Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest 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 Reduction in carrying value of operating right-of-use assets Operating Lease, Right-of-Use Asset, Amortization Expense Operating Lease, Right-of-Use Asset, Amortization Expense 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 Accrued Interest On Short Term Bank Deposits Accrued Interest On Short Term Bank Deposits Interest on non-current operating lease liability Accrued Interest On Operating Lease Liability Accrued Interest On Operating Lease Liability Realized gain on sale of short term investments Gain (Loss) on Sale of Investments Loss/(gain) on re-measurement of financial assets Increase (Decrease) in Equity Securities, FV-NI Stock compensation expense Share-based Payment Arrangement, Noncash Expense Amortization of interest rate hedge Amortization of Deferred Hedge Gains Amortization of financing costs Amortization of Debt Issuance Costs Deferred taxes Deferred Income Taxes and Tax Credits Changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Decrease in accounts receivable Increase (Decrease) in Accounts Receivable Increase in unbilled revenue 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/(increase) in other non-current assets Increase (Decrease) in Other Noncurrent Assets Increase in unearned revenue Increase (Decrease) in Contract with Customer, Liability Increase in other current liabilities Increase (Decrease) in Other Current Liabilities Decrease in operating lease liabilities Increase (Decrease) In Operating Lease Liabilities Increase (Decrease) in Operating Lease Liabilities Increase/(decrease) in other non-current liabilities Increase (Decrease) in Other Noncurrent Liabilities Increase in income taxes payable Increase (Decrease) in Income Taxes Payable (Decrease)/increase 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, Continuing Operations [Abstract] Purchase of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Purchase of subsidiary undertakings Payments to Acquire Businesses, Gross Cash acquired with subsidiary undertaking Cash Acquired from Acquisition Purchase of available for sale investments Payments to Acquire Debt Securities, Available-for-sale Sale of available for sale investments Proceeds from Sale of Debt Securities, Available-for-sale Purchase of investments in equity - long term Payments to Acquire Long-term Investments 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, Continuing Operations [Abstract] Proceeds from exercise of equity compensation Proceeds from Stock Options Exercised 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, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Net (decrease)/increase in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash and cash equivalents at beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash and cash equivalents at end of period Goodwill and Intangible Assets Disclosure [Abstract] Goodwill Goodwill Disclosure [Text Block] Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Ireland IRELAND Rest of Europe Rest Of Europe [Member] Rest Of Europe [Member] U.S. UNITED STATES Rest of World Rest of World [Member] Rest of World [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Revenue Revenue from Contract with Customer, Excluding Assessed Tax Income Tax Disclosure [Abstract] Liability for unrecognized tax benefit Unrecognized Tax Benefits Items generating unrecognized tax benefits Unrecognized Tax Benefits, Current Year Tax Positions Unrecognized Tax Benefits, Current Year Tax Positions Interest and related penalties Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Income Statement [Abstract] Costs and expenses: Cost of Revenue [Abstract] Direct costs Cost of Goods and Services Sold Selling, general and administrative expense Selling, General and Administrative Expense Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction 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 provision for income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Provision for income taxes Income Tax Expense (Benefit) Net income Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Net income attributable to the Group Net Income (Loss) Attributable to Parent Net income per Ordinary Share attributable to the Group (note 10): Earnings Per Share [Abstract] Basic (USD per share) Earnings Per Share, Basic Diluted (USD 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 Accounting Policies [Abstract] Basis of presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Goodwill [Roll Forward] Goodwill [Roll Forward] Opening balance Goodwill Current period acquisitions (Note 6) Goodwill, Acquired During Period Prior period acquisitions Goodwill, Purchase Accounting Adjustments Foreign exchange movement Goodwill, Foreign Currency Translation Gain (Loss) Closing balance Share-based Payment Arrangement [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 [Axis] Award Type [Axis] Award Type [Domain] Award Type [Domain] Employee Stock Option Share-based Payment Arrangement, 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, Outstanding [Roll Forward] Outstanding at beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Canceled/expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding at end of period (in shares) Exercisable at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding at beginning of period (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) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Canceled/expired (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Outstanding at end of period (USD per share) Exercisable at end of