Accounting Changes and Error Corrections [Abstract] Impact of New Accounting Pronouncements New And Recent Accounting Pronouncements Disclosure [Text Block] New And Recent Accounting Pronouncements Disclosure Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Geographical Geographical [Axis] Geographical [Domain] Geographical [Domain] Ireland IRELAND Rest of Europe Rest Of Europe [Member] Rest Of Europe [Member] U.S. UNITED STATES Other Other Countries [Member] Other Countries [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Assets Assets Investments, Debt and Equity Securities [Abstract] Cost and Fair Value of Investments Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Available For Sale Short Term Investments by Major Security Type Available-for-sale Securities [Table Text Block] Accounting Policies [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Direct Cost, Reimbursable Expenses Direct Cost, Reimbursable Expenses [Member] Direct Cost, Reimbursable Expenses [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Senior Notes Senior Notes [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Direct costs, reimbursable expenses Cost of Goods and Services Sold Advertising and promotion costs Marketing and Advertising Expense Long term debt Long-term Line of Credit, Noncurrent Goodwill impairment charge Goodwill, Impairment Loss Carrying value of inventory Inventory, Net Debt instrument, interest rate Debt Instrument, Interest Rate, Stated Percentage Debt instrument, term Debt Instrument, Term Cash proceeds from interest rate hedge Proceeds from Hedge, Financing Activities Foreign Currency [Abstract] Amounts (credited)/ charged Foreign Currency Transaction Gain (Loss), before Tax Property, plant and equipment, net Property, Plant and Equipment, Net Income Statement [Abstract] Statement [Table] Statement [Table] Direct Costs Direct Costs [Member] Direct Costs [Member] Statement [Line Items] Statement [Line Items] Revenue: Revenues [Abstract] Gross revenue Revenue from Contract with Customer, Excluding Assessed Tax, Gross Revenue from Contract with Customer, Excluding Assessed Tax, Gross Reimbursable expenses Revenue from Contract with Customer, Excluding Assessed Tax, Reimbursable Expenses Revenue from Contract with Customer, Excluding Assessed Tax, Reimbursable Expenses Total revenue Revenue from Contract with Customer, Excluding Assessed Tax Costs and expenses: Costs and Expenses [Abstract] Direct costs Selling, general and administrative Selling, General and Administrative Expense Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Restructuring (Note 14) Restructuring Charges Total costs and expenses Costs and Expenses Income from operations Operating Income (Loss) Interest income Investment Income, Interest Interest expense Interest Expense, Debt Income before income taxes expense Income (Loss) Attributable to Parent, before Tax Income tax expense (Note 13) Income Tax Expense (Benefit) Net income Net Income (Loss) Attributable to Parent Net income per ordinary share: Earnings Per Share [Abstract] Basic (dollars per share) Earnings Per Share, Basic Diluted (dollars per share) Earnings Per Share, Diluted Weighted average number of ordinary shares outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Debt Disclosure [Abstract] Long-Term Debt - Senior Notes Long-term Debt [Text Block] Commitments and Contingencies Disclosure [Abstract] Non-cancelable operating leases for facilities expiration period Lessee, Operating Lease, Term of Contract Operating leases rental expense Operating Leases, Rent Expense, Net Retirement Benefits [Abstract] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Domain] Retirement Plan Sponsor Location [Domain] United Kingdom UNITED KINGDOM Switzerland SWITZERLAND Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Expected rate of return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Other comprehensive income Other Comprehensive Income (Loss), Net of Tax [Abstract] Actuarial (gain)/loss - benefit obligation Defined Benefit Plan, Actuarial Losses to be Amortized and Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Pension Cost Defined Benefit Plan, Actuarial Losses to be Amortized and Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Pension Cost Actuarial loss/(gain) – plan assets Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Prior service credit recognized in net periodic benefit cost Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Actuarial gain recognized in net periodic benefit cost Other Comprehensive Income, Amortization of Net Actuarial Gain (Loss), Net of Tax Other Comprehensive Income, Amortization of Net Actuarial Gain (Loss), Net of Tax Amortization of net prior service credit Other Comprehensive Income Amortization of Net Period Service Credit Net of Tax Other Comprehensive Income Amortization of Net Period Service Credit Net of Tax Net prior service cost occurring during the year Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax Stated interest rate Retirement Plan Name [Axis] Retirement Plan Name [Axis] Retirement Plan Name [Domain] Retirement Plan Name [Domain] The Plan Defined Benefit Plan Contributions [Member] Defined Benefit Plan Contributions [Member] U.S. Plan US 401K Plan [Member] US 401K Plan [Member] Defined Benefit Plan, Asset Categories [Axis] Defined Benefit Plan, Asset Categories [Axis] Plan Asset Categories [Domain] Plan Asset Categories [Domain] iboxx Corporate Bonds Iboxx Corporate Bonds [Member] iboxx Corporate Bonds Gilts United Kingdom Treasury Bond Securities [Member] United Kingdom Treasury Bond Securities [Member] Corporate Bonds Corporate Bonds [Member] Corporate Bonds [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Aptiv Solutions Aptiv Solutions [Member] Aptiv Solutions [Member] Contributions percentage of annual compensation Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Employer contributions Defined Contribution Plan, Employer Discretionary Contribution Amount Participant's contributions percentage matches Defined Contribution Plan, Employer Matching Contribution, Percent of Match Participant's contributions percentage of annual compensation Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent Deferred salary arrangement employer contribution Deferred Compensation Arrangement with Individual, Contributions by Employer 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 Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year 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 Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year Defined benefit plan bond maturity period Defined Benefit Plan High Quality Bond Maturity Maximum Range Used To Determine Yield Curve Period Defined Benefit Plan High Quality Bond Maturity Maximum Range Used To Determine Yield Curve Period Basis point increase in benefits obligation discount rate Basis Point Increase In Pension And Postretirement Benefits Discount Rate Basis Point Increase In Pension And Postretirement Benefits Discount Rate Retirement period used as a basis to estimate expected cash flows Expected Retirement period Expected Retirement period Percentage of investments Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Expected long-term rate of return after application of a market value reduction factor (per annum) Defined Benefit Plan, Expected Long Term Return on Assets after Application of Market Value Reduction Factor Defined Benefit Plan, Expected Long Term Return on Assets after Application of Market Value Reduction Factor Contribution to pension fund in the year ending December 31, 2018 Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year Percentage of common stock acquired Business Acquisition, Percentage of Voting Interests Acquired Revenue from Contract with Customer [Abstract] Schedule of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Basis of Consolidation Consolidation, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Revenue Recognition Revenue From Contract With Customer [Policy Text Block] Revenue From Contract With Customer [Policy Text Block] Direct Costs Compensation Related Costs, Policy [Policy Text Block] Advertising Costs Advertising Costs, Policy [Policy Text Block] Foreign Currencies and Translation of Subsidiaries Foreign Currency Transactions and Translations Policy [Policy Text Block] Disclosure of Fair Value of Financial Instruments Fair Value of Financial Instruments, Policy [Policy Text Block] Business Combinations Business Combinations Policy [Policy Text Block] Goodwill and Impairment Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Intangible Assets Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Investment in Debt, Equity and Other Investment, Policy [Policy Text Block] Accounts Receivable, net Receivables, Policy [Policy Text Block] Inventory Inventory, Policy [Policy Text Block] Property, Plant and Equipment Property, Plant and Equipment, Policy [Policy Text Block] Leased Assets Lessee, Leases [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Government Grants Government Grants [Policy Text Block] Government Grants, Policy Research and Development Credits