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Date Fair Value Employee Stock Plan, 2003 Plan Employee Stock Plan, 2003 Plan [Member] Employee Stock Plan, 2003 Plan [Member] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Commitments and contingencies Commitments and Contingencies Repurchase of ordinary shares Payments for Repurchase of Common Stock ASSETS Assets [Abstract] Shares vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Accounting Policies [Abstract] Accounting Policies [Abstract] Cash acquired with subsidiary undertaking Cash Acquired from Acquisition Issue of restricted share units Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Shares vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Accounts payable Accounts Payable, Current Foreign exchange movement Goodwill, Foreign Currency 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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] Stock units to be granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Settlement of liability Payments for Previous Acquisition Equity method investments Equity Method Investments [Table Text Block] Amortization of interest rate hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Share repurchase costs Payments for Share Repurchase Costs Payments for Share Repurchase Costs Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Comprehensive income: Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Entity Central Index Key Entity Central Index Key Restructuring Restructuring and Related Activities Disclosure [Text Block] Available for sale investments Debt Securities, Available-for-sale, Current Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Non-current other liabilities Other Noncurrent Liabilities [Member] Equity method investment, majority ownership percentage Equity Method Investment, Majority Ownership Percentage Equity Method Investment, Majority Ownership Percentage Income taxes receivable Income Taxes Receivable, Current Geographical [Axis] Geographical [Axis] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Non-current income taxes payable Accrued Income Taxes, Noncurrent Accounts receivable and unbilled revenue, net Contracts Receivable, Net Contracts Receivable, Net Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Effect of dilutive share options outstanding (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Clients 6-10 Clients 6-10 [Member] Clients 6-10 [Member] Medpass Medpass Group Limited [Member] Medpass Group Limited [Member] Income Statement Location [Axis] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs, by Report Line Income Statement Location [Axis] Other Undenominated Capital Other Undenominated Capital [Member] Other undenominated capital. 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Purchase of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Document Period End Date Document Period End Date Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Contingent consideration Business Combination, Contingent Consideration, Liability Change in unbilled services (unbilled revenue) Decrease in unbilled services (unbilled revenue) Increase (Decrease) In Contract with Customer, Asset, Gross, Current Increase (Decrease) In Contract with Customer, Asset, Gross, Current Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Percentage Share of equity method investments Loss on equity method investments Income (Loss) from Equity Method Investments Non-vested outstanding at beginning of period (USD per share) Non-vested outstanding at end of period (USD per share) Share-based Compensation Arrangement 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Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Restructuring Plan [Axis] Restructuring Plan [Axis] Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Due within 1 year Lessee, Operating Lease, Liability, Payments, Due Next Rolling Twelve Months Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Amount Amortization of interest rate hedge Amortization of Deferred Hedge Gains Liability for unrecognized tax benefit Unrecognized Tax Benefits Balance Sheet Location [Axis] Balance Sheet Location [Axis] Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 52,858,190 shares issued and outstanding at March 31, 2021 and 52,788,093 shares issued and outstanding at December 31, 2020 Common Stock, Value, Issued Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Document Fiscal Year Focus Document Fiscal Year Focus Investments in equity-long term Other Long-term Investments Other Assets: Assets, Noncurrent [Abstract] Fully vested total intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Issue of restricted share units (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture 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. Decrease/(increase) in unbilled revenue Increase (Decrease) in Contract with Customer, Asset 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 Ordinary shares, shares issued (in shares) Beginning balance (in shares) Ending balance (in shares) Common Stock, Shares, Issued Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Operating lease, liability Total Operating Lease, Liability Segment Reporting [Abstract] Segment Reporting [Abstract] Share Repurchase Program Share Repurchase Program [Axis] Decrease in accounts receivable Increase (Decrease) in Accounts Receivable Total Shareholders' Equity Beginning balance Ending balance Opening retained earnings Stockholders' Equity Attributable to Parent Share issue costs Payments of Stock Issuance Costs Total consideration Business Combination, Consideration Transferred Total Liabilities and Shareholders' Equity Liabilities and Equity Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Income taxes Income Tax Disclosure [Text Block] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Consultants Restricted Stock Units 2019 Consultants Restricted Stock Units Twenty Nineteen [Member] Consultants Restricted Stock Units Twenty Nineteen [Member] Operating lease costs Operating Lease, Cost Net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Type of Adoption [Domain] Accounting Standards Update [Domain] Performance Share Unit (PSUs) Performance Share Unit [Member] Performance Share Unit [Member] (Gain)/loss on disposal of property, plant and equipment Gain (Loss) on Disposition of Property Plant Equipment Equity Component [Domain] Equity Component [Domain] Award Date, 2018 Award Date, 2018 [Member] Award Date, 2018 [Member] (Increase)/decrease in other non-current assets Increase (Decrease) in Other Noncurrent Assets Property, plant and equipment, net Property, Plant and Equipment, Net Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Share-based awards Share-based Payment Arrangement [Text Block] Net income attributable