period (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, 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] Outstanding at beginning of period (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 Granted (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, 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 Canceled/expired (USD per share) Share-based Compensation Arrangement By Share-based Payment Award, Options, Forfeitures And Expirations In Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement By Share-based Payment Award, Options, Forfeitures And Expirations In Period, Weighted Average Grant Date Fair Value Outstanding at end of period (USD per share) Exercisable at end of period (USD per share) Share Based Compensation Arrangement by Share Based Payment Award, Options, Exercisable, Weighted Average Grant Date Fair Value Share Based Compensation Arrangement by Share Based Payment Award, Options, Exercisable, Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Life Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Remaining Contractual Term [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Remaining Contractual Term [Abstract] Outstanding at end of period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Exercisable at end of period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Weighted average grant date fair value 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 Leases [Abstract] Due within 1 year Lessee, Operating Lease, Liability, Payments, Due Next Rolling Twelve Months Due between 1 and 5 years Lessee, Operating Lease, Liability, Payments, Due Between Rolling Year One And Year Five Lessee, Operating Lease, Liability, Payments, Due Between Rolling Year One and Year Five Thereafter Lessee, Operating Lease, Liability, Payments, Due after Rolling Year Five Total future minimum lease payments Lessee, Operating Lease, Liability, Payments, Due Lease imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Operating lease liabilities Operating Lease, Liability Schedule of Components of Income Tax Expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] 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] Customer relationships Customer Relationships [Member] Order backlog Order or Production Backlog [Member] Patient database Database Rights [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Medpass Medpass Group Limited [Member] Medpass Group Limited [Member] Symphony CRN Holdings LLC, Symphony [Member] CRN Holdings LLC, Symphony [Member] MeDiNova MeDiNova Research [Member] MeDiNova Research [Member] MMD MolecularMD Corp [Member] MolecularMD Corp [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Percentage of share capital acquired Business Acquisition, Percentage of Voting Interests Acquired Percentage of interest acquired Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage Cash & cash equivalents 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 Operating right of use assets Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Operating Lease, Right-Of-Use Asset Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Operating Lease, Right-of-Use Asset Goodwill Intangible asset 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 Deferred tax asset Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets Accounts payable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable Unearned revenue 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 Current lease liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Operating Lease Liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Operating Lease Liability Non-current lease liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Operating Lease Liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Operating Lease Liability Non-current deferred tax liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Cash outflows Working capital adjustment Business Combination, Consideration Transferred, Working Capital Adjustments Business Combination, Consideration Transferred, Working Capital Adjustments Redeemable noncontrolling interest Business Combination Acquisition Of Less Than 100 Percent Redeemable Noncontrolling Interest Fair Value Business Combination Acquisition Of Less Than 100 Percent Redeemable Noncontrolling Interest Fair Value Non-current other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Contingent consideration Business Combination, Contingent Consideration, Liability Total consideration Business Combination, Consideration Transferred Goodwill expected to be tax deductible Business Acquisition, Goodwill, Expected Tax Deductible Amount Contingent consideration, maximum amount Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Restructuring and Related Activities [Abstract] Details of Movement in Restructuring Provisions Recognized Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Revenue from Contract with Customer [Abstract] Schedule of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] 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 Program [Axis] Restructuring Program [Axis] Restructuring Program [Axis] Restructuring Program [Domain] Restructuring Program [Domain] Restructuring Program [Domain] Workforce Reduction Workforce Reduction [Member] Workforce Reduction [Member] Onerous Lease Onerous Lease [Member] Onerous Lease [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other liabilities Other Current Liabilities [Member] Non-current other liabilities Other Noncurrent Liabilities [Member] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Restructuring charges Restructuring Charges Restructuring reserve, current Restructuring