest cost 895 929 1,017 Expected return on plan assets (624 ) (586 ) (646 ) Amortization of net loss 248 250 — Expenses — (8 ) 8 Net periodic benefit cost $ 643 $ 697 $ 454
Summary of Assumptions Used in Calculating Net Periodic Benefit Cost	The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : December 31, 2018 December 31, 2017 December 31, 2016 Discount rate 0.80 % 0.75 % 0.95 % Rate of compensation increase 2.00 % 2.00 % 2.00 % Expected rate of return on plan assets 0.80 % 0.75 % 0.95 % Other comprehensive income December 31, 2018 December 31, 2017 December 31, 2016 Actuarial gain - benefit obligation $ (372 ) $ (626 ) $ (1,157 ) Actuarial loss/(gain) – plan assets 240 296 (1,233 ) Prior service credit recognized in net periodic benefit cost 93 215 136 Actuarial gain/(loss) recognized in net periodic benefit cost 69 43 (22 ) Amortization of net prior service credit 8 8 8 Net prior service cost occurring during the year (9 ) (1 ) (89 ) Total $ 29 $ (65 ) $ (2,357 ) The following assumptions were used at the commencement of the year in determining the net periodic pension benefit cost for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : December 31, 2018 December 31, 2017 December 31, 2016 Discount rate 2.5 % 2.7 % 4.0 % Rate of compensation increase 3.7 % 3.9 % 3.7 % Expected rate of return on plan assets 2.0 % 2.1 % 3.0 % Other comprehensive income December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Actuarial (gain)/loss - benefit obligation $ (3,844 ) $ 658 $ 10,057 Actuarial loss/(gain) – plan assets 1,208 (393 ) (5,215 ) Actuarial gain recognized in net periodic benefit cost (248 ) (250 ) — Total $ (2,884 ) $ 15 $ 4,842
Summary of Amounts Recognized in Accumulated Other Comprehensive Income Which Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost	Amounts recognized in accumulated other comprehensive income that have not yet been recognized as components of net periodic benefit cost are as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Net actuarial gain $ (1,254 ) $ (1,283 ) $ (1,218 ) Total $ (1,254 ) $ (1,283 ) $ (1,218 ) Amounts recognized in accumulated other comprehensive income that have not yet been recognized as components of net periodic benefit cost are as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Net actuarial loss $ 4,254 $ 7,138 $ 7,123 Total $ 4,254 $ 7,138 $ 7,123
Summary of Assumptions Used in Calculating Pension Benefit Obligations	The following assumptions were used in determining the benefit obligation at December 31, 2018 and December 31, 2017 : December 31, 2018 December 31, 2017 Discount rate 2.9 % 2.5 % Rate of compensation increase 3.7 % 3.7 % The following assumptions were used in determining the benefit obligation at December 31, 2018 and December 31, 2017 : December 31, 2018 December 31, 2017 Discount rate 0.80 % 0.80 % Rate of compensation increase 2.00 % 2.00 %
Summary of Expected Long Term Rates of Return on Different Asset Classes	The expected long term rates of return on different asset classes are as follows: Asset Category Expected long-term return per annum Corporate Bonds 2.9 % Gilts 1.8 % Cash 2.9 %
Schedule of Underlying Asset Split of Fund	The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2019 678 2020 212 2021 209 2022 204 2023 199 Years 2023 - 2028 $ 908 The underlying asset split of the fund is shown below. Asset Category December 31, 2018 December 31, 2017 Corporate Bonds 25 % 22 % Gilts 71 % 65 % Cash 4 % 13 % 100 % 100 %
Schedule of Plan Asset Fair Value Measurements	Quoted Prices in Active Markets for Identical Assets Level 1 (in thousands) December 31, 2018 December 31, 2017 Cash $ 1,029 $ 4,086 Fixed Income Securities Legal and General Active Corporate Bond – Over 10 Year 6,688 7,188 Legal and General Gilt Funds 7,136 7,611 Legal and General Index Linked Gilt Funds 12,444 13,538 $ 27,297 $ 32,423
Schedule of Annual Benefit Payments which Reflect Expected Future Service	The following annual benefit payments, which reflect expected future service as appropriate, are expected to be paid. (in thousands) 2019 295 2020 311 2021 383 2022 412 2023 403 Years 2024 - 2028 $ 3,451

Equity Incentive Schemes and _2

Equity Incentive Schemes and Stock Compensation Charges (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Transactions for Company's Share Option Plans	The following table summarizes the transactions for the Company's share option plans for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 : Options Granted Under Plans Weighted Average Exercise Price Weighted Average Grant Date Fair Value Outstanding at December 31, 2015 1,626,582 $ 34.87 $ 11.94 Granted 256,191 $ 69.61 $ 20.10 Exercised (393,240 ) $ 25.79 $ 9.84 Cancelled (23,089 ) $ 29.74 $ 11.19 Outstanding at December 31, 2016 1,466,444 $ 43.45 $ 13.94 Granted 219,113 $ 85.98 $ 25.06 Exercised (458,243 ) $ 30.35 $ 10.72 Cancelled (55,921 ) $ 54.35 $ 16.76 Outstanding at December 31, 2017 1,171,393 $ 56.02 $ 17.15 Granted 167,557 $ 118.90 $ 36.84 Exercised (408,699 ) $ 41.12 $ 13.55 Cancelled (9,505 ) $ 32.35 $ 11.39 Outstanding at December 31, 2018 920,746 $ 74.32 $ 22.39 Vested and exercisable at December 31, 2018 397,923 $ 56.10 $ 17.11
Schedule of Non Vested Shares Outstanding	Non-vested shares outstanding as at December 31, 2018 are as follows: Options Outstanding Number of Shares Weighted Average Exercise Price Weighted Average Fair Value Non-vested outstanding at December 31, 2017 694,727 $ 68.06 $ 20.03 Granted 167,557 118.90 36.84 Vested (336,756 ) 62.21 18.51 Forfeited (2,705 ) 56.58 16.84 Non-vested outstanding at December 31, 2018 522,823 $ 88.18 $ 26.41
Summary of Information Concerning Outstanding and Exercisable Share Options	The following table summarizes information concerning outstanding and exercisable share options as of December 31, 2018 : Options Outstanding Options Exercisable Range Exercise Price Number of Shares Weighted Average Remaining Contractual Life Weighted Average Exercise Price Number of Shares Weighted Average Exercise Price $ 20.28 18,190 0.16 18,190 $ 22.30 52,811 1.32 52,811 $ 23.66 1,161 1.57 1,161 $ 26.71 450 1.69 450 $ 32.37 20,848 2.33 20,848 $ 36.22 2,141 2.46 2,141 $ 37.90 920 2.93 920 $ 40.83 40,502 3.39 25,360 $ 47.03 22,685 3.17 14,020 $ 48.67 50,896 3.21 34,906 $ 51.35 2,030 3.60 1,224 $ 65.60 66,752 5.38 24,543 $ 66.47 4,267 4.39 1,572 $ 66.97 1,249 4.45 — $ 68.39 135,400 4.18 79,069 $ 71.95 127,905 5.17 74,473 $ 83.47 127,136 6.17 22,461 $ 90.03 77,846 6.38 23,774 $ 115.11 107,794 7.17 — $ 125.74 59,763 7.38 — $20.28 - $125.74 920,746 5.01 $ 74.32 397,923 $ 56.10
Summary of Weighted Average Fair Values and Assumptions Used	The weighted average fair values and assumptions were as follows: Year Ended December 31, 2018 December 31, 2017 December 31, 2016 Weighted average fair value $ 36.84 $ 25.06 $ 20.10 Assumptions: Expected volatility 30 % 29 % 30 % Dividend yield — % — % — % Risk-free interest rate 2.76 % 1.93 % 1.39 % Expected life 5.0 years 5.0 years 5.0 years
Summary of RSU and PSU Activity	The following table summarizes RSU and PSU activity for the year ended December 31, 2018 : PSU Outstanding Number of Shares PSU Weighted Average Fair Value PSU Weighted Average Remaining Contractual Life RSU Outstanding Number of Shares RSU Weighted Average 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 71,906 $ 116.02 160,113 $ 123.42 Shares vested (215,826 ) $ 68.28 (276,495 ) $ 67.99 Forfeited (116,053 ) $ 70.89 (64,911 ) $ 78.92 Outstanding at December 31, 2018 251,053 $ 85.95 0.96 534,677 $ 89.50 1.22
Schedule of Non-cash Stock Compensation Expense	Non-cash stock compensation expense has been allocated as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Direct costs $ 17,408 $ 18,020 $ 21,903 Selling, general and administrative $ 14,186 $ 12,553 $ 18,440 Total compensation costs $ 31,594 $ 30,573 $ 40,343

Government Grants (Tables)

Government Grants (Tables)	12 Months Ended
	Dec. 31, 2018
Deferred Revenue Disclosure [Abstract]
Government Grant	December 31, 2018 December 31, 2017 (in thousands) Received $ 3,539 $ 3,539 Less accumulated amortization (2,792 ) (2,745 ) Foreign exchange translation adjustment 172 207 Total government grants 919 1,001 Less current portion (42 ) (35 ) Non-current government grants $ 877 $ 966

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Summary of Components of Income Before Provision for Income Taxes	The components of income before income tax expense are as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 243,988 $ 218,306 $ 201,221 United States 27,499 28,426 11,466 Other 93,127 81,325 87,485 Income before provision for income taxes $ 364,614 $ 328,057 $ 300,172
Summary of Components of Provision for Income Taxes	The components of provision for income taxes are as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Provision for income taxes: Current tax expense: Ireland $ 28,042 $ 20,084 $ 22,931 United States 2,885 5,792 7,768 Other 9,379 9,964 5,749 Total current tax expense 40,306 35,840 36,448 Deferred tax expense/(benefit): Ireland 1,054 261 1,284 United States 875 8,980 613 Other (277 ) 1,488 (352 ) Total deferred tax expense 1,652 10,729 1,545 Provision for income taxes 41,958 46,569 37,993 Impact on shareholders equity and other comprehensive income of the tax consequence of : Excess tax benefit on stock compensation — — (4,332 ) Currency impact on long term funding 119 973 (396 ) Fair value of cash flow hedge (148 ) 148 — Total $ 41,929 $ 47,690 $ 33,265
Schedule of Reconciliation of Consolidated Reported Provision for Income Taxes and Statutory Rate	The Company's consolidated reported provision for income taxes differed from the amount that would result from applying the Irish statutory rate as set forth below: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Taxes at Irish statutory rate of 12.5% (2017:12.5%; 2016:12.5%) $ 45,577 $ 41,007 $ 37,522 Foreign and other income taxed at higher rates 7,649 6,324 4,642 Research & development tax incentives (1,243 ) (830 ) (907 ) Movement in valuation allowance 5,667 1,329 1,208 Effects of change in tax rates (147 ) 925 576 Increase/(decrease) in unrecognized tax benefits (5,423 ) 933 (1,521 ) Impact of stock compensation (8,301 ) (9,917 ) (4,121 ) Impact of deemed repatriation under US Tax Reform — 7,694 — Other (1,821 ) (896 ) 594 Provision for income taxes $ 41,958 $ 46,569 $ 37,993
Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities	The tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities are presented below: December 31, 2018 December 31, 2017 (in thousands) Deferred tax liabilities: Property, plant and equipment $ 981 $ 1,139 Goodwill 25,149 22,655 Other intangible assets 9,397 11,801 Other 5,703 4,139 Total deferred tax liabilities recognized 41,230 39,734 Deferred tax assets: Operating loss and tax credits carry-forwards 29,995 24,962 Property, plant and equipment 4,893 4,062 Accrued expenses and payments on account 24,599 24,433 Stock compensation 6,490 5,786 Deferred compensation 2,197 2,548 Deferred revenue 5,681 — Other 2 740 Total deferred tax assets 73,857 62,531 Valuation allowance for deferred tax assets (27,263 ) (22,439 ) Deferred tax assets recognized 46,594 40,092 Overall net deferred tax asset $ 5,364 $ 358
Schedule of Expected Expiry Dates of NOL's	The expected expiry dates of these losses are as follows: Federal NOL's State NOL's (in thousands) 2021-2034 14,323 12,158 2035-2037 766 20,464 $ 15,089 $ 32,622
Schedule of Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of total unrecognized tax benefits is as follows: December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Unrecognized tax benefits at start of year $ 23,720 $ 26,620 $ 28,166 Increase related to prior year tax positions 2,084 — 1,151 Decrease related to prior year tax positions (2,915 ) (3,050 ) (2,483 ) Increase related to current year tax positions 3,065 4,765 1,104 Settlements (182 ) (2,523 ) (837 ) Lapse of statute of limitations (4,339 ) (2,092 ) (481 ) Unrecognized tax benefits at end of year $ 21,433 $ 23,720 $ 26,620