Research and Development Credits [Policy Text Block] Research and Development Credits [Policy Text Block] Pension Costs Pension and Other Postretirement Plans, Policy [Policy Text Block] Net Income per Ordinary Share Earnings Per Share, Policy [Policy Text Block] Share-Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Derivative Financial Instruments Derivatives, Policy [Policy Text Block] Financing Costs and Gain on Interest Rate Hedge Derivatives, Methods of Accounting, Hedging Derivatives [Policy Text Block] Reclassifications Reclassifications [Policy Text Block] Reclassifications [Policy Text Block] Impact of New Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Property, Plant and Equipment [Abstract] Schedule of Property Plant and Equipment Net Property, Plant and Equipment [Table Text Block] Short term investments average maturity period Short Term Available For Sale Investments Average Remaining Maturity Period Short Term Available For Sale Investments Average Remaining Maturity Period Purchase of investments in equity - long term Payments to Acquire Long-term Investments Increase in fair value of investments in equity Equity Securities, FV-NI, Gain (Loss) Equity Securities, FV-NI, Gain (Loss) Carrying value of long-term investments in equity Equity Securities, FV-NI Equity Securities, FV-NI Commitment to acquire future long-term investments Commitment To Acquire Long-Term Investments Commitment To Acquire Long-Term Investments Equity [Abstract] Share Capital Stockholders' Equity Note Disclosure [Text Block] Income Tax Disclosure [Abstract] Reconciliation of Provision of Income Taxes [Table] Reconciliation of Provision of Income Taxes [Table] Reconciliation of Provision of Income Taxes [Table] United States Other Rest Of Europe And Other Countries [Member] Rest Of Europe And Other Countries [Member] Reconciliation of Provision of Income Taxes [Line Items] Reconciliation of Provision of Income Taxes [Line Items] Reconciliation of Provision of Income Taxes [Line Items] Provision for income taxes: Income Tax Expense (Benefit), Continuing Operations [Abstract] Current tax expense: Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Current tax expense Current Income Tax Expense (Benefit) Deferred tax expense/(benefit): Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Deferred tax expense/(benefit) Deferred Income Tax Expense (Benefit) Provision for income taxes Impact on shareholders equity and other comprehensive income of the tax consequence of : Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net [Abstract] Excess tax benefit on stock compensation Income Tax Effects Allocated Directly to Equity, Employee Stock Options Currency impact on long term funding Other Comprehensive Income (Loss), Foreign Currency Translation Gain (Loss) Arising During Period, Tax Fair value of cash flow hedge Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax Total Income Tax Effects Allocated Directly to Equity Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Building Building [Member] Computer equipment and software Computer Equipment and Capitalized Software [Member] Computer Equipment and Capitalized Software [Member] Office furniture and fixtures Furniture and Fixtures [Member] Laboratory equipment Equipment [Member] Motor vehicles Vehicles [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Estimated useful lives Property, Plant and Equipment, Useful Life Supplemental Cash Flow Elements [Abstract] Supplemental Disclosure of Cash Flow Information Cash Flow, Supplemental Disclosures [Text Block] Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Includes Financial Instruments, Property Occupied by, or Other Assets Other Plan Asset Categories [Member] Other Plan Asset Categories [Member] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of net (gain)/loss Defined Benefit Plan, Amortization of Gain (Loss) Expenses Define Benefit Plan Expenses Define Benefit Plan Expenses Amortization of prior service credit Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Settlement Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement Curtailment Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Selling, general and administrative Selling, General and Administrative Expenses [Member] Mapi Development SAS Mapi Development SAS [Member] Mapi Development SAS [Member] Mapi Group Mapi Group [Member] Mapi group. ICON Government & Public Health Solutions ICON Government & Public Health Solutions [Member] ICON Government & Public Health Solutions [Member] PMG PMG Research Incorporated [Member] PMG Research Incorporated [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Goodwill Goodwill Consideration transferred Business Combination, Consideration Transferred Working capital adjustment Business Acquisition Purchase Price Allocation To Working Capital Increase (Decrease) Business Acquisition Purchase Price Allocation To Working Capital Increase (Decrease) Net cash outflows Payments to Acquire Businesses, Net of Cash Acquired Other liabilities assumed Business Combination, Consideration Transferred, Liabilities Incurred Maximum additional consideration that might be payable Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Assessment of valuation of contingent consideration at acquisition Business Combination, Contingent Consideration, Liability Decrease in amount of contingent consideration, liability Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Number of clinical research sites acquired Number of Clinical Research Sites Acquired Number of Clinical Research Sites Acquired Number of active patients via electronic medical records acquired (in excess of) Number of Patients Number of Patients Schedule of Pension Costs Schedule of Defined Benefit Plans Disclosures [Table Text Block] Schedule of Funded Status Schedule of Net Funded Status [Table Text Block] Schedule of Components of Net Periodic Benefit Cost Schedule of Net Benefit Costs [Table Text Block] Summary of Assumptions Used in Calculating Net Periodic Benefit Cost Schedule of Assumptions Used in Calculating Net Periodic Benefit Cost [Table Text Block] Schedule of Assumptions Used in Calculating Net Periodic Benefit Cost [Table Text Block] Summary of Amounts Recognized in Accumulated Other Comprehensive Income Which Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] Summary of Assumptions Used in Calculating Pension Benefit Obligations Schedule of Assumptions Used [Table Text Block] Summary of Expected Long Term Rates of Return on Different Asset Classes Expected Long Term Rate of Return on Plan Assets [Table Text Block] Expected Long Term Rate of Return on Plan Assets Schedule of Underlying Asset Split of Fund Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets [Table Text Block] Schedule of Plan Asset Fair Value Measurements Schedule of Allocation of Plan Assets [Table Text Block] Schedule of Annual Benefit Payments which Reflect Expected Future Service Schedule of Expected Benefit Payments [Table Text Block] Statement of Comprehensive Income [Abstract] Net income Other comprehensive income, net of tax Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Currency impact of long-term funding Other Comprehensive Income (Loss), Foreign Currency Transaction And Translation Adjustment, Net Of Tax, Long-Term Funding Adjustment that results from the process of translating the impact of long-term funding into the functional currency of the reporting entity. Unrealized capital (loss)/gain– investments Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Actuarial gain/(loss) on defined benefit pension plan Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax Amortization of interest rate hedge Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Fair value of cash flow hedge Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax Total comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Schedule of Accumulated Other Comprehensive Income Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Order book Order or Production Backlog [Member] Customer lists Customer Lists [Member] Brand Brand [Member] Brand [Member] Cash Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Property, plant and equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Finite-lived intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Accounts receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Unbilled revenue Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Unbilled Revenue Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Unbilled Revenue Prepayments and other current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Other receivables Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Other Receivables Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Other Receivables Income taxes receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Income Taxes Receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Income Taxes Receivable Accounts payable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable Payments on account Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Contract With Customer, Liability, Current Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Contract