Reserve, Current Restructuring reserve, noncurrent Restructuring Reserve, Noncurrent Redeemable noncontrolling interests and equity Redeemable Noncontrolling Interests And Equity [Policy Text Block] Redeemable Noncontrolling Interests And Equity [Policy Text Block] New accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Options Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Non-vested outstanding at beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Granted (in shares) 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 Non-vested outstanding at end of period (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 Non-vested outstanding at beginning of period (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 Non-vested outstanding at end of period (USD per share) Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Non-vested outstanding at beginning of period (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price 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 Non-vested outstanding at end of period (USD per share) Operating leases Lessee, Operating Leases [Text Block] Restructuring Reserve [Roll Forward] Restructuring Reserve [Roll Forward] Provision, beginning balance Restructuring Reserve Utilization Restructuring Reserve Utilizations Restructuring Reserve Utilizations Provision, ending balance Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current Assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Available for sale investments Debt Securities, Available-for-sale, Current Accounts receivable, net of allowance for credit losses Accounts Receivable, after Allowance for Credit Loss, Current Unbilled revenue Contract with Customer, Asset, Gross, Current Other receivables Nontrade Receivables, Current Prepayments and other current assets Prepaid Expense and Other Assets, Current Income taxes receivable Income Taxes Receivable, Current Total current assets Assets, Current Other Assets: Assets, Noncurrent [Abstract] Property, plant and equipment, net Property, Plant and Equipment, Net Operating right-of-use assets Operating Lease, Right-of-Use Asset Other non-current assets Other Assets, Noncurrent Non-current income taxes receivable Income Taxes Receivable, Noncurrent Non-current deferred tax asset Deferred Income Tax Assets, Net Investments in equity-long term Long-term Investments Intangible assets Finite-Lived Intangible Assets, Net Total Assets Assets LIABILITIES AND SHAREHOLDERS’ EQUITY Liabilities and Equity [Abstract] Current Liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable, Current Unearned revenue Contract with Customer, Liability, Current Other liabilities Other Liabilities, Current Income taxes payable Accrued Income Taxes, Current Current bank credit lines and loan facilities Line of Credit, Current Total current liabilities Liabilities, Current Other Liabilities: Liabilities, Noncurrent [Abstract] Non-current operating lease liabilities Operating Lease, Liability, Noncurrent Non-current other liabilities Other Liabilities, Noncurrent Non-current government grants Capital Government Grants Noncurrent Noncurrent capital government grants, which are amortized and recognized in earnings over the grant period. Non-current income taxes payable Accrued Income Taxes, Noncurrent Non-current deferred tax liability Deferred Income Tax Liabilities, Net Commitments and contingencies 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, 52,508,751 shares issued and outstanding at March 31, 2020 and 53,622,206 shares issued and outstanding at December 31, 2019 Common Stock, Value, Issued Additional paid‑in capital Additional Paid in Capital, Common Stock Other undenominated capital Other Undenominated Capital Other undenominated capital. Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total Shareholders' Equity Stockholders' Equity Attributable to Parent Redeemable noncontrolling interest Redeemable Noncontrolling Interest, Equity, Carrying Amount Total Shareholders' Equity and Redeemable Noncontrolling Interest Stockholders Equity Attributable To Parent And Redeemable Noncontrolling Interest, Equity, Carrying Amount Stockholders Equity Attributable To Parent And Redeemable Noncontrolling Interest, Equity, Carrying Amount Total Liabilities and Shareholders' Equity and Redeemable Noncontrolling Interest Liabilities and Equity 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 Revenue Revenue from Contract with Customer [Text Block] Performance Share Unit (PSUs) Performance Share Unit [Member] Performance Share Unit [Member] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] 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) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 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 end of period (in shares) Weighted Average Grant Date 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 (in years) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Significant accounting policies Significant Accounting Policies [Text Block] Equity [Abstract] Share capital Stockholders' Equity Note Disclosure [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] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Maximum Maximum [Member] Equity, Class of Treasury Stock [Line Items] Equity, Class of Treasury Stock [Line Items] Share repurchase program, authorized percentage Stock Repurchase Program Authorized Percentage Stock Repurchase Program Authorized Percentage Share repurchase program, number of shares authorized to be repurchased (in shares) Stock Repurchase Program, Number of Shares Authorized to be Repurchased Shares redeemed, value Stock Repurchased