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Dec. 31, 2018
Restructuring and Related Activities [Abstract]
Schedule of Restructuring Charges	Restructuring and other items recognized during the year ended December 31, 2018 comprise: Year Ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Restructuring charges $ 12,490 7,753 $ 8,159 Net charge $ 12,490 7,753 $ 8,159
Schedule of Restructuring Reserve by Type of Cost	Workforce (in thousands) Restructuring charges $ 7,753 Utilized (4,656 ) Provision at December 31, 2017 $ 3,097 Utilized (1,015 ) Provision at December 31, 2018 $ 2,082 Workforce reductions Onerous Lease Total (in thousands) Total provision recognized $ 9,684 $ 2,806 $ 12,490 Utilized (5,399 ) (672 ) $ (6,071 ) Foreign exchange — — — Provision at December 31, 2018 $ 4,285 $ 2,134 $ 6,419 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 — (317 ) (317 ) Provision at December 31, 2018 $ — $ — $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Commitments for Operating Leases with Non-cancelable Terms	Future minimum rental commitments for operating leases with non-cancelable terms in excess of one year are as follows: Minimum rental payments (in thousands) 2019 32,634 2020 26,839 2021 21,306 2022 15,781 2023 10,829 Thereafter 19,194 Total $ 126,583

Disaggregation of Revenue (Tabl

Disaggregation of Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Revenue	Revenue disaggregated by customer profile is as follows: YTD December 31, 2018 December 31, 2017 (in thousands) Top client $ 352,335 $ 503,875 Clients 2-5 671,723 505,818 Clients 6-10 385,741 348,841 Clients 11-25 461,351 421,759 Other 724,627 622,028 Total $2,595,777 $2,402,321

Trade Accounts Receivable, Un_2

Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities) (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of Contracts with Customers, Asset and Liabilities	Trade accounts receivables and unbilled revenue are as follows: December 31, 2018 December 31, 2017 (in thousands) Billed services (accounts receivable) $ 423,680 $ 388,431 Unbilled services (unbilled revenue) 362,926 268,509 Trade accounts receivable and unbilled revenue 786,606 656,940 Allowance for doubtful accounts (note 18) (8,889 ) (8,930 ) Trade accounts receivable and unbilled revenue, net $ 777,717 $ 648,010 Unbilled services and payments on account (contract assets and liabilities) were as follows: (in thousands, except percentages) December 31, 2018 December 31, 2017 $ Change % Change Unbilled services (unbilled revenue) $ 362,926 $ 268,509 $ 94,417 35.2 % Unearned revenue (payments on account) (274,468 ) (298,992 ) 24,524 (8.2 )% Net balance $ 88,458 $ (30,483 ) $ 118,941 390.2 %

Provision for Doubtful Debts (T

Provision for Doubtful Debts (Tables)	12 Months Ended
	Dec. 31, 2018
Accounts Receivable, Net [Abstract]
Schedule of Provision for Doubtful Debts	The Company does business with most major international pharmaceutical companies. Provision for doubtful debts at December 31, 2018 comprises: December 31, 2018 December 31, 2017 (in thousands) Opening provision $ 8,930 $ 9,450 Amounts used during the year (995 ) (2,733 ) Amounts provided during the year 3,083 5,116 Amounts released during the year (2,355 ) (3,106 ) Foreign exchange 226 203 Closing provision $ 8,889 $ 8,930

Business Segment and Geograph_2

Business Segment and Geographical Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Schedule of Distribution of Net Revenue by Geographical Area	The distribution of revenue by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 Restated* Restated* (in thousands) Ireland $ 1,066,200 $ 944,130 $ 995,365 Rest of Europe 379,883 355,552 319,935 U.S. 894,978 881,829 866,901 Other 254,716 220,810 182,755 Total $ 2,595,777 $ 2,402,321 $ 2,364,956 * 2017 and 2016 restated to reflect gross revenue.
Schedule of Distribution of Income from Operations by Geographical Area	The distribution of income from operations, including restructuring and other items, by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 257,089 $ 232,032 $ 216,149 Rest of Europe 36,280 26,493 34,200 U.S. 58,561 58,322 41,348 Other 21,427 21,491 19,997 Total $ 373,357 $ 338,338 $ 311,694 c) The distribution of income from operations, excluding restructuring and other items, by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 269,196 $ 240,115 $ 218,334 Rest of Europe 36,904 26,351 36,509 U.S. 58,340 58,164 44,590 Other 21,407 21,461 20,420 Total $ 385,847 $ 346,091 $ 319,853
Schedule of Distribution of Property, Plant and Equipment, Net, by Geographical Area	The distribution of property, plant and equipment, net, by geographical area was as follows: December 31, 2018 December 31, 2017 (in thousands) Ireland $ 106,206 $ 111,329 Rest of Europe 9,807 9,026 U.S. 25,535 27,797 Other 17,121 14,899 Total $ 158,669 $ 163,051
Schedule of Distribution of Depreciation and Amortization by Geographical Area	The distribution of depreciation and amortization by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 34,721 $ 26,277 $ 25,766 Rest of Europe 5,331 6,857 6,914 U.S. 21,605 24,246 23,462 Other 4,259 3,917 3,433 Total $ 65,916 $ 61,297 $ 59,575
Schedule of Distribution of Total Assets by Geographical Area	The distribution of total assets by geographical area was as follows: Year Ended December 31, 2018 December 31, 2017 (in thousands) Ireland $ 1,073,411 $ 880,378 Rest of Europe 514,010 504,418 U.S. 646,512 650,681 Other 120,322 111,141 Total $ 2,354,255 $ 2,146,618
Schedule of Distribution of Capital Expenditures by Geographical Area	The distribution of capital expenditures by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 30,942 $ 24,468 $ 27,670 Rest of Europe 2,590 2,819 2,851 U.S. 9,311 11,027 8,432 Other 5,554 6,403 3,648 Total $ 48,397 $ 44,717 $ 42,601
Schedule of Clients Representing Company's Net Revenue	The following table sets forth the clients which represented 10% or more of the Company's net revenue in each of the periods set out below. Year ended December 31, 2018 December 31, 2017 December 31, 2016 Client A 14 % 21 % 26 %
Schedule of Distribution of Interest Income by Business Segment	The distribution of interest income by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 2,620 $ 1,084 $ 407 Rest of Europe 1,750 1,222 1,040 U.S. 17 16 2 Other 372 24 35 Total $ 4,759 $ 2,346 $ 1,484
Schedule of Distribution of Tax Charge by Geographical Area	The distribution of the income tax charge by geographical area was as follows: Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Ireland $ 29,096 $ 20,345 $ 24,215 Rest of Europe (434 ) 1,921 5,528 U.S. 3,761 14,772 8,381 Other 9,535 9,531 (131 ) Total $ 41,958 $ 46,569 $ 37,993

Supplemental Disclosure of Ca_2

Supplemental Disclosure of Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Schedule of Supplemental Disclosure of Cash Flow Information	Year ended December 31, 2018 December 31, 2017 December 31, 2016 (in thousands) Cash paid for interest $ 13,060 $ 13,094 $ 13,615 Cash paid for income taxes $ 18,558 $ 12,305 $ 10,205

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Accumulated Other Comprehensive Income	Year ended December 31, 2018 December 31, 2017 (in thousands) Currency translation adjustments $ (62,710 ) $ (36,188 ) Currency impact on long term funding (5,016 ) (182 ) Actuarial loss on defined benefit pension plan (note 9) (3,000 ) (5,855 ) Unrealized capital loss – investments (note 3) (450 ) (295 ) Realized gain on interest rate hedge 4,658 4,658 Amortization of interest rate hedge (2,810 ) (1,887 ) Fair value of cash flow hedge — 1,036 Total $ (69,328 ) $ (38,713 )

Impact of Change in Accountin_2

Impact of Change in Accounting Policies (Tables)	12 Months Ended
	Dec. 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 twelve months ended December 31, 2018 . ICON plc CONDENSED CONSOLIDATED BALANCE SHEETS AS AT DECEMBER 31, 2018 December 31, 2018 As reported Adjustments Balance without adoption of Topic 606 ASSETS (in thousands) Current Assets: Cash and cash equivalents $ 395,851 $ — $ 395,851 Available for sale investments 59,910 — 59,910 Accounts receivable, net 414,791 — 414,791 Unbilled revenue 362,926 46,642 409,568 Other receivables 40,459 (12,580 ) 27,879 Prepayments and other current assets 36,801 — 36,801 Income taxes receivable 19,445 — 19,445 Total current assets 1,330,183 34,062 1,364,245 Other Assets: Property, plant and equipment, net 158,669 — 158,669 Goodwill 756,260 — 756,260 Other non-current assets 14,525 — 14,525 Non-current income taxes receivable 20,023 — 20,023 Non-current deferred tax asset 13,577 (5,680 ) 7,897 Investments in equity- long term 6,963 — 6,963 Intangible assets 54,055 — 54,055 Total Assets $ 2,354,255 $ 28,382 $ 2,382,637 LIABILITIES AND SHAREHOLDERS’ EQUITY Current Liabilities: Accounts payable $ 13,288 $ — $ 13,288 Payments on account 274,468 59,242 333,710 Other liabilities 317,143 (84,342 ) 232,801 Income taxes payable 5,724 1,669 7,393 Total current liabilities 610,623 (23,431 ) 587,192 Other Liabilities: Non-current bank credit lines and loan facilities 349,264 — 349,264 Non-current other liabilities 13,446 — 13,446 Non-current government grants 877 — 877 Non-current income taxes payable 17,551 — 17,551 Non-current deferred tax liability 8,213 — 8,213 Commitments and contingencies — — — Total Liabilities 999,974 (23,431 ) 976,543 Shareholders' Equity: Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017. 4,658 — 4,658 Additional paid‑in capital 529,642 — 529,642 Other undenominated capital 983 — 983 Accumulated other comprehensive income (69,328 ) — (69,328 ) Retained earnings 888,326 51,813 940,139 Total Shareholders' Equity 1,354,281 51,813 1,406,094 Total Liabilities and Shareholders' Equity $ 2,354,255 $ 28,382 $ 2,382,637 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS FOR THE TWELVE MONTHS ENDED DECEMBER 31, 2018 YTD December 31, 2018 December 31, 2018 December 31, 2018 As reported Adjustments Balance without adoption of Topic 606 (in thousands except share and per share data) Revenue: Revenue $ 2,595,777 $ 4,657 $ 2,600,434 Reimbursable expenses — (702,812 ) $ (702,812 ) (698,155 ) 1,897,622 Costs and expenses: Direct costs 1,818,220 (702,812 ) 1,115,408 Selling, general and administrative expense 325,794 472 326,266 Depreciation and amortization 65,916 — 65,916 Restructuring 12,490 — 12,490 Total costs and expenses 2,222,420 (702,340 ) 1,520,080 Income from operations 373,357 4,185 377,542 Interest income 4,759 — 4,759 Interest expense (13,502 ) — (13,502 ) Income before income tax expense 364,614 4,185 368,799 Income tax expense (Note 13) (41,958 ) (476 ) (42,434 ) Net income $ 322,656 $ 3,709 $ 326,365 ICON plc CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME 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 — — — — — 322,656 322,656 Impact of change in accounting policy — — — — — 3,709 3,709 Currency translation adjustment — — — — (26,522 ) — (26,522 ) Currency impact of long-term funding — — — — (4,834 ) — (4,834 ) Unrealized capital loss - investments — — — — (155 ) — (155 ) Actuarial gain on defined benefit pension plan — — — — 2,855 — 2,855 Amortization of interest rate hedge — — — — (923 ) — (923 ) Fair value of cash flow hedge — — — — (1,036 ) — (1,036 ) Total comprehensive income — — — — — — 295,750 Exercise of share options 408,699 29 16,777 — — — 16,806 Issue of restricted share units / performance share units 489,568 36 — — — — 36 Share based compensation expense — — 31,544 — — — 31,544 Share issue costs — — (16 ) — — — (16 ) Repurchase of ordinary shares (1,008,162 ) (71 ) — 71 — (128,960 ) (128,960 ) Share repurchase costs — — — — — (66 ) (66 ) Balance at December 31, 2018 53,971,706 4,658 529,642 983 (69,328 ) 940,139 1,406,094 There is no impact of adoption of ASC 606 on the CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY AND COMPREHENSIVE INCOME. An adjustment to retained earnings as at January 1, 2018 arises on adoption of ASC 606 and this is presented in the primary statement reflecting ASC 606 adoption. ICON plc CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS FOR THE TWELVE MONTHS ENDED DECEMBER 31, 2018 As Reported Adjustments Balance without adoption of Topic 606 (in thousands) Cash flows from operating activities: Net income 322,656 3,709 326,365 Adjustments to reconcile net income to net cash provided by operating activities: Loss on disposal of property, plant and equipment 70 — 70 Depreciation expense 50,565 — 50,565 Amortization of intangibles 15,351 — 15,351 Amortization of government grants (47 ) — (47 ) Interest on short term investments (1,329 ) — (1,329 ) Realized (gain)/loss on sale of short term investments (56 ) — (56 ) Gain on re-measurement of financial assets (800 ) — (800 ) Amortization of gain on interest rate hedge (923 ) — (923 ) Amortization of financing costs 812 — 812 Stock compensation expense 31,594 — 31,594 Deferred tax expense 1,652 5,680 7,332 Changes in assets and liabilities: — — Increase in accounts receivable (37,557 ) — (37,557 ) Increase in unbilled revenue (98,510 ) 1,462 (97,048 ) Decrease in other receivables 3,107 12,580 15,687 Increase in prepayments and other current assets (3,237 ) — (3,237 ) Decrease in other non-current assets 856 — 856 Decrease in payments on account (6,253 ) 59,242 52,989 Increase in other current liabilities 2,009 (84,342 ) (82,333 ) Decrease in other non-current liabilities (1,034 ) — (1,034 ) Decrease in income taxes payable (5,220 ) 1,669 (3,551 ) Decrease in accounts payable (5,067 ) — (5,067 ) Net cash provided by operating activities 268,639 — 268,639 Cash flows from investing activities: — — Purchase of property, plant and equipment (48,397 ) — (48,397 ) Purchase of subsidiary undertakings (1,645 ) — (1,645 ) Sale of available for sale investments 99,865 — 99,865 Purchase of available for sale investments (80,956 ) — (80,956 ) Purchase of investments in equity - long term (6,163 ) — (6,163 ) Net cash used in investing activities (37,296 ) — (37,296 ) Cash flows from financing activities: Financing costs (823 ) — (823 ) Proceeds from the exercise of equity compensation 16,842 — 16,842 Share issue costs (16 ) — (16 ) Repurchase of ordinary shares (128,960 ) — (128,960 ) Share repurchase costs (66 ) — (66 ) Net cash used in financing activities (113,023 ) — (113,023 ) Effect of exchange rate movements on cash (5,328 ) — (5,328 ) Net increase in cash and cash equivalents 112,992 — 112,992 Cash and cash equivalents at beginning of year 282,859 — 282,859 Cash and cash equivalents at end of year $ 395,851 $ — $ 395,851