With Customer, Liability, Current Other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Non-current other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Non-current deferred tax liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent Net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Cash outflows Payments to Acquire Businesses, Gross Total consideration Other liabilities adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Liabilities, Other Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Liabilities, Other Property and equipment adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment Accounts receivable adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Income taxes receivable adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Income Taxes Receivable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Income Taxes Receivable Unbilled revenue adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Unbilled Revenue Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Unbilled Revenue Prepayments and other current assets adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Prepaid Expense And Other Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Prepaid Expense And Other Assets Other receivables adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Other Receivables Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Current Asset, Other Receivables Payments on account adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Contract With Customer, Liability, Current Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Contract With Customer, Liability, Current Deferred tax liabilities adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Tax Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Tax Liabilities Accounts payable adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable Schedule of Components of Income Before Income Tax Expense (Benefit) [Table] Schedule of Components of Income Before Income Tax Expense (Benefit) [Table] Schedule of Components of Income Before Income Tax Expense (Benefit) [Table] Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items] Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items] Schedule of Components of Income Before Income Tax Expense (Benefit) [Line Items] Income before provision for income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Revenue, remaining performance obligation Revenue, Remaining Performance Obligation Revenue, remaining performance obligation, percent Revenue, Remaining Performance Obligation, Percent Revenue, Remaining Performance Obligation, Percent Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, remaining performance obligation, expected timing of satisfaction, period Revenue Remaining Performance Obligation Expected Timing Of Satisfaction Period1 Revenue Remaining Performance Obligation Expected Timing Of Satisfaction Period1 Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] MediMedia Pharma Solutions Medi Media Pharma Solutions [Member] Medi Media Pharma Solutions [Member] Clinical Trial Services of Cross Country Healthcare Inc. Clinical trial services [Member] Clinical trial services [Member] PriceSpective PriceSpective Llc [Member] PriceSpective Llc [Member] BeijingWits Medical BeijingWits Medical Limited [Member] BeijingWits Medical Limited [Member] Firecrest Clinical Limited Firecrest Clinical [Member] Firecrest Clinical [Member] Oxford Outcomes Limited Oxford Outcomes Limited [Member] Oxford Outcomes Limited [Member] Prevalere Life Sciences Prevalere Life Sciences [Member] Prevalere Life Sciences [Member] Healthcare Discoveries Healthcare Discoveries Inc [Member] Healthcare Discoveries Inc [Member] Customer relationships acquired Customer Relationships [Member] Order backlog Non-compete arrangements Noncompete Agreements [Member] Technology asset acquired Technology Assets [Member] Technology Assets [Member] Trade names acquired Trade Names [Member] Volunteer list acquired Volunteer List [Member] Volunteer List [Member] Indefinite-lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Value of amortizable intangible asset acquired Finite-lived Intangible Assets Acquired Amortization period Finite-Lived Intangible Asset, Useful Life Amount amortized since date of acquisition Finite-Lived Intangible Assets, Accumulated Amortization Statement of Financial Position [Abstract] Ordinary shares, par value (in EUR per share) Common Stock, Par or Stated Value Per Share Ordinary shares, shares authorized (in shares) Common Stock, Shares Authorized Ordinary shares, shares issued (in shares) Common Stock, Shares, Issued Ordinary shares, shares outstanding (in shares) Common Stock, Shares, Outstanding Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Employee Stock Option Employee Stock Option [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Options Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Beginning Balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares Ending Balance (in shares) Weighted Average Exercise Price Share Based Compensation Arrangement By Share Based Payment Award Nonvested Options Outstanding Exercise Price Abstract Share Based Compensation Arrangement By Share Based Payment Award Nonvested Options Outstanding Exercise Price Abstract Beginning balance (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Outstanding Weighted Average Exercise Price Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Outstanding Weighted Average Exercise Price Granted (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Granted in Period Weighted Average Exercise Price Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Granted in Period Weighted Average Exercise Price Vested (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Vested in Period Weighted Average Exercise Price Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Vested in Period Weighted Average Exercise Price Forfeited (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Forfeited in Period Weighted Average Exercise Price Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Forfeited in Period Weighted Average Exercise Price Ending balance (usd per share) Weighted Average Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Beginning Balance (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value Granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value Forfeited (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value Ending Balance (usd per share) Deferred Revenue Disclosure [Abstract] Government Grant Government Grants [Table Text Block] Government Grants Organization, Consolidation and Presentation of Financial Statements [Abstract] Description of Business Nature of Operations [Text Block] Schedule of Distribution of Net Revenue by Geographical Area Revenue from External Customers by Geographic Areas [Table Text Block] Schedule of Distribution of Income from Operations by Geographical Area Operating Income by Geography [Table Text Block] Operating Income by Geography. Schedule of Distribution of Property, Plant and Equipment, Net, by Geographical Area Long-lived Assets by Geographic Areas [Table Text Block] Schedule of Distribution of Depreciation and Amortization by Geographical Area Schedule Of Depreciation And Amortization Expense By Geographical Area [Table Text Block] Schedule Of Depreciation And Amortization Expense By Geographical Area. Schedule of Distribution of Total Assets by Geographical Area Reconciliation of Assets from Segment to Consolidated [Table Text Block] Schedule of Distribution of Capital Expenditures by Geographical Area Schedule Of Capital Expenditures by Geographic Area [Table Text Block] Schedule Of Capital Expenditures by Geographic Area Schedule of Clients Representing Company's Net Revenue Schedule of Revenue by Major Customers by Reporting Segments [Table Text Block] Schedule of Distribution of Interest Income by Business Segment Schedule Of Interest Incom by Segment [Table Text Block] Schedule Of Interest Incom by Segment Schedule of Distribution of Tax Charge by Geographical Area Schedule Of Income Tax Expense Benefit by Geographic Area [Table Text Block] Schedule Of Income Tax Expense Benefit by Geographic Area Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Fair value of plan assets at beginning of year Defined Benefit Plan, Fair Value of Plan Assets Expected return on plan assets Actual return on plan assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Scheme contributions Defined Benefit Plan, Scheme Contributions Defined Benefit Plan, Scheme Contributions Employer contributions Defined Benefit Plan, Plan Assets, Contributions by Employer Plan participants' contributions Defined Benefit Plan, Plan Assets, Contributions by Plan Participant Transferred (benefits paid)/balances Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan Benefits paid Defined Benefit Plan, Plan Assets, Benefits Paid Settlement Defined Benefit Plan, Plan Assets, Payment for Settlement Foreign currency exchange rate changes Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss) Fair value of plan assets at end of year