and Retired During Period, Value Shares redeemed (in shares) Stock Repurchased and Retired During Period, Shares Schedule of Goodwill Schedule of Goodwill [Table Text Block] Accounting Changes and Error Corrections [Abstract] Impact of change in accounting policies New Accounting Pronouncements and Changes in Accounting Principles [Text Block] 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 Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Income from operations, including restructuring Operating lease costs Operating Lease, Cost Income from sub-leases Sublease Income Net operating lease costs Lease, Cost Schedule of Contracts with Customers, Asset and Liabilities Contract with Customer, Asset and Liability [Table Text Block] Restructuring Restructuring and Related Activities Disclosure [Text Block] Net income Noncontrolling interest adjustment to redemption amount Temporary Equity, Accretion to Redemption Value, Adjustment Net income attributable to the Group (including NCI redemption adjustment) Net Income (Loss) Available to Common Stockholders, Basic Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment): Earnings Per Share, Basic and Diluted [Abstract] 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) Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Title of Individual [Axis] Title of Individual [Axis] Title of Individual [Domain] Title of Individual [Domain] Non-executive directors Non-executive Director [Member] Non-executive Director [Member] Award Date [Axis] Award Date [Axis] Award Date [Domain] Award Date [Domain] Award Date, All years excluding 2018 Award Date, All Years Excluding 2018 [Member] Award Date, Prior to 2018 [Member] Award Date, 2018 Award Date, 2018 [Member] Award Date, 2018 [Member] Consultants Stock Plan, 2008 Plan Consultants Stock Plan, 2008 Plan [Member] Consultants Stock Plan, 2008 Plan [Member] Employee Stock Plan, 2008 Plan Employee Stock Plan, 2008 Plan [Member] Employee Stock Plan, 2008 Plan [Member] Option Plans 2008 Option Plans 2008 [Member] Option Plans 2008 [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] Consultants Restricted Stock Units 2019 Consultants Restricted Stock Units Twenty Nineteen [Member] Consultants Restricted Stock Units Twenty Nineteen [Member] PSUs Based on Service and EPS Targets Performance Based Grants [Member] Performance Based Grants [Member] Minimum Minimum [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] Ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Limit of shares issued or to be issued pursuant to options granted (in shares) Limit Of Shares Issued Or To Be Issued Pursuant To Options Granted Limit of Shares Issued or To Be Issued Pursuant to Options Granted Maximum number of shares per employee (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares Per Employee 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 Options outstanding fair value (USD per share) Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Option Price Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Option Price Weighted average fair value per option (USD per share) Share Based Compensation Arrangement by Share Based Payment Award, Options, Grants in Period, Weighted Average Option Price Share Based Compensation Arrangement by Share Based Payment Award, Options, Grants in Period, Weighted Average Option Price Exercisable - number of shares Exercisable - weighted average remaining contractual life Exercisable - weighted average exercise price (USD per share) Fully vested total intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Intrinsic value of option exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized 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 to be granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Non-cash stock compensation expense not yet recognized Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Unrecognized stock-based compensation expense, weighted average period Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Schedule of Lease Costs Lease, Cost [Table Text Block] Schedule of Operating Lease Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Net income per ordinary share Earnings Per Share [Text Block] Provision for income taxes Business segment information Segment Reporting Disclosure [Text Block] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] 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] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Income taxes Income Tax Disclosure [Text Block] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] 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 2016-02 Accounting Standards Update 2016-02 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Liability due to former noncontrolling interest holders Shareholders' equity Share-based awards Share-based Payment Arrangement [Text Block] Billed services (accounts receivable) Accounts Receivable, before Allowance for Credit Loss, Current Unbilled services (unbilled revenue) Accounts receivable and unbilled revenue Contracts Receivable, Gross Contracts Receivable Allowance for credit losses Accounts Receivable, Allowance for Credit Loss Accounts receivable and unbilled revenue, net Contracts Receivable, Net Contracts Receivable, Net Unearned revenue (payments on account) Net balance Advance Payments Netted Against Unbilled Contracts Receivable Advance Payments Netted Against Unbilled Contracts Receivable Change in unbilled services (unbilled revenue) Increase (Decrease) In Contract with Customer, Asset, Gross, Current Increase (Decrease) In Contract with Customer, Asset, Gross, Current Change in unearned revenue (payments