Description of Business (Detail

Description of Business (Details)	12 Months Ended
	Dec. 31, 2018EmployeeCountryLocation
Product Information [Line Items]
Number of employees | Employee	13,670
Number of locations in which company operates | Location	89
Number of countries in which company operates | Country	37
Geographic Concentration Risk | United States | Net Revenue
Product Information [Line Items]
Percentage of company revenue	34.50%
Geographic Concentration Risk | Europe | Net Revenue
Product Information [Line Items]
Percentage of company revenue	55.70%
Geographic Concentration Risk | Rest of World | Net Revenue
Product Information [Line Items]
Percentage of company revenue	9.80%

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Advertising and promotion costs			$ 6,516,637	$ 6,744,333	$ 7,167,050
Long term debt			349,264,000	348,888,000
Goodwill impairment charge			0	0	$ 0
Carrying value of inventory			$ 2,300,000	$ 2,200,000
Senior Notes
Debt Instrument [Line Items]
Long term debt	$ 350,000,000
Debt instrument, interest rate	3.64%		3.64%
Debt instrument, term	5 years		5 years
Cash proceeds from interest rate hedge		$ 4,600,000
Direct Cost, Reimbursable Expenses
Debt Instrument [Line Items]
Direct costs, reimbursable expenses			$ 702,800,000

Significant Accounting Polici_5

Significant Accounting Policies - Adjustments Resulting from Foreign Currency Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Foreign Currency [Abstract]
Amounts (credited)/ charged	$ (3,876)	$ 7,760	$ 2,094

Significant Accounting Polici_6

Significant Accounting Policies - Estimated Useful Lives of Assets (Details)	12 Months Ended
	Dec. 31, 2018
Building
Property, Plant and Equipment [Line Items]
Estimated useful lives	40 years
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years
Office furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful lives	8 years
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years
Motor vehicles
Property, Plant and Equipment [Line Items]
Estimated useful lives	5 years

Significant Accounting Polici_7

Significant Accounting Policies - Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares)	54,118,764	54,129,439	55,248,900
Effect of dilutive share options outstanding (in shares)	671,899	719,607	1,158,236
Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares)	54,790,663	54,849,046	56,407,136

Investments - Available For Sal

Investments - Available For Sale Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Debt Securities, Available-For-Sale [Roll Forward]
At start of year	$ 77,589	$ 68,046
Purchases	80,956	41,701	$ 22,030
Sales and maturities	(99,865)	(33,086)	(40,858)
Interest on short term investments	1,329	1,088	823
Realized gain on sale of short term investments	56	112	50
Unrealized capital loss - investments	(155)	(272)	11
At end of year	$ 59,910	$ 77,589	$ 68,046

Investments - Additional Inform

Investments - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Short term investments average maturity period	1 year 2 months 18 days	1 year 6 months 29 days
Purchase of investments in equity - long term	$ 6,163	$ 0	$ 0
Increase in fair value of investments in equity	800
Carrying value of long-term investments in equity	6,963	$ 0
Commitment to acquire future long-term investments	$ 21,300

Investments - Available For S_2

Investments - Available For Sale Short Term Investments by Major Security Type (Details) $ in Thousands	Dec. 31, 2018USD ($)
Schedule of Available-for-sale Securities [Line Items]
Cost Total	$ 61,170
Unrealized gains / (losses)	(1,260)
Fair Value Total	59,910
Maturity by period, less than 1 year	18,140
Maturity by period, 1 to 5 years	41,770
US government debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost Total	14,910
Unrealized gains / (losses)	(70)
Fair Value Total	14,840
Maturity by period, less than 1 year	5,960
Maturity by period, 1 to 5 years	8,880
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Cost Total	44,920
Unrealized gains / (losses)	(1,190)
Fair Value Total	43,730
Maturity by period, less than 1 year	11,480
Maturity by period, 1 to 5 years	32,250
Term deposits
Schedule of Available-for-sale Securities [Line Items]
Cost Total	1,340
Unrealized gains / (losses)	0
Fair Value Total	1,340
Maturity by period, less than 1 year	700
Maturity by period, 1 to 5 years	$ 640

Goodwill - Schedule of Goodwill

Goodwill - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Opening goodwill	$ 769,058	$ 616,088
Current year acquisitions	0	129,222
Prior period acquisition (note 4 (a))	1,048	1,393
Foreign exchange movement	(13,846)	22,355
Closing goodwill	$ 756,260	$ 769,058

Goodwill - Additional Informati

Goodwill - Additional Information (Details) Patient in Millions	Jul. 27, 2017USD ($)	Sep. 15, 2016USD ($)	Dec. 04, 2015USD ($)PatientSite	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Business Acquisition [Line Items]
Goodwill				$ 769,058,000	$ 756,260,000			$ 616,088,000
Mapi Development SAS
Business Acquisition [Line Items]
Goodwill				130,300,000
Mapi Group
Business Acquisition [Line Items]
Goodwill	$ 130,270,000
Consideration transferred	145,776,000
Working capital adjustment	$ 1,645,000
ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Goodwill		$ 35,969,000
Consideration transferred		58,396,000
Net cash outflows		52,400,000
Other liabilities assumed		9,200,000
Maximum additional consideration that might be payable		$ 12,000,000
Assessment of valuation of contingent consideration at acquisition						$ 0	$ 6,000,000
PMG
Business Acquisition [Line Items]
Consideration transferred			$ 65,400,000
Other liabilities assumed			$ 10,100,000
Number of clinical research sites acquired | Site			52
Number of active patients via electronic medical records acquired (in excess of) | Patient			2
Selling, general and administrative | ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Decrease in amount of contingent consideration, liability				$ 6,000,000

Goodwill - Summary of Estimates

Goodwill - Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Jul. 27, 2017	Sep. 15, 2016	Dec. 31, 2018	Jul. 27, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 15, 2016
Business Acquisition [Line Items]
Goodwill			$ 756,260		$ 769,058	$ 616,088
Cash outflows			$ 1,645		$ 144,131	$ 54,209
Mapi Group
Business Acquisition [Line Items]
Cash	$ 19,649
Property, plant and equipment	4,872
Goodwill	130,270
Accounts receivable	15,874
Unbilled revenue	6,984
Prepayments and other current assets	2,587
Other receivables	1,430
Income taxes receivable	4,262
Accounts payable	(2,994)
Payments on account	(31,445)
Other liabilities	(24,952)
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
Other liabilities adjustment				$ 3,900
Property and equipment adjustment				1,700
Accounts receivable adjustment				1,700
Income taxes receivable adjustment				1,500
Unbilled revenue adjustment				(4,800)
Prepayments and other current assets adjustment				(1,900)
Other receivables adjustment				(1,000)
Payments on account adjustment				(2,600)
Deferred tax liabilities adjustment				$ (9,100)
ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Cash		$ 3,168					$ 3,168
Property, plant and equipment		939					939
Goodwill		35,969					35,969
Accounts receivable		11,431					11,431
Unbilled revenue		3,868					3,868
Prepayments and other current assets		1,673					1,673
Accounts payable		(165)					(165)
Other liabilities		(5,569)					(5,569)
Non-current other liabilities		(7)					(7)
Net assets acquired		58,396					58,396
Total consideration		58,396
Other liabilities adjustment							1,100
Accounts receivable adjustment							(300)
Unbilled revenue adjustment							1,100
Accounts payable adjustment							(500)
Order book | Mapi Group
Business Acquisition [Line Items]
Finite-lived intangible assets	13,012
Order book | ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Finite-lived intangible assets		1,668					1,668
Customer lists | Mapi Group
Business Acquisition [Line Items]
Finite-lived intangible assets	$ 18,392
Customer lists | ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Finite-lived intangible assets		4,012					4,012
Brand | ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Finite-lived intangible assets		$ 1,409					$ 1,409

Goodwill - Summary of Proforma

Goodwill - Summary of Proforma Effect in Net Revenue, Net Income and Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mapi Group
Business Acquisition [Line Items]
Net revenue	$ 1,811,018	$ 1,750,643
Net income	$ 284,903	$ 263,101
Basic earnings per share (dollars per share)	$ 5.26	$ 4.76
Diluted earnings per share (dollars per share)	$ 5.19	$ 4.66
ICON Government & Public Health Solutions
Business Acquisition [Line Items]
Net revenue		$ 1,713,245	$ 1,639,085
Net income		$ 266,148	$ 244,167
Basic earnings per share (dollars per share)		$ 4.82	$ 4.16
Diluted earnings per share (dollars per share)		$ 4.72	$ 4.05

Intangible Assets - Summary (De

Intangible Assets - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cost
Total cost	$ 151,506	$ 155,103
Foreign exchange movement	(5,085)	(2,389)
Accumulated amortization	(100,249)	(84,898)
Foreign exchange movement	2,798	1,423
Net book value	54,055	71,628
Customer relationships acquired
Cost
Total cost	109,622	91,230
Technology asset acquired
Cost
Total cost	11,169	11,169
Order backlog
Cost
Total cost	31,220	18,208
Trade names/ brands acquired
Cost
Total cost	2,766	2,766
Volunteer list acquired
Cost
Total cost	1,325	1,325
Non-compete arrangements
Cost
Total cost	489	489
Mapi intangible asset
Cost
Total cost	$ 0	$ 32,305

Intangible Assets - Additional

Intangible Assets - Additional Information (Details) $ in Thousands

Jul. 27, 2017USD ($)