Accounts Receivable, Net [Abstract] Provision for Doubtful Debts Allowance For Doubtful Accounts [Text Block] Allowance For Doubtful Accounts [Text Block] 2019 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year Two 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Four 2023 Defined Benefit Plan, Expected Future Benefit Payment, Year Five Years 2024 - 2028 Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter ASSETS Assets [Abstract] Current Assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Available for sale investments (Note 3a) Available-for-sale Securities, Debt Securities, Current Accounts receivable, net (Note 17) Accounts Receivable, Net, Current Unbilled revenue (Note 17) Contract with Customer, Asset, Net Other receivables Nontrade Receivables, Current Prepayments and other current assets Prepaid Expense and Other Assets, Current Income taxes receivable (Note 13) Income Taxes Receivable, Current Total current assets Assets, Current Other Assets: Assets, Noncurrent [Abstract] Property, plant and equipment, net (Note 6) Goodwill (Note 4) Other non-current assets Other Assets, Noncurrent Non-current income taxes receivable (Note 13) Income Taxes Receivable, Noncurrent Non-current deferred tax asset (Note 13) Deferred Income Tax Assets, Net Investments in equity- long term (Note 3b) Intangible assets (Note 5) Finite-Lived Intangible Assets, Net Total Assets LIABILITIES AND SHAREHOLDERS' EQUITY Liabilities and Equity [Abstract] Current Liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable, Current Payments on account (Note 17) Contract with Customer, Liability, Current Other liabilities (Note 7) Other Liabilities, Current Income taxes payable (Note 13) Accrued Income Taxes, Current Total current liabilities Liabilities, Current Other Liabilities: Liabilities, Noncurrent [Abstract] Non-current bank credit lines and loan facilities (Note 22) Non-current other liabilities (Note 8) Other Liabilities, Noncurrent Non-current government grants (Note 11) Capital Government Grants Noncurrent Noncurrent capital government grants, which are amortized and recognized in earnings over the grant period. Non-current income taxes payable (Note 13) Accrued Income Taxes, Noncurrent Non-current deferred tax liability (Note 13) Deferred Income Tax Liabilities, Net Commitments and contingencies (Note 15) Commitments and Contingencies Total Liabilities Liabilities Shareholders' Equity: Stockholders' Equity Attributable to Parent [Abstract] 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. Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Other undenominated capital (Note 12 (a)) Other Undenominated Capital Other undenominated capital. Accumulated other comprehensive income (Note 21) Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total Shareholders' Equity Stockholders' Equity Attributable to Parent Total Liabilities and Shareholders' Equity Liabilities and Equity Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Schedule of Supplemental Disclosure of Cash Flow Information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Trade names/ brands acquired Trade Name and Brands [Member] Trade Name and Brands [Member] Mapi intangible asset Mapi intangible asset [Member] Mapi intangible asset [Member] Acquired Finite-Lived Intangible Assets [Line Items] Acquired Finite-Lived Intangible Assets [Line Items] Cost Finite-Lived Intangible Assets, Net [Abstract] Total cost Finite-Lived Intangible Assets, Gross Foreign exchange movement Finite Lived Intangible Assets Foreign Exchange Movement Finite Lived Intangible Assets Foreign Exchange Movement Accumulated amortization Foreign exchange movement Finite Lived Intangible Assets Accumulated Amortizations Foreign Exchange Movement Finite Lived Intangible Assets Accumulated Amortizations Foreign Exchange Movement Net book value Land Land Building Buildings and Improvements, Gross Computer equipment and software Capitalized Computer Software, Gross Office furniture and fixtures Furniture and Fixtures, Gross Laboratory equipment Machinery and Equipment, Gross Leasehold improvements Leasehold Improvements, Gross Motor vehicles Vehicles, Gross Vehicles, Gross Property, plant and equipment, gross Property, Plant and Equipment, Gross Less accumulated depreciation and asset write offs Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Property, plant and equipment (net) Capital expenditures Payments to Acquire Property, Plant, and Equipment Schedule of Intangible Assets Schedule of Acquired Finite-Lived Intangible Assets by Major Class [Table Text Block] Schedule of Future Intangible Asset Amortization Expense Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Deferred tax liabilities: Deferred Tax Liabilities, Gross [Abstract] Property, plant and equipment Deferred Tax Liabilities, Property, Plant and Equipment Goodwill Deferred Tax Liabilities, Goodwill Other intangible assets Deferred Tax Liabilities, Other Finite-Lived Assets Other Deferred Tax Liabilities, Other Total deferred tax liabilities recognized Deferred Tax Liabilities, Net Deferred tax assets: Deferred Tax Assets, Net [Abstract] Operating loss and tax credits carry-forwards Deferred Tax Assets Operating Loss And Tax Credits Carryforwards Deferred Tax Assets Operating Loss And Tax Credits Carryforwards Property, plant and equipment Deferred Tax Assets, Property, Plant and Equipment Accrued expenses and payments on account Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities Stock compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Other Deferred compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits Deferred revenue Deferred Tax Asset, Contract With Customer, Liability Deferred Tax Asset, Contract With Customer, Liability Other Deferred Tax Assets, Tax Deferred Expense, Other Total deferred tax assets Deferred Tax Assets, Gross Valuation allowance for deferred tax assets Deferred Tax Assets, Valuation Allowance Deferred tax assets recognized Deferred Tax Assets, Net of Valuation Allowance Overall net deferred tax asset Deferred Tax Assets, Net Statement of Stockholders' Equity [Abstract] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Ordinary Shares Common Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Other Undenominated Capital Other Undenominated Capital [Member] Other undenominated capital. Accumulated Other Comprehensive Income AOCI Attributable to Parent [Member] Retained Earnings Retained Earnings [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2016-09 Accounting Standards Update 2016-09 [Member] Accounting Standards Update 2014-09 Accounting Standards Update 2014-09 [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning balance (in shares) Beginning balance Adjusted beginning balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Comprehensive Income: Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Unrealized capital loss - investments Exercise of share options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Exercise of share options Stock Issued During Period, Value, Stock Options Exercised Issue of restricted share units / performance share units (in shares) Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Issue of restricted share units / performance share units Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Share based compensation expense Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Share issue costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Repurchase of ordinary shares (in shares) Stock Repurchased and Retired During Period, Shares Repurchase of ordinary shares Stock Repurchased and Retired During Period, Value Share repurchase costs Adjustment to Retained Earning Share Repurchase costs Adjustment to Retained Earning Share Repurchase costs Excess income tax benefit on exercise of equity compensation Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net Cumulative effect adjustment from adoption of new accounting principal Cumulative Effect of New Accounting Principle in Period of Adoption Ending balance (in shares) Ending balance Reconciliation Of Debt Securities, Available-For-Sale [Roll Forward] Reconciliation Of Debt Securities, Available-For-Sale [Roll Forward] Reconciliation Of Debt Securities, Available-For-Sale [Roll Forward] At start of year Purchases Payments to Acquire Available-for-sale Securities, Debt Sales and maturities Proceeds from Sale of Available-for-sale Securities, Debt Interest on short term investments Interest On Short Term Investments Interest On Short Term Investments Realized gain on sale of short term investments Gain (Loss) on Sale of Investments At end of year Property, Plant and Equipment, Net Property, Plant and Equipment Disclosure [Text Block] Taxes at Irish statutory rate of 12.5% (2017:12.5%; 2016:12.5%) Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Foreign and other income taxed at higher rates Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount Research & development tax incentives Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount Movement in valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Effects of change in tax