on account) Increase (Decrease) In Contract With Customer, Liability, Current Increase (Decrease) In Contract With Customer, Liability, Current Change in net balance Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in unbilled services (unbilled revenue), percent Increase (Decrease) In Contract with Customer, Asset, Gross, Current, Percent Increase (Decrease) In Contract with Customer, Asset, Gross, Current, Percent Change in unearned revenue (payments on account), percent Increase (Decrease) In Contract With Customer, Liability, Gross, Current, Percent Increase (Decrease) In Contract with Customer, Liability, Gross, Current, Percent Change in net balance, percent Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] 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] Redeemable Noncontrolling Interest Redeemable Noncontrolling Interest [Member] Redeemable Noncontrolling Interest [Member] Statement [Line Items] Statement [Line Items] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning balance (in shares) Beginning balance Comprehensive income: Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Currency impact of long term funding (net of tax) 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. Transfer to realized capital gain Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Amortization of interest rate hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Fair value of cash flow hedge (net of tax) Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax Total comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Exercise of share options (in shares) Exercise of share options Stock Issued During Period, Value, Stock Options Exercised Issue of restricted share units (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Issue of restricted share units Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Non-cash stock compensation expense APIC, Share-based Payment Arrangement, Increase for Cost Recognition Share issuance costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Share repurchase program (in shares) Share repurchase program Share repurchase costs Adjustment to Retained Earning Share Repurchase costs Adjustment to Retained Earning Share Repurchase costs Noncontrolling interest adjustment to redemption amount Redeemable Noncontrolling Interest, Accretion To Redemption Value Redeemable Noncontrolling Interest, Accretion To Redemption Value Ending balance (in shares) Ending balance Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Beginning balance Net income attributable to redeemable noncontrolling interest Temporary Equity, Net Income Total comprehensive income Temporary Equity, Comprehensive Income (Loss) Temporary Equity, Comprehensive Income (Loss) Noncontrolling interest adjustment to redemption amount Temporary Equity, Accretion to Redemption Value Exercise of call option on noncontrolling interest shares Redeemable Noncontrolling Interest, Decrease From Redemptions Or Purchase Of Interests Redeemable Noncontrolling Interest, Decrease From Redemptions Or Purchase Of Interests Ending balance Schedule of Share Option Activity Share-based Payment Arrangement, Option, Activity [Table Text Block] Schedule of Movement in Non-vested Share Options Schedule of Nonvested Share Activity [Table Text Block] Schedule of Weighted Average Fair Values and Assumptions Used Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Schedule 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. Assets Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Amount 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, percent Revenue, Remaining Performance Obligation, Percentage Revenue, remaining performance obligation, expected timing of satisfaction, period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs, by Report Line Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Direct costs Cost of Sales [Member] Selling, general and administrative Selling, General and Administrative Expenses [Member] Business combinations Business Combination Disclosure [Text Block] Long-lived assets, net Long-Lived Assets Schedule of Summary of Estimates of Fair Values of Assets Acquired and Liabilities Assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Number of operating segments Number of Operating Segments Depreciation, amortization and reduction in carrying value of right-of-use assets Depreciation, Amortization And Operating Lease, Right-of-Use Asset, Amortization Expense Depreciation, Amortization And Operating Lease, Right-of-Use Asset, Amortization Expense Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Income Statement Location [Axis] Selling, general and administration costs Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Total lease cost Right-of-use assets obtained in exchange for lease obligations Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Weighted average remaining lease term (in years) Operating Lease, Weighted Average Remaining Lease Term Weighted average discount rate Operating Lease, Weighted Average Discount Rate, Percent Operating lease liabilities Operating Lease, Liability, Current Distribution of Revenue by Geographical Area Revenue from External Customers by Geographic Areas [Table Text Block] Distribution of Income from Operations by Geographical Area Operating Income by Geography [Table Text Block] Operating Income by Geography. Distribution of Long-lived Assets, Net, by Geographical Area Long-lived Assets by Geographic Areas [Table Text Block] 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. Distribution of Total Assets by Geographical Area Reconciliation of Assets from Segment to Consolidated [Table Text Block]