Sep. 15, 2016USD ($)

Dec. 04, 2015USD ($)Site

Feb. 27, 2015USD ($)

May 07, 2014USD ($)

Feb. 15, 2013USD ($)

Feb. 28, 2012USD ($)

Feb. 15, 2012USD ($)

Jul. 14, 2011USD ($)

Jan. 14, 2011USD ($)

Nov. 14, 2008USD ($)

Feb. 11, 2008USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

$ 100,249

$ 84,898

Mapi Group

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

5,800

Mapi Group | Customer lists

Indefinite-lived Intangible Assets [Line Items]

Finite-lived intangible assets

$ 18,392

Amortization period

8 years

Mapi Group | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Finite-lived intangible assets

$ 13,012

Amortization period

8 years 6 months

ICON Government & Public Health Solutions

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

4,400

ICON Government & Public Health Solutions | Customer lists

Indefinite-lived Intangible Assets [Line Items]

Finite-lived intangible assets

$ 4,012

ICON Government & Public Health Solutions | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 4,000

Amortization period

7 years

ICON Government & Public Health Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Finite-lived intangible assets

$ 1,668

Value of amortizable intangible asset acquired

$ 1,700

Amortization period

2 years

ICON Government & Public Health Solutions | Brand

Indefinite-lived Intangible Assets [Line Items]

Finite-lived intangible assets

$ 1,409

Value of amortizable intangible asset acquired

$ 1,400

Amortization period

5 years

PMG

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

6,000

Number of clinical research sites acquired | Site

52

PMG | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 6,900

Amortization period

7 years

PMG | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 3,000

Amortization period

2 years

MediMedia Pharma Solutions

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

15,000

MediMedia Pharma Solutions | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 22,800

Amortization period

7 years

MediMedia Pharma Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 2,500

Amortization period

1 year

Aptiv Solutions

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

22,200

Aptiv Solutions | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 21,400

Amortization period

7 years

Aptiv Solutions | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 7,900

Amortization period

3 years

Clinical Trial Services of Cross Country Healthcare Inc.

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

3,900

Clinical Trial Services of Cross Country Healthcare Inc. | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 3,300

Amortization period

3 years

Clinical Trial Services of Cross Country Healthcare Inc. | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 600

Amortization period

1 year

PriceSpective

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

7,800

PriceSpective | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 10,200

Amortization period

10 years

PriceSpective | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 400

Amortization period

9 months 18 days

PriceSpective | Non-compete arrangements

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 400

Amortization period

3 years

BeijingWits Medical

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

1,700

BeijingWits Medical | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 1,800

Amortization period

10 years

BeijingWits Medical | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 400

Amortization period

4 years

BeijingWits Medical | Non-compete arrangements

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 10

Amortization period

5 years

Firecrest Clinical Limited

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

16,400

Firecrest Clinical Limited | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 5,200

Amortization period

7 years 6 months

Firecrest Clinical Limited | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 1,200

Amortization period

1 year 2 months 12 days

Firecrest Clinical Limited | Technology asset acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 11,200

Amortization period

7 years 6 months

Firecrest Clinical Limited | Trade names acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 1,400

Amortization period

4 years 6 months

Oxford Outcomes Limited | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 6,600

Amortization period

6 years 6 months

Oxford Outcomes Limited | Order backlog

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 600

Amortization period

2 years

Prevalere Life Sciences

Indefinite-lived Intangible Assets [Line Items]

Amount amortized since date of acquisition

$ 7,200

Prevalere Life Sciences | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 7,400

Prevalere Life Sciences | Minimum | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Amortization period

7 years

Prevalere Life Sciences | Maximum | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Amortization period

11 years

Healthcare Discoveries | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 1,600

Healthcare Discoveries | Volunteer list acquired

Indefinite-lived Intangible Assets [Line Items]

Value of amortizable intangible asset acquired

$ 1,300

Healthcare Discoveries | Minimum | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Amortization period

2 years

Healthcare Discoveries | Minimum | Volunteer list acquired

Indefinite-lived Intangible Assets [Line Items]

Amortization period

6 years

Healthcare Discoveries | Maximum | Customer relationships acquired

Indefinite-lived Intangible Assets [Line Items]

Amortization period

9 years

Intangible Assets - Future Inta

Intangible Assets - Future Intangible Asset Amortization Expense (Details) $ in Thousands	Dec. 31, 2018USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,019	$ 13,183
2,020	12,895
2,021	10,857
2,022	6,069
2,023	4,221
Finite Lived Intangible Assets, Amortization Expense, Net, Total	$ 47,225

Property, Plant and Equipment_3

Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Land	$ 3,476	$ 3,464
Building	86,621	88,411
Computer equipment and software	399,192	358,874
Office furniture and fixtures	83,215	78,372
Laboratory equipment	36,092	34,918
Leasehold improvements	25,827	24,097
Motor vehicles	144	42
Property, plant and equipment, gross	634,567	588,178
Less accumulated depreciation and asset write offs	(475,898)	(425,127)
Property, plant and equipment (net)	$ 158,669	$ 163,051

Other Liabilities (Details)

Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Liabilities Disclosure [Abstract]
Personnel related liabilities	$ 171,866	$ 168,964
Facility related liabilities	14,012	13,061
General trade and overhead liabilities	118,845	41,789
Other liabilities	4,289	4,628
Short term government grants (note 11)	42	35
Restructuring (note 14)	8,089	5,026
Other liabilities	317,143	$ 233,503
Amounts due to third parties for reimbursable expenses	$ 85,600

Non-Current Other Liabilities_2

Non-Current Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Defined benefit pension obligations, net (note 9)	$ 3,320	$ 6,061
Other non-current liabilities	10,126	11,050
Non-current other liabilities	$ 13,446	$ 17,111

Employee Benefits - Additional

Employee Benefits - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 07, 2014
The Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions percentage of annual compensation	8.00%
Employer contributions	$ 25,241,000	$ 20,355,000	$ 20,952,000
U.S. Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions percentage of annual compensation	3.00%
Participant's contributions percentage matches	50.00%
Participant's contributions percentage of annual compensation	4.50%
Deferred salary arrangement employer contribution	$ 15,532,000	$ 14,946,000	$ 15,223,000
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss) for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	(100,000)
Net prior service cost for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ 0
Retirement period used as a basis to estimate expected cash flows	10 years
Expected long-term rate of return after application of a market value reduction factor (per annum)	2.10%
Contribution to pension fund in the year ending December 31, 2018	$ 200,000
United Kingdom | iboxx Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan bond maturity period	15 years
Basis point increase in benefits obligation discount rate	10.00%
United Kingdom | Gilts
Defined Benefit Plan Disclosure [Line Items]
Percentage of investments	75.00%
United Kingdom | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Percentage of investments	25.00%
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss) for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ 95,000
Net prior service cost for defined benefit pension plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost over the next year	$ 9,000
Retirement period used as a basis to estimate expected cash flows	10 years
Contribution to pension fund in the year ending December 31, 2018	$ 100,000
Switzerland | Aptiv Solutions
Defined Benefit Plan Disclosure [Line Items]
Percentage of common stock acquired				100.00%

Employee Benefits - Funded Stat

Employee Benefits - Funded Status (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ (30,045)	$ (37,759)
Fair value of plan assets	27,297	32,423	$ 24,876
Funded status	(2,748)	(5,336)
Non-current other liabilities (note 8)	(2,748)	(5,336)
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	(5,279)	(5,927)
Fair value of plan assets	4,707	5,202	$ 6,006
Funded status	(572)	(725)
Non-current other liabilities (note 8)	$ (572)	$ (725)

Employee Benefits - Change in B

Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United Kingdom
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	$ 37,759	$ 32,906
Service cost	124	112	$ 75
Interest cost	895	929	1,017
Plan participants' contributions	24	22
Expenses	0	(8)	8
Benefits paid	(3,049)	(68)
Actuarial (gain)/loss	(3,844)	658
Foreign currency exchange rate changes	(1,864)	3,208
Benefit obligation at end of year	30,045	37,759	32,906
Switzerland
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,927	6,928
Service cost	138	243
Interest cost	47	54
Plan participants' contributions	83	120
Benefits paid	(77)	(76)
Actuarial (gain)/loss	(372)	(626)
Foreign currency exchange rate changes	(50)	303
Settlement	(409)	(1,019)
Prior service cost	(8)	0
Benefit obligation at end of year	$ 5,279	$ 5,927	$ 6,928

Employee Benefits - Change in P

Employee Benefits - Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United Kingdom
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 32,423	$ 24,876
Expected return on plan assets	624	586	$ 646
Actual return on plan assets	(584)	979
Employer contributions	153	4,008
Plan participants' contributions	24	22
Benefits paid	(3,049)	(68)
Foreign currency exchange rate changes	(1,670)	2,606
Fair value of plan assets at end of year	27,297	32,423	24,876
Switzerland
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	5,202	6,006
Expected return on plan assets	41	47
Actual return on plan assets	(240)	(296)
Scheme contributions	109	157
Plan participants' contributions	83	120
Transferred (benefits paid)/balances	(77)	(76)
Settlement	(409)	(1,019)
Foreign currency exchange rate changes	(2)	263
Fair value of plan assets at end of year	$ 4,707	$ 5,202	$ 6,006

Employee Benefits - Components

Employee Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 124	$ 112	$ 75
Interest cost	895	929	1,017
Expected return on plan assets	(624)	(586)	(646)
Amortization of net (gain)/loss	248	250	0
Expenses	0	(8)	8
Net periodic benefit cost	643	697	454
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Service cost	138	243
Interest cost	47	54
Expected return on plan assets	(41)	(47)
Amortization of prior service credit	(8)	0
Includes Financial Instruments, Property Occupied by, or Other Assets | Switzerland
Defined Benefit Plan Disclosure [Line Items]
Service cost	138	243	352
Interest cost	47	54	82
Expected return on plan assets	(41)	(47)	(48)
Amortization of net (gain)/loss	(69)	(43)	22
Amortization of prior service credit	(8)	(8)	(8)
Settlement	(93)	(214)	(136)
Curtailment	0	0	0
Net periodic benefit cost	$ (26)	$ (15)	$ 264

Employee Benefits - Net Periodi

Employee Benefits - Net Periodic Pension Benefit Cost Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.50%	2.70%	4.00%
Rate of compensation increase	3.70%	3.90%	3.70%
Expected rate of return on plan assets	2.00%	2.10%	3.00%
Other comprehensive income
Actuarial (gain)/loss - benefit obligation	$ (3,844)	$ 658	$ 10,057
Actuarial loss/(gain) – plan assets	1,208	(393)	(5,215)
Actuarial gain recognized in net periodic benefit cost	(248)	(250)	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	$ (2,884)	$ 15	$ 4,842
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Discount rate	0.80%	0.75%	0.95%
Rate of compensation increase	2.00%	2.00%	2.00%
Expected rate of return on plan assets	0.80%	0.75%	0.95%
Other comprehensive income
Actuarial (gain)/loss - benefit obligation	$ (372)	$ (626)	$ (1,157)
Actuarial loss/(gain) – plan assets	240	296	(1,233)
Prior service credit recognized in net periodic benefit cost	93	215	136
Actuarial gain recognized in net periodic benefit cost	69	43	(22)
Amortization of net prior service credit	8	8	8
Net prior service cost occurring during the year	(9)	(1)	(89)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	$ 29	$ (65)	$ (2,357)

Employee Benefits - Amounts Rec

Employee Benefits - Amounts Recognized in Accumulated Other Comprehensive Income not yet Recognized (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total	$ 3,000	$ 5,855
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	4,254	7,138	$ 7,123
Total	4,254	7,138	7,123
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(1,254)	(1,283)	(1,218)
Total	$ (1,254)	$ (1,283)	$ (1,218)

Employee Benefits - Assumptions

Employee Benefits - Assumptions Used in Determining Benefit Obligation (Details)	Dec. 31, 2018	Dec. 31, 2017
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.90%	2.50%
Rate of compensation increase	3.70%	3.70%
Switzerland
Defined Benefit Plan Disclosure [Line Items]
Discount rate	0.80%	0.80%
Rate of compensation increase	2.00%	2.00%

Employee Benefits - Expected Lo

Employee Benefits - Expected Long Term Rates of Return on Different Asset Classes (Details) - United Kingdom	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	2.00%	2.10%	3.00%
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	2.90%
Gilts
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	1.80%
Cash
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return per annum	2.90%