rates Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Increase/(decrease) in unrecognized tax benefits Unrecognized Tax Benefits, Period Increase (Decrease) Impact of stock compensation Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount Impact of deemed repatriation under US Tax Reform Tax Cuts And Jobs Act of 2017, Incomplete Accounting, Transition Tax For Accumulated Foreign Earnings, Provisional Income Tax Expense Tax Cuts And Jobs Act of 2017, Incomplete Accounting, Transition Tax For Accumulated Foreign Earnings, Provisional Income Tax Expense Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Schedule of New Accounting Pronouncements and Changes in Accounting Principles Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block] Class of Treasury Stock [Table] Class of Treasury Stock [Table] Share Repurchase Program Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] Buyback Program Buyback Program [Member] Buyback Program [Member] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Performance Share Unit (PSUs) Performance Share Unit [Member] Performance Share Unit [Member] Equity, Class of Treasury Stock [Line Items] Equity, Class of Treasury Stock [Line Items] Options exercised by employees (in shares) Average exercise price of option per share (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Total proceeds from exercise of stock options by employees Proceeds from Stock Options Exercised Ordinary shares issued in respect of certain RSUs previously awarded by the Company (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures Ordinary shares issued in respect of certain PSUs previously awarded by the Company (in shares) Stock Issued During Period, Shares, Share-based Compensation, Gross Stock repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Ordinary shares redeemed (in shares) Ordinary shares redeemed, value Income from operations, excluding restructuring and other items Operating Income Loss Before Restructuring Charges Operating Income Loss Before Restructuring Charges Schedule of Contracts with Customers, Asset and Liabilities Contract with Customer, Asset and Liability [Table Text Block] Allowance for Doubtful Accounts Receivable [Roll Forward] Allowance for Doubtful Accounts Receivable [Roll Forward] Opening provision Allowance for Doubtful Accounts Receivable Amounts used during the year Provision for Doubtful Accounts Amounts provided during the year Allowance for Doubtful Accounts Receivable, Recoveries Amounts released during the year Allowance for Doubtful Accounts Receivable, Write-offs Foreign exchange Allowance For Doubtful Accounts Foreign Currency Translation Adjustments Allowance For Doubtful Accounts Foreign Currency Translation Adjustments Closing provision Disaggregation of Revenue Revenue from Contract with Customer [Text Block] Ordinary shares, shares issued (in shares) Income from operations Restructuring and Related Activities [Abstract] Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Restructuring Plan [Axis] Restructuring Plan [Axis] Restructuring Plan [Domain] Restructuring Plan [Domain] Resource Rationalizations 2018 Resource Rationalizations 2018 [Member] Resource Rationalizations 2018 [Member] Restructuring Type [Axis] Restructuring Type [Axis] Type of Restructuring [Domain] Type of Restructuring [Domain] Workforce Reductions Employee Severance [Member] Onerous Lease Onerous Lease [Member] Onerous Lease [Member] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Restructuring charges Utilized Restructuring Reserve Utilizations Restructuring Reserve Utilizations Foreign exchange Restructuring Reserve, Foreign Currency Translation Gain (Loss) Restructuring Reserve Restructuring Reserve Resource Rationalizations 2017 Resource Rationalizations 2017 [Member] Resource Rationalizations 2017 [Member] Resource Rationalizations 2016 Resource Rationalizations 2016 [Member] Resource Rationalizations 2016 [Member] Restructuring reserve, current Restructuring Reserve, Current Restructuring reserve, noncurrent Restructuring Reserve, Noncurrent Cash Cash [Member] Expected long-term return per annum Number of operating segments Number of Operating Segments Income tax charge New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Accounting Standards Update 2016-02 Accounting Standards Update 2016-02 [Member] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Scenario, Forecast Scenario, Forecast [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Operating lease, right-of-use asset Operating Lease, Right-of-Use Asset Operating lease, liability Operating Lease, Liability Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest Rate Contract Interest Rate Contract [Member] Foreign Exchange Forward Foreign Exchange Forward [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Currency translation adjustments Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Currency impact on long term funding Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Currency Impact on Long Term Funding, Net of Tax Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Currency Impact on Long Term Funding, Net of Tax Actuarial loss on defined benefit pension plan (note 9) Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax Unrealized capital loss – investments (note 3) Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Hedges Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax Amortization of interest rate hedge Accumulated Other Comprehensive Income (Loss), Amortization Of Cash Flow Hedges, Effect Net of Tax Accumulated Other Comprehensive Income (Loss), Amortization Of Cash Flow Hedges, Effect Net of Tax Total Schedule of Provision for Doubtful Debts Allowance for Doubtful Accounts [Table Text Block] Allowance for Doubtful Accounts, Table Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Employee Stock Plan, 2008 Plan Employee Stock Plan, 2008 Plan [Member] Employee Stock Plan, 2008 Plan [Member] Restricted Stock Units 2008 Restricted Stock Units Twenty Zero Eight [Member] Restricted Stock Units Twenty Zero Eight [Member] Employee Stock Plan, 2003 Plan Employee Stock Plan, 2003 Plan [Member] Employee Stock Plan, 2003 Plan [Member] Restricted Stock Units 2013 Restricted Stock Units Twenty Thirteen [Member] Restricted Stock Units Twenty Thirteen [Member] PSUs Based on Service and EPS Targets Performance Based Grants [Member] Performance Based Grants [Member] Plan Name Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Individual Employee Employee [Member] Employee [Member] Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Vested and Unvested Options Vested And Non Vested Options [Member] Vested And Non Vested Options [Member] Compensation Related Costs Share Based Payments Disclosure [Line Items] Compensation Related Costs Share Based Payments Disclosure [Line Items] Compensation Related Costs Share Based Payments Disclosure [Line Items] Additional number of ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Percentage of option price for fair value of ordinary share Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Maximum number of award as percentage of shares outstanding Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum Shares vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Shares expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Weighted average contractual term of options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Exercisable - weighted average remaining contractual life Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term Options expected to vest (in shares) Share Based Compensation Arrangement By Share Based Payment Award Options Expected To Vest Outstanding Number Share Based Compensation Arrangement By Share Based Payment Award Options Expected To Vest Outstanding Number Intrinsic value of option exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Intrinsic value of option outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Intrinsic value of option exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Options vesting percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Fair value of stock units vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Stock units granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Stock units to be granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Total compensation costs Share-based Compensation Non-cash stock compensation expense not yet recognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Unrecognized stock-based compensation expense, weighted average period (years) Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Income tax benefit related to stock compensation Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Cash tax benefit related to stock options exercised Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Table] Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Table] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Foreign Country Foreign Tax Authority [Member] State NOL's State and Local Jurisdiction [Member] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Domain] Other Comprehensive Income Location [Domain] Other Comprehensive Income