Employee Benefits - Actual Plan

Employee Benefits - Actual Plan Asset Allocation (Details) - United Kingdom	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	100.00%	100.00%
Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	25.00%	22.00%
Gilts
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	71.00%	65.00%
Cash
Defined Benefit Plan Disclosure [Line Items]
Actual plan asset allocations	4.00%	13.00%

Employee Benefits - Plan Asset

Employee Benefits - Plan Asset Fair Value Measurements (Details) - United Kingdom - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 27,297	$ 32,423	$ 24,876
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,297	32,423
Fair Value, Inputs, Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,029	4,086
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Active Corporate Bond – Over 10 Year
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,688	7,188
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Gilt Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,136	7,611
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Index Linked Gilt Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12,444	$ 13,538

Employee Benefits - Annual Bene

Employee Benefits - Annual Benefit Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
2,019	$ 295
2,020	311
2,021	383
2,022	412
2,023	403
Years 2024 - 2028	3,451
Switzerland
Defined Benefit Plan Disclosure [Line Items]
2,019	678
2,020	212
2,021	209
2,022	204
2,023	199
Years 2024 - 2028	$ 908

Equity Incentive Schemes and _3

Equity Incentive Schemes and Stock Compensation Charges - Additional Information (Details) - USD ($) $ in Thousands	Feb. 14, 2017	May 11, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Jul. 21, 2008
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Weighted average contractual term of options outstanding			5 years 3 days	4 years 10 months 10 days
Exercisable - weighted average remaining contractual life			3 years 10 months	3 years 5 months 9 days
Options expected to vest (in shares)			241,149	255,198		241,149
Intrinsic value of option exercised			$ 38,200
Intrinsic value of option outstanding			48,600			$ 48,600
Intrinsic value of option exercisable			28,300			28,300
Total compensation costs			31,594	$ 30,573	$ 40,343
Non-cash stock compensation expense not yet recognized			$ 43,100			$ 43,100
Unrecognized stock-based compensation expense, weighted average period (years)			2 years 2 months 9 days
Income tax benefit related to stock compensation			$ 12,700	9,300	3,500
Cash tax benefit related to stock options exercised			$ 3,600	3,200	$ 3,400
Employee Stock Plan, 2008 Plan
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)			6,000,000			6,000,000	1,000,000
Employee Stock Plan, 2008 Plan | Individual Employee
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)			400,000			400,000
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 (in shares)	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 (in shares)	1,000,000
Restricted Stock Units 2008
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Ordinary shares which have been reserved for issuance (in shares)			1,000,000			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 (in shares)			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 (in shares)			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
Employee Stock Option | Vested and Unvested Options
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Options vesting percentage			20.00%
Employee Stock Option | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Weighted average contractual term of options outstanding			8 years
Restricted Stock Units 2013
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Additional number of ordinary shares which have been reserved for issuance (in shares)		2,500,000
Ordinary shares which have been reserved for issuance (in shares)		4,100,000
Restricted Stock Units (RSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 18,800	16,600
Stock units granted (in shares)			160,113
Performance Share Unit (PSUs)
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Fair value of stock units vested			$ 14,700	$ 15,000
Stock units granted (in shares)			71,906
PSUs Based on Service and EPS Targets
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units granted (in shares)						147,630
PSUs Based on Service and EPS Targets | Maximum
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Stock units to be granted (in shares)			103,423			103,423

Equity Incentive Schemes and _4

Equity Incentive Schemes and Stock Compensation Charges - Summary of Stock Option Activity (Details) - Stock Option And Award Plans - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Granted Under Plans
Beginning balance (in shares)	1,171,393	1,466,444	1,626,582
Granted (in shares)	167,557	219,113	256,191
Exercised (in shares)	(408,699)	(458,243)	(393,240)
Cancelled (in shares)	(9,505)	(55,921)	(23,089)
Ending balance (in shares)	920,746	1,171,393	1,466,444
Vested and exercisable at end of period (in shares)	397,923
Weighted Average Exercise Price
Beginning balance (usd per share)	$ 56.02	$ 43.45	$ 34.87
Granted (usd per share)	118.90	85.98	69.61
Exercised (usd per share)	41.12	30.35	25.79
Cancelled (usd per share)	32.35	54.35	29.74
Ending balance (usd per share)	74.32	56.02	43.45
Vested and exercisable at end of period (usd per share)	56.10
Weighted Average Grant Date Fair Value
Beginning balance (usd per share)	17.15	13.94	11.94
Granted (usd per share)	36.84	25.06	20.10
Exercised (usd per share)	13.55	10.72	9.84
Cancelled (usd per share)	11.39	16.76	11.19
Ending balance (usd per share)	22.39	$ 17.15	$ 13.94
Vested and exercisable at end of period (usd per share)	$ 17.11

Equity Incentive Schemes and _5

Equity Incentive Schemes and Stock Compensation Charges - Summary of Movement in Non-Vested Share Options (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Outstanding Number of Shares
Beginning Balance (in shares)	694,727
Granted (in shares)	167,557
Vested (in shares)	(336,756)
Forfeited (in shares)	(2,705)
Ending Balance (in shares)	522,823	694,727
Weighted Average Exercise Price
Beginning balance (usd per share)	$ 68.06
Granted (usd per share)	118.90
Vested (usd per share)	62.21
Forfeited (usd per share)	56.58
Ending balance (usd per share)	88.18	$ 68.06
Weighted Average Fair Value
Beginning Balance (usd per share)	20.03
Granted (usd per share)	36.84	25.06	$ 20.10
Vested (usd per share)	18.51
Forfeited (usd per share)	16.84
Ending Balance (usd per share)	$ 26.41	$ 20.03

Equity Incentive Schemes and _6

Equity Incentive Schemes and Stock Compensation Charges - Outstanding and Exercisable Share Options (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 125.74
Number of Shares (in shares) | shares	920,746
Weighted Average Remaining Contractual Life	5 years 3 days
Weighted Average Exercise Price (usd per share)	$ 74.32
Number of Shares (in shares) | shares	397,923
Weighted Average Exercise Price (usd per share)	$ 56.10
Minimum Range Exercise Price (usd per share)	20.28
Range 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 20.28
Number of Shares (in shares) | shares	18,190
Weighted Average Remaining Contractual Life	1 month 27 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	18,190
Weighted Average Exercise Price (usd per share)
Range 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 22.30
Number of Shares (in shares) | shares	52,811
Weighted Average Remaining Contractual Life	1 year 3 months 26 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	52,811
Weighted Average Exercise Price (usd per share)
Range 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 23.66
Number of Shares (in shares) | shares	1,161
Weighted Average Remaining Contractual Life	1 year 6 months 25 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,161
Weighted Average Exercise Price (usd per share)
Range 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 26.71
Number of Shares (in shares) | shares	450
Weighted Average Remaining Contractual Life	1 year 8 months 9 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	450
Weighted Average Exercise Price (usd per share)
Range 5
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 32.37
Number of Shares (in shares) | shares	20,848
Weighted Average Remaining Contractual Life	2 years 3 months 29 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	20,848
Weighted Average Exercise Price (usd per share)
Range 6
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 36.22
Number of Shares (in shares) | shares	2,141
Weighted Average Remaining Contractual Life	2 years 5 months 16 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	2,141
Weighted Average Exercise Price (usd per share)
Range 7
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 37.90
Number of Shares (in shares) | shares	920
Weighted Average Remaining Contractual Life	2 years 11 months 4 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	920
Weighted Average Exercise Price (usd per share)
Range 8
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 40.83
Number of Shares (in shares) | shares	40,502
Weighted Average Remaining Contractual Life	3 years 4 months 20 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	25,360
Weighted Average Exercise Price (usd per share)
Range 9
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 47.03
Number of Shares (in shares) | shares	22,685
Weighted Average Remaining Contractual Life	3 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	14,020
Weighted Average Exercise Price (usd per share)
Range 10
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 48.67
Number of Shares (in shares) | shares	50,896
Weighted Average Remaining Contractual Life	3 years 2 months 16 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	34,906
Weighted Average Exercise Price (usd per share)
Range 11
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 51.35
Number of Shares (in shares) | shares	2,030
Weighted Average Remaining Contractual Life	3 years 7 months 6 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,224
Weighted Average Exercise Price (usd per share)
Range 12
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 65.60
Number of Shares (in shares) | shares	66,752
Weighted Average Remaining Contractual Life	5 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	24,543
Weighted Average Exercise Price (usd per share)
Range 13
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 66.47
Number of Shares (in shares) | shares	4,267
Weighted Average Remaining Contractual Life	4 years 4 months 20 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	1,572
Weighted Average Exercise Price (usd per share)
Range 14
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 66.97
Number of Shares (in shares) | shares	1,249
Weighted Average Remaining Contractual Life	4 years 5 months 12 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)
Range 15
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 68.39
Number of Shares (in shares) | shares	135,400
Weighted Average Remaining Contractual Life	4 years 2 months 5 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	79,069
Weighted Average Exercise Price (usd per share)
Range 16
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 71.95
Number of Shares (in shares) | shares	127,905
Weighted Average Remaining Contractual Life	5 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	74,473
Weighted Average Exercise Price (usd per share)
Range 17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 83.47
Number of Shares (in shares) | shares	127,136
Weighted Average Remaining Contractual Life	6 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	22,461
Weighted Average Exercise Price (usd per share)
Range 18
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 90.03
Number of Shares (in shares) | shares	77,846
Weighted Average Remaining Contractual Life	6 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	23,774
Weighted Average Exercise Price (usd per share)
Range 19
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 115.11
Number of Shares (in shares) | shares	107,794
Weighted Average Remaining Contractual Life	7 years 2 months 1 day
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)
Range 20
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Range Exercise Price (usd per share)	$ 125.74
Number of Shares (in shares) | shares	59,763
Weighted Average Remaining Contractual Life	7 years 4 months 17 days
Weighted Average Exercise Price (usd per share)
Number of Shares (in shares) | shares	0
Weighted Average Exercise Price (usd per share)

Equity Incentive Schemes and _7

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Weighted Average Fair Values and Assumptions Used (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value (usd per share)	$ 36.84	$ 25.06	$ 20.10
Assumptions:
Expected volatility	30.00%	29.00%	30.00%
Dividend yield	0.00%	0.00%	0.00%
Risk-free interest rate	2.76%	1.93%	1.39%
Expected life	5 years	5 years	5 years

Equity Incentive Schemes and _8

Equity Incentive Schemes and Stock Compensation Charges - Summary of RSU and PSU Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Performance Share Unit (PSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	511,026
Granted (in shares)	71,906
Shares vested (in shares)	(215,826)
Forfeited (in shares)	(116,053)
Outstanding at ending of period (in shares)	251,053	511,026
Weighted Average Fair Value
Outstanding at beginning of period (usd per share)	$ 72.07
Granted (usd per share)	116.02
Shares vested (usd per share)	68.28
Forfeited (usd per share)	70.89
Outstanding at end of period (usd per share)	$ 85.95	$ 72.07
Weighted Average Remaining Contractual Life
Outstanding at end of period (usd per share)	11 months 15 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)	160,113
Shares vested (in shares)	(276,495)
Forfeited (in shares)	(64,911)
Outstanding at ending of period (in shares)	534,677	715,970
Weighted Average Fair Value
Outstanding at beginning of period (usd per share)	$ 72.65
Granted (usd per share)	123.42
Shares vested (usd per share)	67.99
Forfeited (usd per share)	78.92
Outstanding at end of period (usd per share)	$ 89.50	$ 72.65
Weighted Average Remaining Contractual Life
Outstanding at end of period (usd per share)	1 year 2 months 19 days	1 year 3 months 11 days

Equity Incentive Schemes and _9

Equity Incentive Schemes and Stock Compensation Charges - Schedule of Non-cash Stock Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 31,594	$ 30,573	$ 40,343
Direct costs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	17,408	18,020	21,903
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation costs	$ 14,186	$ 12,553	$ 18,440

Government Grants - Summary (De

Government Grants - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Received	$ 3,539	$ 3,539
Less accumulated amortization	(2,792)	(2,745)
Foreign exchange translation adjustment	172	207
Less current portion	(274,468)	(298,992)
Government Grants
Disaggregation of Revenue [Line Items]
Total government grants	919	1,001
Less current portion	(42)	(35)
Non-current government grants	$ 877	$ 966

Government Grants - Additional

Government Grants - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Revenue Disclosure [Abstract]
Amortization of government grants	$ 47	$ 44	$ 44
Restricted retained earnings	$ 3,200

Share Capital - Additional Info

Share Capital - Additional Information (Details) - USD ($)	Jul. 31, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Oct. 03, 2016
Equity, Class of Treasury Stock [Line Items]
Total proceeds from exercise of stock options by employees		$ 16,842,000	$ 13,950,000	$ 10,180,000
Ordinary shares redeemed, value		$ 128,960,000	$ 133,106,000	$ 110,000,000
Employee Stock Option
Equity, Class of Treasury Stock [Line Items]
Options exercised by employees (in shares)		408,699	458,243	393,240
Average exercise price of option per share (usd per share)		$ 41.12	$ 30.35	$ 25.79
Total proceeds from exercise of stock options by employees		$ 16,800,000	$ 13,900,000	$ 10,100,000
Restricted Stock Units (RSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain RSUs previously awarded by the Company (in shares)		276,495	361,102	296,386
Performance Share Unit (PSUs)
Equity, Class of Treasury Stock [Line Items]
Ordinary shares issued in respect of certain PSUs previously awarded by the Company (in shares)		215,826	320,640	311,492
Buyback Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount	$ 400,000,000						$ 400,000,000
Ordinary shares redeemed (in shares)	5,316,062	1,008,162			6,198,481	4,026,576
Ordinary shares redeemed, value	$ 400,000,000	$ 129,000,000			$ 457,900,000	$ 372,100,000

Income Taxes - Components of In

Income Taxes - Components of Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before provision for income taxes	$ 364,614	$ 328,057	$ 300,172
Ireland
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before provision for income taxes	243,988	218,306	201,221
United States
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before provision for income taxes	27,499	28,426	11,466
Other
Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items]
Income before provision for income taxes	$ 93,127	$ 81,325	$ 87,485

Income Taxes - Components of Pr

Income Taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current tax expense:
Current tax expense	$ 40,306	$ 35,840	$ 36,448
Deferred tax expense/(benefit):
Deferred tax expense/(benefit)	1,652	10,729	1,545
Provision for income taxes	41,958	46,569	37,993
Impact on shareholders equity and other comprehensive income of the tax consequence of :
Excess tax benefit on stock compensation	0	0	(4,332)
Currency impact on long term funding	119	973	(396)
Fair value of cash flow hedge	(148)	148	0
Total	41,929	47,690	33,265
Ireland
Current tax expense:
Current tax expense	28,042	20,084	22,931
Deferred tax expense/(benefit):
Deferred tax expense/(benefit)	1,054	261	1,284
Provision for income taxes	29,096	20,345	24,215
United States
Current tax expense:
Current tax expense	2,885	5,792	7,768
Deferred tax expense/(benefit):
Deferred tax expense/(benefit)	875	8,980	613
Provision for income taxes	3,761	14,772	8,381
Other
Current tax expense:
Current tax expense	9,379	9,964	5,749
Deferred tax expense/(benefit):
Deferred tax expense/(benefit)	$ (277)	$ 1,488	$ (352)

Income Taxes - Consolidated Rep

Income Taxes - Consolidated Reported Provision for Income Taxes Differed from Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Taxes at Irish statutory rate of 12.5% (2017:12.5%; 2016:12.5%)	$ 45,577	$ 41,007	$ 37,522
Foreign and other income taxed at higher rates	7,649	6,324	4,642
Research & development tax incentives	(1,243)	(830)	(907)
Movement in valuation allowance	5,667	1,329	1,208
Effects of change in tax rates	(147)	925	576
Increase/(decrease) in unrecognized tax benefits	(5,423)	933	(1,521)
Impact of stock compensation	(8,301)	(9,917)	(4,121)
Impact of deemed repatriation under US Tax Reform	0	7,694	0
Other	(1,821)	(896)	594
Provision for income taxes	$ 41,958	$ 46,569	$ 37,993

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Impact of deemed repatriation under US Tax Reform	$ 0	$ 7,694	$ 0
Tax cuts and jobs act of 2017, incomplete accounting, provisional income tax expense (benefit)		500
Valuation allowance for deferred tax assets	27,263	22,439	20,300
Net change in the total valuation allowance	4,800	2,100
Tax credit carry forward, valuation allowance	6,200
Deferred tax liabilities, undistributed foreign earnings	4,900	3,100
Unrecognized tax benefit, potentially expire in 2019	1,300
Total unrecognized tax benefits net of potential benefits	21,400	23,700	26,600
Interest and penalties recognized as an expense	1,300	900	$ 100
Total accrued interest and penalties	1,100	2,400
Provision for income taxes
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	5,600	500
Other Comprehensive Income
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Net change in the total valuation allowance	800	1,600
Foreign Country
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Ireland subsidiaries additional tax credit carryforward for income tax	4,000	4,500
Other non-U.S subsidiaries operating loss carryforwards for income tax	72,900	77,200
Other non-U.S subsidiaries tax credit carryforwards for income tax	4,900	4,800
Foreign Country | Tax Year 2019 to 2025
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	4,700	4,700
Foreign Country | Tax Year 2026 to 2035
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	3,200	$ 0
United States
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. Federal net operating loss carry forwards currently available for offset	2,200
Alternative minimum tax credit carry forwards	400
Business credit carry forwards that are available to offset	300
United States | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	15,089
U.S. federal net operating loss carry forwards	12,900
Tax credit carry forward	21,900
United States | Tax Year 2018 to 2020 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	4,900
United States | Tax Year 2021 to 2025 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	7,500
United States | Tax Year 2026 to 2035 | Federal NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
U.S. federal net operating loss carry forwards	500
United States | State NOL's
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Additional operating loss carryforward	$ 32,622

Income Taxes - Tax Effects of T

Income Taxes - Tax Effects of Temporary Differences That Give Rise to Significant Portions of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Property, plant and equipment	$ 981	$ 1,139
Goodwill	25,149	22,655
Other intangible assets	9,397	11,801
Other	5,703	4,139
Total deferred tax liabilities recognized	41,230	39,734
Deferred tax assets:
Operating loss and tax credits carry-forwards	29,995	24,962
Property, plant and equipment	4,893	4,062
Accrued expenses and payments on account	24,599	24,433
Stock compensation	6,490	5,786
Deferred compensation	2,197	2,548
Deferred revenue	5,681	0
Other	2	740
Total deferred tax assets	73,857	62,531
Valuation allowance for deferred tax assets	(27,263)	(22,439)	$ (20,300)
Deferred tax assets recognized	46,594	40,092
Overall net deferred tax asset	$ 5,364	$ 358

Income Taxes - Expected Expiry

Income Taxes - Expected Expiry Dates of NOL's (Details) - United States $ in Thousands	Dec. 31, 2018USD ($)
Federal NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 15,089
Federal NOL's | 2021-2034
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	14,323
Federal NOL's | 2035-2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	766
State NOL's
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	32,622
State NOL's | 2021-2034
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	12,158
State NOL's | 2035-2037
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 20,464

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at start of year	$ 23,720	$ 26,620	$ 28,166
Increase related to prior year tax positions	2,084	0	1,151
Decrease related to prior year tax positions	(2,915)	(3,050)	(2,483)
Increase related to current year tax positions	3,065	4,765	1,104
Settlements	(182)	(2,523)	(837)
Lapse of statute of limitations	(4,339)	(2,092)	(481)
Unrecognized tax benefits at end of year	$ 21,433	$ 23,720	$ 26,620

Restructuring Charges - Recogni

Restructuring Charges - Recognized Restructuring and Other Items, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 12,490	$ 7,753	$ 8,159
Resource Rationalizations 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges	12,490
Utilized	(6,071)
Foreign exchange	0
Restructuring Reserve	6,419
Resource Rationalizations 2018 | Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring charges	9,684
Foreign exchange	0
Restructuring Reserve	4,285
Resource Rationalizations 2018 | Onerous Lease
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2,806
Utilized	(672)
Foreign exchange	0
Restructuring Reserve	$ 2,134

Restructuring Charges - Additio

Restructuring Charges - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 12,490	$ 7,753	$ 8,159
Restructuring reserve, current	7,200
Restructuring reserve, noncurrent	1,300
Resource Rationalizations 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges	12,490
Resource Rationalizations 2018 | Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring charges	9,684
Resource Rationalizations 2018 | Onerous Lease
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 2,806
Resource Rationalizations 2017
Restructuring Cost and Reserve [Line Items]
Restructuring charges			7,753
Resource Rationalizations 2016
Restructuring Cost and Reserve [Line Items]
Restructuring charges			8,159
Resource Rationalizations 2016 | Workforce Reductions
Restructuring Cost and Reserve [Line Items]
Restructuring charges			6,190
Resource Rationalizations 2016 | Onerous Lease
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 1,969

Restructuring Charges - Prior P

Restructuring Charges - Prior Period Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Restructuring charges	$ 12,490	$ 7,753	$ 8,159
Resource Rationalizations 2017
Restructuring Reserve [Roll Forward]
Closing provision	3,097
Restructuring charges			7,753
Utilized	(1,015)	(4,656)
Closing provision	2,082	3,097
Resource Rationalizations 2017 | Workforce Reductions
Restructuring Reserve [Roll Forward]
Utilized	(5,399)
Resource Rationalizations 2016
Restructuring Reserve [Roll Forward]
Closing provision	317	1,791
Restructuring charges			8,159
Utilized	(317)	(1,474)	(6,305)
Foreign exchange			(63)
Closing provision	0	317	1,791
Resource Rationalizations 2016 | Workforce Reductions
Restructuring Reserve [Roll Forward]
Closing provision	0	393
Restructuring charges			6,190
Utilized	0	(393)	(5,734)
Foreign exchange			(63)
Closing provision	0	0	393
Resource Rationalizations 2016 | Onerous Lease
Restructuring Reserve [Roll Forward]
Closing provision	317	1,398
Restructuring charges			1,969
Utilized	(317)	(1,081)	(571)
Foreign exchange			0
Closing provision	$ 0	$ 317	$ 1,398

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Non-cancelable operating leases for facilities expiration period	13 years
Operating leases rental expense	$ 40.3	$ 44	$ 44

Commitments and Contingencies_2

Commitments and Contingencies - Future Minimum Rental Commitments for Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,019	$ 32,634
2,020	26,839
2,021	21,306
2,022	15,781
2,023	10,829
Thereafter	19,194
Total	$ 126,583

Disaggregation of Revenue (Deta

Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 2,595,777	$ 1,758,439	$ 1,666,487
Gross revenue	2,595,777	2,402,321	$ 2,364,956
Top client
Disaggregation of Revenue [Line Items]
Revenue	352,335
Gross revenue		503,875
Clients 2-5
Disaggregation of Revenue [Line Items]
Revenue	671,723
Gross revenue		505,818
Clients 6-10
Disaggregation of Revenue [Line Items]
Revenue	385,741
Gross revenue		348,841
Clients 11-25
Disaggregation of Revenue [Line Items]
Revenue	461,351
Gross revenue		421,759
Other
Disaggregation of Revenue [Line Items]
Revenue	$ 724,627
Gross revenue		$ 622,028

Trade Accounts Receivable, Un_3

Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities) - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer [Abstract]
Billed services (accounts receivable)	$ 423,680	$ 388,431
Trade accounts receivable and unbilled revenue	786,606	656,940
Allowance for doubtful accounts	(8,889)	(8,930)	$ (9,450)
Trade accounts receivable and unbilled revenue, net	777,717	648,010
Unbilled services (unbilled revenue)	362,926	268,509
Unearned revenue (payments on account)	(274,468)	(298,992)
Net balance	88,458	$ (30,483)
Change in unbilled receivables	94,417
Change in unearned revenue (payments on account)	24,524
Change in advance payments netted against unbilled contracts receivable	$ 118,941
Change in unbilled receivables, percent	35.20%
Change in unearned revenue (payments on account), percent	(8.20%)
Change in advance payments netted against unbilled contracts receivable, percent	390.20%
Amounts due to third parties for reimbursable expenses	$ 85,600
Credit loss expense	$ 700

Trade Accounts Receivable, Un_4

Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities) - Revenue Remaining Performance Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation	$ 5,272
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation, percent	40.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year

Provision for Doubtful Debts (D

Provision for Doubtful Debts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Opening provision	$ 8,930	$ 9,450
Amounts used during the year	(995)	(2,733)
Amounts provided during the year	3,083	5,116
Amounts released during the year	(2,355)	(3,106)
Foreign exchange	226	203
Closing provision	$ 8,889	$ 8,930