Other Comprehensive Income (Loss) [Member] Tax Period [Axis] Tax Period [Axis] Tax Period [Domain] Tax Period [Domain] Tax Year 2018 to 2020 Tax Year 2018 to 2020 [Member] Tax Year 2018 to 2020 [Member] Tax Year 2021 to 2025 Tax Year 2021 to 2025 [Member] Tax Year 2021 to 2025 [Member] Tax Year 2019 to 2025 Tax Year 2019 to 2025 [Member] Tax Year 2019 to 2025 [Member] Tax Year 2026 to 2035 Tax Year 2026 to 2035 [Member] Tax Year 2026 to 2035 [Member] Provision for income taxes Income Tax Benefit [Member] Income Tax Benefit [Member] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Domain] Income Tax Authority, Name [Domain] Federal NOL's Internal Revenue Service (IRS) [Member] Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] Tax cuts and jobs act of 2017, incomplete accounting, provisional income tax expense (benefit) Tax Cuts And Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Tax Cuts And Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Ireland subsidiaries additional tax credit carryforward for income tax Deferred Tax Assets, Additional Tax Credit Carryforwards Not Subject to Expiration Deferred Tax Assets, Additional Tax Credit Carryforwards Not Subject to Expiration Additional operating loss carryforward Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration U.S. Federal net operating loss carry forwards currently available for offset Operating Loss Carryforwards, US Federal Operating Loss Carryforwards, US Federal U.S. federal net operating loss carry forwards Operating Loss Carryforwards Tax credit carry forward Tax Credit Carryforward, Amount Other non-U.S subsidiaries operating loss carryforwards for income tax Deferred Tax Assets, Operating Loss Carryforwards, Not Subject to Expiration Other non-U.S subsidiaries tax credit carryforwards for income tax Deferred Tax Assets Tax Credit Carry Forwards Not Subject To Expiration Deferred Tax Assets Tax Credit Carry Forwards Not Subject To Expiration Alternative minimum tax credit carry forwards Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax Business credit carry forwards that are available to offset Deferred Tax Assets, Tax Credit Carryforwards, General Business Valuation allowance for deferred tax assets Net change in the total valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Tax credit carry forward, valuation allowance Tax Credit Carryforward, Valuation Allowance Deferred tax liabilities, undistributed foreign earnings Deferred Tax Liabilities, Undistributed Foreign Earnings Unrecognized tax benefit, potentially expire in 2019 Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit Total unrecognized tax benefits net of potential benefits Unrecognized Tax Benefits that Would Impact Effective Tax Rate Interest and penalties recognized as an expense Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Total accrued interest and penalties Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Revenue, Initial Application Period Cumulative Effect Transition [Table] Revenue, Initial Application Period Cumulative Effect Transition [Table] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Domain] Initial Application Period Cumulative Effect Transition [Domain] Adjustments Difference between Revenue Guidance in Effect before and after Topic 606 [Member] Balance without adoption of Topic 606 Calculated under Revenue Guidance in Effect before Topic 606 [Member] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] Available for sale investments Unbilled revenue Unbilled revenue Unbilled Contracts Receivable Income taxes receivable Non-current income taxes receivable Non-current deferred tax asset Intangible assets Other liabilities Income taxes payable Non-current bank credit lines and loan facilities Non-current other liabilities Non-current government grants Non-current income taxes payable Non-current deferred tax liability Commitments and contingencies Other undenominated capital Accumulated other comprehensive income Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Unrecognized tax benefits at start of year Unrecognized Tax Benefits Increase related to prior year tax positions Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Decrease related to prior year tax positions Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Increase related to current year tax positions Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Settlements Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities Lapse of statute of limitations Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Unrecognized tax benefits at end of year Related Party Transactions [Abstract] Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Subsidiaries Subsidiary of Common Parent [Member] Mr. Ciaran Murray Related Party, Two [Member] Related Party, Two [Member] Mr. Thomas Lynch Related Party, One [Member] Related Party, One [Member] Related Party Transaction [Axis] Related Party Transaction [Axis] Related Party Transaction [Domain] Related Party Transaction [Domain] DS Biopharma Limited DS Biopharma Limited [Member] DS Biopharma Limited [Member] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Revenue earned from related parties Revenue from Related Parties Amounts due from related parties Due from Related Parties Expenses incurred from related party transactions Related Party Transaction, Expenses from Transactions with Related Party Fair value of asset transferred to related party Related Party Transaction, Asset Transferred to Related Party, Fair Value Related Party Transaction, Asset Transferred to Related Party, Fair Value Related party transaction amounts Related Party Transaction, Amounts of Transaction Amounts due to related parties Due to Related Parties Government Grants Government Grants Disclosure [Text Block] Government Grants Disclosure Exercise Price Range [Axis] Exercise Price Range [Axis] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] Range 1 Range 1 [Member] Range 1 [Member] Range 2 Range 2 [Member] Range 2 [Member] Range 3 Range 3 [Member] Range 3 [Member] Range 4 Range 4 [Member] Range 4 [Member] Range 5 Range 5 [Member] Range 5 [Member] Range 6 Range 6 [Member] Range 6 [Member] Range 7 Range 7 [Member] Range 7 [Member] Range 8 Range 8 [Member] Range 8 [Member] Range 9 Range 9 [Member] Range 9 [Member] Range 10 Range 10 [Member] Range 10 [Member] Range 11 Range 11 [Member] Range 11 [Member] Range 12 Range 12 [Member] Range 12 [Member] Range 13 Range 13 [Member] Range 13 [Member] Range 14 Range 14 [Member] Range 14 [Member] Range 15 Range 15 [Member] Range 15 [Member] Range 16 Range 16 [Member] Range 16 [Member] Range 17 Range 17 [Member] Range 17 [Member] Range 18 Range 18 [Member] Range 18 [Member] Range 19 Range 19 [Member] Range 19 [Member] Range 20 Range 20 [Member] Range 20 [Member] Maximum Range Exercise Price (usd per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit Number of Shares (in shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options Weighted Average Remaining Contractual Life Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term Weighted Average Exercise Price (usd per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price Number of Shares (in shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options Weighted Average Exercise Price (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Minimum Range Exercise Price (usd per share) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Government Grants Government Grants [Member] Government Grants [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Received Deferred Items Grants From Outside Parties Deferred Items Grants From Outside Parties Less accumulated amortization Accumulated Amortization, Capital Government Grants Accumulated Amortization, Capital Government Grants Foreign exchange translation adjustment Foreign Currency Translation Adjustment Foreign Currency Translation Adjustment Total government grants Contract with Customer, Liability Less current portion Non-current government grants Contract with Customer, Liability, Noncurrent Cash paid for interest Interest Paid, Net Cash paid for income taxes Income Taxes Paid 2019 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five Finite Lived Intangible Assets, Amortization Expense, Net, Total Finite Lived Intangible Assets, Amortization Expense, Net Finite Lived Intangible Assets, Amortization Expense, Net Business Combinations [Abstract] Net revenue Business Acquisition, Pro Forma Revenue Net income Business Acquisition, Pro Forma Net Income (Loss) Basic earnings per share (dollars per share) Business Acquisition, Pro Forma Earnings Per Share, Basic Diluted earnings per share (dollars per share) Business Acquisition, Pro Forma Earnings Per Share, Diluted Other Liabilities Disclosure [Abstract] Personnel related liabilities Employee-related Liabilities, Current Facility related liabilities Accrued Utilities, Current General trade and overhead liabilities Other Sundry Liabilities, Current Other liabilities Other Accrued Liabilities, Current Short term government grants (note 11) Capital Government Grants Current Capital Government Grants Current Restructuring (note 