Business Segment and Geograph_3

Business Segment and Geographical Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2018segment
Segment Reporting [Abstract]
Number of operating segments	1

Business Segment and Geograph_4

Business Segment and Geographical Information - Distribution of Net Revenue by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross revenue	$ 2,595,777	$ 2,402,321	$ 2,364,956
Ireland
Segment Reporting Information [Line Items]
Gross revenue	1,066,200	944,130	995,365
Rest of Europe
Segment Reporting Information [Line Items]
Gross revenue	379,883	355,552	319,935
U.S.
Segment Reporting Information [Line Items]
Gross revenue	894,978	881,829	866,901
Other
Segment Reporting Information [Line Items]
Gross revenue	$ 254,716	$ 220,810	$ 182,755

Business Segment and Geograph_5

Business Segment and Geographical Information - Distribution of Income from Operations, Including Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Income from operations	$ 373,357	$ 338,338	$ 311,694
Ireland
Segment Reporting Information [Line Items]
Income from operations	257,089	232,032	216,149
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations	36,280	26,493	34,200
U.S.
Segment Reporting Information [Line Items]
Income from operations	58,561	58,322	41,348
Other
Segment Reporting Information [Line Items]
Income from operations	$ 21,427	$ 21,491	$ 19,997

Business Segment and Geograph_6

Business Segment and Geographical Information - Distribution of Income from Operations, Excluding Restructuring, by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 385,847	$ 346,091	$ 319,853
Ireland
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	269,196	240,115	218,334
Rest of Europe
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	36,904	26,351	36,509
U.S.
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	58,340	58,164	44,590
Other
Segment Reporting Information [Line Items]
Income from operations, excluding restructuring and other items	$ 21,407	$ 21,461	$ 20,420

Business Segment and Geograph_7

Business Segment and Geographical Information - Distribution of Property, Plant and Equipment, Net, by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 158,669	$ 163,051
Ireland
Segment Reporting Information [Line Items]
Property, plant and equipment, net	106,206	111,329
Rest of Europe
Segment Reporting Information [Line Items]
Property, plant and equipment, net	9,807	9,026
U.S.
Segment Reporting Information [Line Items]
Property, plant and equipment, net	25,535	27,797
Other
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 17,121	$ 14,899

Business Segment and Geograph_8

Business Segment and Geographical Information - Distribution of Depreciation and Amortization by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Depreciation and amortization	$ 65,916	$ 61,297	$ 59,575
Ireland
Segment Reporting Information [Line Items]
Depreciation and amortization	34,721	26,277	25,766
Rest of Europe
Segment Reporting Information [Line Items]
Depreciation and amortization	5,331	6,857	6,914
U.S.
Segment Reporting Information [Line Items]
Depreciation and amortization	21,605	24,246	23,462
Other
Segment Reporting Information [Line Items]
Depreciation and amortization	$ 4,259	$ 3,917	$ 3,433

Business Segment and Geograph_9

Business Segment and Geographical Information - Distribution of Total Assets by Geographical Area (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 2,354,255	$ 2,146,618
Ireland
Segment Reporting Information [Line Items]
Assets	1,073,411	880,378
Rest of Europe
Segment Reporting Information [Line Items]
Assets	514,010	504,418
U.S.
Segment Reporting Information [Line Items]
Assets	646,512	650,681
Other
Segment Reporting Information [Line Items]
Assets	$ 120,322	$ 111,141

Business Segment and Geograp_10

Business Segment and Geographical Information - Distribution of Capital Expenditure by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Capital expenditures	$ 48,397	$ 44,717	$ 42,601
Ireland
Segment Reporting Information [Line Items]
Capital expenditures	30,942	24,468	27,670
Rest of Europe
Segment Reporting Information [Line Items]
Capital expenditures	2,590	2,819	2,851
U.S.
Segment Reporting Information [Line Items]
Capital expenditures	9,311	11,027	8,432
Other
Segment Reporting Information [Line Items]
Capital expenditures	$ 5,554	$ 6,403	$ 3,648

Business Segment and Geograp_11

Business Segment and Geographical Information - Clients Representing Company's Net Revenue (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Customer Concentration Risk | Net Revenue | Client A
Revenue, Major Customer [Line Items]
Clients which represented 10% or more of the company's net revenue	14.00%	21.00%	26.00%

Business Segment and Geograp_12

Business Segment and Geographical Information - Distribution of Interest Income by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Interest income	$ 4,759	$ 2,346	$ 1,484
Ireland
Segment Reporting Information [Line Items]
Interest income	2,620	1,084	407
Rest of Europe
Segment Reporting Information [Line Items]
Interest income	1,750	1,222	1,040
U.S.
Segment Reporting Information [Line Items]
Interest income	17	16	2
Other
Segment Reporting Information [Line Items]
Interest income	$ 372	$ 24	$ 35

Business Segment and Geograp_13

Business Segment and Geographical Information - Distribution of Income Tax Charge by Geographical Area (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Income tax charge	$ 41,958	$ 46,569	$ 37,993
Ireland
Segment Reporting Information [Line Items]
Income tax charge	29,096	20,345	24,215
Rest of Europe
Segment Reporting Information [Line Items]
Income tax charge	(434)	1,921	5,528
U.S.
Segment Reporting Information [Line Items]
Income tax charge	3,761	14,772	8,381
Other
Segment Reporting Information [Line Items]
Income tax charge	$ 9,535	$ 9,531	$ (131)

Supplemental Disclosure of Ca_3

Supplemental Disclosure of Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 13,060	$ 13,094	$ 13,615
Cash paid for income taxes	$ 18,558	$ 12,305	$ 10,205

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation adjustments	$ (62,710)	$ (36,188)
Currency impact on long term funding	(5,016)	(182)
Actuarial loss on defined benefit pension plan (note 9)	(3,000)	(5,855)
Unrealized capital loss – investments (note 3)	(450)	(295)
Total	(69,328)	(38,713)
Interest Rate Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Hedges	4,658	4,658
Amortization of interest rate hedge	(2,810)	(1,887)
Foreign Exchange Forward
Accumulated Other Comprehensive Income (Loss) [Line Items]
Hedges	$ 0	$ 1,036

Long-Term Debt - Senior Notes -

Long-Term Debt - Senior Notes - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long term debt			$ 349,264,000	$ 348,888,000
Senior Notes
Debt Instrument [Line Items]
Long term debt	$ 350,000,000
Stated interest rate	3.64%		3.64%
Debt instrument, term	5 years		5 years
Cash proceeds from interest rate hedge		$ 4,600,000

Impact of New Accounting Pron_2

Impact of New Accounting Pronouncements (Details) - Accounting Standards Update 2016-02 - Scenario, Forecast $ in Millions	Dec. 31, 2019USD ($)
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset	$ 100
Operating lease, liability	100
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset	110
Operating lease, liability	$ 110

Related Parties (Details)

Related Parties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2018EUR (€)	Dec. 31, 2017USD ($)	Nov. 01, 2017USD ($)
Subsidiaries | DS Biopharma Limited
Related Party Transaction [Line Items]
Revenue earned from related parties	$ 633		$ 743
Amounts due from related parties	338
Mr. Ciaran Murray
Related Party Transaction [Line Items]
Expenses incurred from related party transactions			$ 178
Fair value of asset transferred to related party				$ 77
Mr. Thomas Lynch
Related Party Transaction [Line Items]
Related party transaction amounts | €		€ 231,750
Amounts due to related parties	$ 64

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event - USD ($) shares in Millions, $ in Millions	Jan. 25, 2019	Jan. 08, 2019
Subsequent Event [Line Items]
Number of shares authorized to be repurchased (in shares)		1
MolecularMD Corp.
Subsequent Event [Line Items]
Percentage of equity share capital acquired	100.00%
Consideration transferred	$ 42

Impact of Change in Accountin_3

Impact of Change in Accounting Policies - Additional Information (Details) $ in Millions	Dec. 31, 2018USD ($)
Revenue from Contract with Customer [Abstract]
Commission costs	$ 12
Deferred tax asset, cumulative impact of non-tax adjustments from adoption of ASC 606	$ 6.9

Impact of Change in Accountin_4

Impact of Change in Accounting Policies - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 395,851	$ 282,859	$ 192,541	$ 103,911
Available for sale investments	59,910	77,589	68,046
Accounts receivable, net (Note 17)	414,791	379,501
Unbilled revenue	362,926	268,509
Unbilled revenue	362,926	268,509
Other receivables	40,459	33,798
Prepayments and other current assets	36,801	34,377
Income taxes receivable	19,445	24,385
Total current assets	1,330,183	1,101,018
Other Assets:
Property, plant and equipment, net	158,669	163,051
Goodwill	756,260	769,058	616,088
Other non-current assets	14,525	15,393
Non-current income taxes receivable	20,023	18,396
Non-current deferred tax asset	13,577	8,074
Investments in equity- long term (Note 3b)	6,963	0
Intangible assets	54,055	71,628
Total Assets	2,354,255	2,146,618
Current Liabilities:
Accounts payable	13,288	18,590
Payments on account (Note 17)	274,468	298,992
Other liabilities	317,143	233,503
Income taxes payable	5,724	14,973
Total current liabilities	610,623	566,058
Other Liabilities:
Non-current bank credit lines and loan facilities	349,264	348,888
Non-current other liabilities	13,446	17,111
Non-current government grants	877	966
Non-current income taxes payable	17,551	14,879
Non-current deferred tax liability	8,213	7,716
Commitments and contingencies	0	0
Total Liabilities	999,974	955,618
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, (Note 12), 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017.	4,658	4,664
Additional paid-in capital	529,642	481,337
Other undenominated capital	983	912
Accumulated other comprehensive income	(69,328)	(38,713)
Retained earnings	888,326	742,800
Total Shareholders' Equity	1,354,281	1,191,000	$ 945,174	$ 763,096
Total Liabilities and Shareholders' Equity	2,354,255	2,146,618
Adjustments | Accounting Standards Update 2014-09
Current Assets:
Cash and cash equivalents	0	0
Available for sale investments	0
Accounts receivable, net (Note 17)	0
Unbilled revenue	46,642
Other receivables	(12,580)
Prepayments and other current assets	0
Income taxes receivable	0
Total current assets	34,062
Other Assets:
Property, plant and equipment, net	0
Goodwill	0
Other non-current assets	0
Non-current income taxes receivable	0
Non-current deferred tax asset	(5,680)
Investments in equity- long term (Note 3b)	0
Intangible assets	0
Total Assets	28,382
Current Liabilities:
Accounts payable	0
Payments on account (Note 17)	59,242
Other liabilities	(84,342)
Income taxes payable	1,669
Total current liabilities	(23,431)
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	(23,431)
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, (Note 12), 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017.	0
Additional paid-in capital	0
Other undenominated capital	0
Accumulated other comprehensive income	0
Retained earnings	51,813
Total Shareholders' Equity	51,813
Total Liabilities and Shareholders' Equity	28,382
Balance without adoption of Topic 606
Current Assets:
Cash and cash equivalents	395,851	$ 282,859
Available for sale investments	59,910
Accounts receivable, net (Note 17)	414,791
Unbilled revenue	409,568
Other receivables	27,879
Prepayments and other current assets	36,801
Income taxes receivable	19,445
Total current assets	1,364,245
Other Assets:
Property, plant and equipment, net	158,669
Goodwill	756,260
Other non-current assets	14,525
Non-current income taxes receivable	20,023
Non-current deferred tax asset	7,897
Investments in equity- long term (Note 3b)	6,963
Intangible assets	54,055
Total Assets	2,382,637
Current Liabilities:
Accounts payable	13,288
Payments on account (Note 17)	333,710
Other liabilities	232,801
Income taxes payable	7,393
Total current liabilities	587,192
Other Liabilities:
Non-current bank credit lines and loan facilities	349,264
Non-current other liabilities	13,446
Non-current government grants	877
Non-current income taxes payable	17,551
Non-current deferred tax liability	8,213
Commitments and contingencies	0
Total Liabilities	976,543
Shareholders' Equity:
Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, (Note 12), 53,971,706 shares issued and outstanding at December 31, 2018 and 54,081,601 shares issued and outstanding at December 31, 2017.	4,658
Additional paid-in capital	529,642
Other undenominated capital	983
Accumulated other comprehensive income	(69,328)
Retained earnings	940,139
Total Shareholders' Equity	1,406,094
Total Liabilities and Shareholders' Equity	$ 2,382,637

Impact of Change in Accountin_5

Impact of Change in Accounting Policies - Balance Sheet - Phantom (Details) - € / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Ordinary shares, par value (in EUR per share)	€ 0.06	€ 0.06
Ordinary shares, shares authorized (in shares)	100,000,000	100,000,000
Ordinary shares, shares issued (in shares)	53,971,706	54,081,601
Ordinary shares, shares outstanding (in shares)	53,971,706	54,081,601

Impact of Change in Accountin_6

Impact of Change in Accounting Policies - Statement of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Gross revenue	$ 2,595,777	$ 2,402,321	$ 2,364,956
Reimbursable expenses		(643,882)	(698,469)
Total revenue	2,595,777	1,758,439	1,666,487
Costs and expenses:
Selling, general and administrative	325,794	323,741	325,726
Depreciation and amortization	65,916	61,297	59,575
Restructuring charges	12,490	7,753	8,159
Total costs and expenses	2,222,420	1,420,101	1,354,793
Income from operations	373,357	338,338	311,694
Interest income	4,759	2,346	1,484
Interest expense	(13,502)	(12,627)	(13,006)
Income before income taxes expense	364,614	328,057	300,172
Income tax expense (Note 13)	(41,958)	(46,569)	(37,993)
Net income	322,656	$ 281,488	$ 262,179
Adjustments
Costs and expenses:
Net income	3,709
Adjustments | Accounting Standards Update 2014-09
Revenue:
Gross revenue	4,657
Reimbursable expenses	(702,812)
Total revenue	(698,155)
Costs and expenses:
Direct costs	(702,812)
Selling, general and administrative	472
Depreciation and amortization	0
Restructuring charges	0
Total costs and expenses	(702,340)
Income from operations	4,185
Interest income	0
Interest expense	0
Income before income taxes expense	4,185
Income tax expense (Note 13)	(476)
Net income	3,709
Balance without adoption of Topic 606
Revenue:
Gross revenue	2,600,434
Reimbursable expenses	(702,812)
Total revenue	1,897,622
Costs and expenses:
Direct costs	1,115,408
Selling, general and administrative	326,266
Depreciation and amortization	65,916
Restructuring charges	12,490
Total costs and expenses	1,520,080
Income from operations	377,542
Interest income	4,759
Interest expense	(13,502)
Income before income taxes expense	368,799
Income tax expense (Note 13)	(42,434)
Net income	$ 326,365

Impact of Change in Accountin_7

Impact of Change in Accounting Policies - Statement of Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	54,081,601
Beginning balance	$ 1,191,000	$ 945,174	$ 763,096
Comprehensive Income:
Net income	322,656	281,488	262,179
Currency translation adjustment	(26,522)	33,966	(12,839)
Currency impact of long-term funding	(4,834)	13,730	(8,428)
Unrealized capital loss - investments	(155)	(272)	11
Actuarial gain/(loss) on defined benefit pension plan	2,855	50	(2,485)
Amortization of interest rate hedge	(923)	(923)	(923)
Fair value of cash flow hedge	(1,036)	1,036	0
Total comprehensive income	292,041	329,075	237,515
Exercise of share options	16,806	13,906	10,139
Issue of restricted share units / performance share units	36	44	41
Share based compensation expense	31,544	29,351	40,343
Share issue costs	(16)	(15)	(17)
Repurchase of ordinary shares	(128,960)	(133,106)	(110,000)
Share repurchase costs	$ (66)	$ (106)	(275)
Ending balance (in shares)	53,971,706	54,081,601
Ending balance	$ 1,354,281	$ 1,191,000	$ 945,174
Ordinary Shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	54,081,601	54,530,843	54,958,912
Beginning balance	$ 4,664	$ 4,692	$ 4,719
Comprehensive Income:
Exercise of share options (in shares)	408,699	458,243	393,240
Exercise of share options	$ 29	$ 31	$ 26
Issue of restricted share units / performance share units (in shares)	489,568	681,742	607,878
Issue of restricted share units / performance share units	$ 36	$ 44	$ 41
Repurchase of ordinary shares (in shares)	(1,008,162)	(1,589,227)	(1,429,187)
Repurchase of ordinary shares	$ (71)	$ (103)	$ (94)
Ending balance (in shares)	53,971,706	54,081,601	54,530,843
Ending balance	$ 4,658	$ 4,664	$ 4,692
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	481,337	438,126	383,355
Comprehensive Income:
Exercise of share options	16,777	13,875	10,113
Share based compensation expense	31,544	29,351	40,343
Share issue costs	(16)	(15)	(17)
Ending balance	529,642	481,337	438,126
Other Undenominated Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	912	809	715
Comprehensive Income:
Repurchase of ordinary shares	71	103	94
Ending balance	983	912	809
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(38,713)	(86,300)	(61,636)
Comprehensive Income:
Currency translation adjustment	(26,522)	33,966	(12,839)
Currency impact of long-term funding	(4,834)	13,730	(8,428)
Unrealized capital loss - investments	(155)	(272)	11
Actuarial gain/(loss) on defined benefit pension plan	2,855	50	(2,485)
Amortization of interest rate hedge	(923)	(923)	(923)
Fair value of cash flow hedge	(1,036)	1,036
Ending balance	(69,328)	(38,713)	(86,300)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	742,800	587,847	435,943
Comprehensive Income:
Net income	322,656	281,488	262,179
Repurchase of ordinary shares	(128,960)	(133,106)	(110,000)
Share repurchase costs	(66)	(106)	(275)
Ending balance	888,326	$ 742,800	$ 587,847
Adjustments
Comprehensive Income:
Net income	3,709
Adjustments | Retained Earnings
Comprehensive Income:
Net income	3,709
Balance without adoption of Topic 606
Comprehensive Income:
Net income	326,365
Total comprehensive income	295,750
Ending balance	$ 1,406,094
Balance without adoption of Topic 606 | Ordinary Shares
Comprehensive Income:
Ending balance (in shares)	53,971,706
Ending balance	$ 4,658
Balance without adoption of Topic 606 | Additional Paid-in Capital
Comprehensive Income:
Ending balance	529,642
Balance without adoption of Topic 606 | Other Undenominated Capital
Comprehensive Income:
Ending balance	983
Balance without adoption of Topic 606 | Accumulated Other Comprehensive Income
Comprehensive Income:
Ending balance	(69,328)
Balance without adoption of Topic 606 | Retained Earnings
Comprehensive Income:
Ending balance	$ 940,139

Impact of Change in Accountin_8

Impact of Change in Accounting Policies - Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 322,656	$ 281,488	$ 262,179
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	70	228	151
Depreciation expense	50,565	43,436	42,125
Amortization of intangibles	15,351	17,861	17,450
Amortization of government grants	(47)	(44)	(44)
Interest on short term investments	(1,329)	(1,088)	(823)
Realized (gain)/loss on sale of short term investments	(56)	(112)	(50)
Gain on re-measurement of financial assets	(800)	0	0
Amortization of gain on interest rate hedge	(923)	(923)	(923)
Amortization of financing costs	812	556	566
Stock compensation expense	31,594	30,573	40,343
Deferred tax expense	1,652	10,729	1,545
Changes in assets and liabilities:
Increase in accounts receivable	(37,557)	57,747	2,526
Increase in unbilled revenue	(98,510)	(62,491)	(16,753)
Decrease in other receivables	3,107	1,771	(1,829)
Increase in prepayments and other current assets	(3,237)	4,359	1,872
Decrease in other non-current assets	856	(1,524)	(2,157)
Decrease in payments on account	(6,253)	(7,174)	(45,754)
Increase in other current liabilities	2,009	6,679	(44,713)
Decrease in other non-current liabilities	(1,034)	(3,710)	3,008
Decrease in income taxes payable	(5,220)	(2,293)	(690)
Decrease in accounts payable	(5,067)	7,014	1,175
Net cash provided by operating activities	268,639	383,082	259,204
Cash flows from investing activities:
Purchase of property, plant and equipment	(48,397)	(44,717)	(42,601)
Purchase of subsidiary undertakings	(1,645)	(144,131)	(54,209)
Sale of available for sale investments	99,865	33,086	40,858
Purchase of available for sale investments	(80,956)	(41,701)	(22,030)
Purchase of investments in equity - long term	(6,163)	0	0
Net cash used in investing activities	(37,296)	(177,814)	(74,814)
Cash flows from financing activities:
Financing costs	(823)	0	0
Proceeds from the exercise of equity compensation	16,842	13,950	10,180
Share issue costs	(16)	(15)	(17)
Repurchase of ordinary shares	(128,960)	(133,106)	(110,000)
Share repurchase costs	(66)	(106)	(275)
Net cash used in financing activities	(113,023)	(119,277)	(93,710)
Effect of exchange rate movements on cash	(5,328)	4,327	(2,050)
Net increase in cash and cash equivalents	112,992	90,318	88,630
Cash and cash equivalents at beginning of year	282,859	192,541	103,911
Cash and cash equivalents at end of year	395,851	282,859	$ 192,541
Adjustments
Cash flows from operating activities:
Net income	3,709
Adjustments | Accounting Standards Update 2014-09
Cash flows from operating activities:
Net income	3,709
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
Realized (gain)/loss on sale of short term investments	0
Gain on re-measurement of financial assets	0
Amortization of gain on interest rate hedge	0
Amortization of financing costs	0
Stock compensation expense	0
Deferred tax expense	5,680
Changes in assets and liabilities:
Increase in accounts receivable	0
Increase in unbilled revenue	1,462
Decrease in other receivables	12,580
Increase in prepayments and other current assets	0
Decrease in other non-current assets	0
Decrease in payments on account	59,242
Increase in other current liabilities	(84,342)
Decrease in other non-current liabilities	0
Decrease in income taxes payable	1,669
Decrease in accounts payable	0
Net cash provided by operating activities	0
Cash flows from investing activities:
Purchase of property, plant and equipment	0
Purchase of subsidiary undertakings	0
Sale of available for sale investments	0
Purchase of available for sale investments	0
Purchase of investments in equity - long term	0
Net cash used in investing activities	0
Cash flows from financing activities:
Financing costs	0
Proceeds from the 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	0
Net increase in cash and cash equivalents	0
Cash and cash equivalents at beginning of year	0
Cash and cash equivalents at end of year	0	0
Balance without adoption of Topic 606
Cash flows from operating activities:
Net income	326,365
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on disposal of property, plant and equipment	70
Depreciation expense	50,565
Amortization of intangibles	15,351
Amortization of government grants	(47)
Interest on short term investments	(1,329)
Realized (gain)/loss on sale of short term investments	(56)
Gain on re-measurement of financial assets	(800)
Amortization of gain on interest rate hedge	(923)
Amortization of financing costs	812
Stock compensation expense	31,594
Deferred tax expense	7,332
Changes in assets and liabilities:
Increase in accounts receivable	(37,557)
Increase in unbilled revenue	(97,048)
Decrease in other receivables	15,687
Increase in prepayments and other current assets	(3,237)
Decrease in other non-current assets	856
Decrease in payments on account	52,989
Increase in other current liabilities	(82,333)
Decrease in other non-current liabilities	(1,034)
Decrease in income taxes payable	(3,551)
Decrease in accounts payable	(5,067)
Net cash provided by operating activities	268,639
Cash flows from investing activities:
Purchase of property, plant and equipment	(48,397)
Purchase of subsidiary undertakings	(1,645)
Sale of available for sale investments	99,865
Purchase of available for sale investments	(80,956)
Purchase of investments in equity - long term	(6,163)
Net cash used in investing activities	(37,296)
Cash flows from financing activities:
Financing costs	(823)
Proceeds from the exercise of equity compensation	16,842
Share issue costs	(16)
Repurchase of ordinary shares	(128,960)
Share repurchase costs	(66)
Net cash used in financing activities	(113,023)
Effect of exchange rate movements on cash	(5,328)
Net increase in cash and cash equivalents	112,992
Cash and cash equivalents at beginning of year	282,859
Cash and cash equivalents at end of year	$ 395,851	$ 282,859

Uncategorized Items - iclr-2018

Label	Element	Value
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 1,142,896,000
AOCI Attributable to Parent [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	(38,713,000)
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 694,696,000
Common Stock [Member]
Common Stock, Shares, Issued	us-gaap_CommonStockSharesIssued	54,081,601
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 4,664,000
Additional Paid-in Capital [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	481,337,000
Other Undenominated Capital [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	912,000
Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	(48,104,000)
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (48,104,000)