14) Restructuring Reserve and Other Liabilities, Current Restructuring Reserve and Other Liabilities, Current Other liabilities Amounts due to third parties for reimbursable expenses Other Liabilities, Amount Payable To Third Parties For Reimbursable Expenses Other Liabilities, Amount Payable To Third Parties For Reimbursable Expenses Accumulated Other Comprehensive Income Accumulated Other Comprehensive Income Loss [Text Block] Disclosure of the components of accumulated other comprehensive income (loss). Summary of Components of Income Before Provision for Income Taxes Income Before Income Tax [Table Text Block] Income Before Income Tax, Table Summary of Components of Provision for Income Taxes Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Schedule of Reconciliation of Consolidated Reported Provision for Income Taxes and Statutory Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Expected Expiry Dates of NOL's Summary of Operating Loss Carryforwards [Table Text Block] Schedule of Reconciliation of Beginning and Ending Amount of Total Unrecognized Tax Benefits Summary of Income Tax Contingencies [Table Text Block] Schedule of Revenue by Major Customers, by Reporting Segments [Table] Schedule of Revenue by Major Customers, by Reporting Segments [Table] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer Concentration Risk Customer Concentration Risk [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Net Revenue Sales Revenue, Net [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Client A Customer A [Member] Customer A [Member] Revenue, Major Customer [Line Items] Revenue, Major Customer [Line Items] Clients which represented 10% or more of the company's net revenue Concentration Risk, Percentage Intangible Assets Intangible Assets Disclosure [Text Block] Other Liabilities Other Liabilities Disclosure [Text Block] Commission costs Capitalized Contract Cost, Net Deferred tax asset, cumulative impact of non-tax adjustments from adoption of ASC 606 Deferred Tax Asset, Cumulative Impact Of Non-Tax Adjustments From Adoption Of ASC 606 Deferred Tax Asset, Cumulative Impact Of Non-Tax Adjustments From Adoption Of ASC 606 Schedule of Other Liabilities Other Liabilities [Table Text Block] Restructuring Charges Restructuring and Related Activities Disclosure [Text Block] Direct costs Cost of Sales [Member] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total Operating Leases, Future Minimum Payments Due Schedule of Goodwill Schedule of Goodwill [Table Text Block] Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Summary of Proforma Effect in Net Revenue Net Income and Earnings Per Share Business Acquisition, Pro Forma Information [Table Text Block] Related Parties Related Party Transactions Disclosure [Text Block] Restructuring Reserve [Roll Forward] Restructuring Reserve [Roll Forward] Closing provision Closing provision Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Loss on disposal of property, plant and equipment Gain (Loss) on Disposition of Property Plant Equipment Depreciation expense Depreciation Amortization of intangibles Amortization of Intangible Assets Amortization of government grants Recovery of Amortization of Government Grants Amortization of Government Grants. Interest on short term investments Realized (gain)/loss on sale of short term investments Gain on re-measurement of financial assets Increase (Decrease) In Equity Securities, FV-NI Increase (Decrease) In Equity Securities, FV-NI Amortization of gain on interest rate hedge Amortization of Deferred Hedge Gains Amortization of financing costs Amortization of Debt Issuance Costs Stock compensation expense Deferred tax expense Deferred Income Taxes and Tax Credits Changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Increase in accounts receivable Increase (Decrease) in Accounts Receivable Increase in unbilled revenue Increase (Decrease) in Contract with Customer, Asset Increase (Decrease) in Contract with Customer, Asset Decrease in other receivables Increase (Decrease) in Other Receivables Increase in prepayments and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Decrease in other non-current assets Increase (Decrease) in Other Noncurrent Assets Decrease in payments on account Increase (Decrease) In Contract With Customer Liability Increase (Decrease) In Contract With Customer Liability Increase in other current liabilities Increase (Decrease) in Other Current Liabilities Decrease in other non-current liabilities Increase (Decrease) in Other Noncurrent Liabilities Decrease in income taxes payable Increase (Decrease) in Income Taxes Payable Decrease in accounts payable Increase (Decrease) in Accounts Payable Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Purchase of property, plant and equipment Purchase of subsidiary undertakings Sale of available for sale investments Purchase of available for sale investments Purchase of investments in equity - long term Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Financing costs Payments of Financing Costs Proceeds from the exercise of equity compensation Share issue costs Payments of Stock Issuance Costs Repurchase of ordinary shares Payments for Repurchase of Common Stock Share repurchase costs Payments for Share Repurchase Costs Payments for Share Repurchase Costs Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Effect of exchange rate movements on cash Effect of Exchange Rate on Cash and Cash Equivalents Net increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of year Cash and cash equivalents at end of year Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Benefit obligation at beginning of year Defined Benefit Plan, Benefit Obligation Plan participants' contributions Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant Benefits paid Defined Benefit Plan, Benefit Obligation, Benefits Paid Actuarial (gain)/loss Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Foreign currency exchange rate changes Defined Benefit Plan, Benefit Obligation, Foreign Currency Translation Gain (Loss) Settlement Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement Prior service cost Benefit obligation at end of year Schedule of Product Information [Table] Schedule of Product Information [Table] Geographic Concentration Risk Geographic Concentration Risk [Member] Europe Europe [Member] Rest of World Other Geographic Locations [Member] Other Geographic Locations [Member] Product Information [Line Items] Product Information [Line Items] Number of employees Entity Number of Employees Number of locations in which company operates Number of Locations Number of Locations Number of countries in which company operates Number of Countries in which Entity Operates Percentage of company revenue Equity Incentive Schemes and Stock Compensation Charges Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Amortization of government grants Restricted retained earnings Retained Earnings, Appropriated Top client Top Client [Member] Top Client [Member] Clients 2-5 Clients 2-5 [Member] Clients 2-5 [Member] Clients 6-10 Clients 6-10 [Member] Clients 6-10 [Member] Clients 11-25 Clients 11-25 [Member] Clients 11-25 [Member] Other Other Customers [Member] Other Customers [Member] Revenue Goodwill [Roll Forward] Goodwill [Roll Forward] Opening goodwill Current year acquisitions Goodwill, Acquired During Period Prior period acquisition (note 4 (a)) Goodwill, Purchase Accounting Adjustments Foreign exchange movement Goodwill, Foreign Currency Translation Gain (Loss) Closing goodwill Direct costs Direct Costs, Other Direct Costs, Other Income tax expense (Note 13) Billed services (accounts receivable) Billed Contracts Receivable Trade accounts receivable and unbilled revenue Contracts Receivable, Gross Contracts Receivable Allowance for doubtful accounts Trade accounts receivable and unbilled revenue, net Contracts Receivable, Net Contracts Receivable, Net Unbilled services (unbilled revenue) Unearned revenue (payments on account) Net balance Advance Payments Netted Against Unbilled Contracts Receivable Advance Payments Netted Against Unbilled Contracts Receivable Change in unbilled receivables Change in Unbilled Receivables Change in Unbilled Receivables Change in unearned revenue (payments on account) Increase (Decrease) In Contracts With Customers, Liability, Current Increase (Decrease) In Contracts With Customers, Liability, Current Change in advance payments netted against unbilled contracts receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in unbilled receivables, percent Change in Unbilled Receivables, Percent Change in Unbilled Receivables, Percent Change in unearned revenue (payments on account), percent Contracts with Customers, Liability, Current, Percent Contracts with Customers, Liability, Current, Percent Change in advance payments netted against unbilled contracts receivable, percent Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Credit loss expense Contract with Customer, Asset, Credit Loss Expense Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Outstanding at beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Shares vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Outstanding at ending of period (in shares) Weighted Average Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Outstanding at beginning of period (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Shares vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Forfeited (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Outstanding at end of period (usd per share) Weighted Average Remaining Contractual Life Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] Outstanding at end of period (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Income Taxes Income Tax Disclosure [Text Block] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] MolecularMD Corp. MolecularMD Corp. [Member] MolecularMD Corp. [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Number of shares authorized to be repurchased (in shares) Stock Repurchase Program, Number of Shares Authorized to be Repurchased Percentage of equity share capital acquired Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares) Effect of dilutive share options outstanding (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares) Schedule of Future Minimum Rental Commitments for Operating Leases with Non-cancelable Terms Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Projected benefit obligation Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation Fair value of plan assets Funded status Defined Benefit Plan, Funded (Unfunded) Status of Plan Non-current other liabilities (note 8) Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position Document And Entity Information [Abstract] Document And Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Current Reporting Status Entity Current Reporting Status Entity Well Known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Filler Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Actual plan asset allocations Defined Benefit Plan, Actual Plan Asset Allocations Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] 2021-2034 Tax Year 2021 to 2034 [Member] Tax Year 2021 to 2034 [Member] 2035-2037 Tax Year 2035 to 2037 [Member] Tax Year 2035 to 2037 [Member] Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Net operating loss carryforward Discount rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Employee Benefits Pension and Other Postretirement Benefits Disclosure [Text Block] Weighted average fair value (usd per share) Assumptions: Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Risk-free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected life Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Payables and Accruals [Abstract] Non-Current Other Liabilities Other Noncurrent Liabilities [Table Text Block] Significant Accounting Policies Significant Accounting Policies [Text Block] Schedule of Restructuring Charges Restructuring and Related Costs [Table Text Block] Schedule of Restructuring Reserve by Type of Cost Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Statement of Cash Flows [Abstract] (Increase)/decrease in accounts receivable Increase in unbilled revenue Decrease/(increase) in other receivables (Increase)/decrease in prepayments and other current assets Decrease/(increase) in other non-current assets Decrease in payments on account Increase/(decrease) in other current liabilities (Decrease)/increase in other non-current liabilities (Decrease)/increase in accounts payable Purchase of subsidiary undertakings Cash acquired with subsidiary undertaking Cash Acquired from Acquisition Purchase of available for sale investments Drawdown of credit lines and facilities Proceeds from Lines of Credit Repayment of credit lines and facilities Repayments of Lines of Credit Excess tax benefit on exercise of equity compensation Excess Tax Benefit from Share-based Compensation, Financing Activities Business Segment and Geographical Information Segment Reporting Disclosure [Text Block] Impact of change in accounting policies New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Defined benefit pension obligations, net (note 9) Liability, Defined Benefit Pension Plan, Noncurrent Other non-current liabilities Other Accrued Liabilities, Noncurrent Non-current other liabilities Net actuarial loss Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax Total Trade Accounts Receivable, Unbilled Services (Contract Assets) and Payments on Account (Contract Liabilities) Goodwill Goodwill Disclosure [Text Block] Schedule of Transactions for Company's Share Option Plans Share-based Compensation, Stock Options, Activity [Table Text Block] Schedule of Non Vested Shares Outstanding Schedule of Nonvested Share Activity [Table Text Block] Summary of Information Concerning Outstanding and Exercisable Share Options Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block] Summary of Weighted Average Fair Values and Assumptions Used Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of RSU and PSU Activity Schedule Of Share Based Compensation Restricted Stock Units Performance Stock Units Award Activity [Table Text Block] Schedule Of Share Based Compensation Restricted Stock Units Performance Stock Units Award Activity [Table Text Block] Schedule of Non-cash Stock Compensation Expense Schedule of Non Cash Stock Compensation Expense [Table Text Block] Schedule of Non Cash Stock Compensation Expense. Adjustments Resulting From Foreign Currency Translations Foreign Currency Disclosure [Table Text Block] Foreign Currency Disclosure Estimated Useful Lives of Assets Schedule Of Useful Lives For Property Plant Equipment [Table Text Block] Schedule Of Useful Lives For Property Plant Equipment Reconciliation of Number of Shares Used in Computation of Basic and Diluted Net Income Per Ordinary Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Financial Instruments [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] US government debt securities US Government Agencies Debt Securities [Member] Corporate securities Corporate Debt Securities [Member] Term deposits Term Deposits [Member] Term Deposits [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] Cost Total Available-for-sale Debt Securities, Amortized Cost Basis Unrealized gains / (losses) Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax Fair Value Total Available-for-sale Securities, Debt Securities Maturity by period, less than 1 year Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Maturity by period, 1 to 5 years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Stock Option And Award Plans Stock Option And Award Plans [Member] Stock Option And Award Plans [Member] Options Granted Under Plans Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Exercised (in shares) Cancelled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Ending balance (in shares) Vested and exercisable at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Beginning balance (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised (usd per share) Cancelled (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price Ending balance (usd per share) Vested and exercisable at end of period (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Grant Date Fair Value [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Grant Date Fair Value [Abstract] Beginning balance (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Options, Weighted Average Grant Date Fair Value Share Based Compensation Arrangement by Share Based Payment Award, Options, Weighted Average Grant Date Fair Value Exercised (usd per share) Share Based Compensation Arrangement By Share Based Payment Award Options Exercised In Period Weighted Average Grant Date Fair Value Share Based Compensation Arrangement By Share Based Payment Award Options Exercised In Period Weighted Average Grant Date Fair Value Cancelled (usd per share) Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period Weighted Average Grant Date Fair Value Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period Weighted Average Grant Date Fair Value Ending balance (usd per share) Vested and exercisable at end of period (usd per share) Share Based Compensation Arrangement by Share Based Payment Award, Options, Exercisable and Vested, Weighted Average Grant Date Fair Value Share Based Compensation Arrangement by Share Based Payment Award, Options, Exercisable and Vested, Weighted Average Grant Date Fair Value Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Fair Value, Inputs, Level 1 Fair Value, Inputs, Level 1 [Member] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Fixed Income Securities Fixed Income Securities [Member] Legal and General Active Corporate Bond – Over 10 Year Legal And General Active Corporate Bond Over Ten Year [Member] Legal And General Active Corporate Bond Over Ten Year [Member] Legal and General Gilt Funds Legal and General Gilt Funds [Member] Legal and General Gilt Funds [Member] Legal and General Index Linked Gilt Funds Legal and General Index Linked Gilt Funds [Member] Legal and General Index Linked Gilt Funds [Member] Non-Current Other Liabilities Accounts Payable, Accrued Liabilities, and Other Liabilities Disclosure, Noncurrent [Text Block]