to the Group Net income Net income attributable to the Group Net Income (Loss) Attributable to Parent Decrease in other receivables Increase (Decrease) in Other Receivables Changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Other liabilities Other Liabilities, Current 2021 Lessee, Operating Lease, Liability, to be Paid, Remainder of Fiscal Year Schedule of Non-cash Stock Compensation Expense Schedule of Non Cash Stock Compensation Expense [Table Text Block] Schedule of Non Cash Stock Compensation Expense. Leases [Abstract] Leases [Abstract] Total current assets Assets, Current Current Fiscal Year End Date Current Fiscal Year End Date Goodwill Goodwill Disclosure [Text Block] Net income per Ordinary Share attributable to the Group (note 11): Earnings Per Share [Abstract] Restructuring Charges Restructuring Charges Other Other Customers [Member] Other Customers [Member] 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 Rest of Europe Rest Of Europe [Member] Rest Of Europe [Member] Workforce reductions Workforce Reduction [Member] Workforce Reduction [Member] Change in unearned revenue (payments on account) Decrease in unearned revenue (payments on account) Increase (Decrease) In Contract With Customer, Liability, Current Increase (Decrease) In Contract With Customer, Liability, Current Options Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Shareholders' Equity: Stockholders' Equity Attributable to Parent [Abstract] Options Outstanding Number of Shares  Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Document Fiscal Period Focus Document Fiscal Period Focus 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] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Rest of World Rest of World [Member] Rest of World [Member] Ordinary Shares Common Stock [Member] Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Outstanding at beginning of period (USD per share) Outstanding at end 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 Top client Top Client [Member] Top Client [Member] Thereafter Lessee, Operating Lease, Liability, To Be Paid, After Year Four Lessee, Operating Lease, Liability, To Be Paid, After Year Four Class of Treasury Stock [Table] Class of Treasury Stock [Table] Revenue, remaining performance obligation, expected timing of satisfaction, period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Exercise of share options (in shares) Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Net increase in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Business combinations Business Combination Disclosure [Text Block] Schedule of Movement in Non-vested Share Options Schedule of Nonvested Share Activity [Table Text Block] 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] Schedule of Operating Lease Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Employee Stock Option Share-based Payment Arrangement, Option [Member] Equity, Class of Treasury Stock [Line Items] Equity, Class of Treasury Stock [Line Items] 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 Basic (in shares) Weighted average number of ordinary shares outstanding for basic net income per ordinary share (in shares) Weighted Average Number of Shares Outstanding, Basic Risk-free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Resource rationalizations 2018 Resource Rationalizations 2018 [Member] Resource Rationalizations 2018 [Member] Restructuring reserve, current Restructuring Reserve, Current Buyback Program Buyback Program [Member] Buyback Program [Member] Equity method investment, ownership percentage Equity Method Investment, Ownership Percentage Revenue Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) Revenue from Contract with Customer [Text Block] (Decrease)/increase in other current liabilities Increase (Decrease) in Other Current Liabilities Right-of-use assets obtained in exchange for lease obligations Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Basis of presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Provision for income taxes Provision for income taxes Income Tax Expense (Benefit) 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 Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents New Accounting Pronouncements or Change in Accounting Principle [Table] Accounting Standards Update and Change in Accounting Principle [Table] 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] Title of Individual [Domain] Title of Individual [Domain] Consultants Stock Plan, 2008 Plan Consultants Stock Plan, 2008 Plan [Member] Consultants Stock Plan, 2008 Plan [Member] Diluted (USD per share) Earnings Per Share, Diluted Sale of available for sale investments Proceeds from Sale of Debt Securities, Available-for-sale Purchase of subsidiary undertakings Cash outflows Payments to Acquire Businesses, Gross Cumulative Effect, Period of Adoption [Domain] Cumulative Effect, Period of Adoption [Domain] Distribution of Total Assets by Geographical Area Reconciliation of Assets from Segment to Consolidated [Table Text Block] 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 Intrinsic value of option exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value 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 Depreciation expense Depreciation Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Exercise of share options Stock Issued During Period, Value, Stock Options Exercised Contingent consideration, maximum amount Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Award Type [Axis] Award Type [Axis] 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] Early termination options reasonably certain to be exercised Early Termination Options Reasonably Certain To Be Exercised, Right Of Use Asset Obtained In Exchange For Operating Lease Liability Early Termination Options Reasonably Certain To Be Exercised, Right Of Use Asset Obtained In Exchange For Operating Lease Liability 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 Maximum Maximum [Member] Amortization of financing costs Amortization of Debt Issuance Costs Percentage change in unearned revenue (payments on account) Increase (Decrease) In Contract With Customer, Liability, Gross, Current, Percent Increase (Decrease) In Contract with Customer, Liability, Gross, Current, Percent Retained Earnings Retained Earnings [Member] Significant accounting policies Significant Accounting Policies [Text Block] Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Weighted average contractual term of options outstanding Outstanding at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Allowance for credit losses Accounts Receivable, Allowance for Credit Loss Award Type [Domain] Award Type [Domain] Other Liabilities: Liabilities, Noncurrent [Abstract] Non-executive directors Non-executive Director [Member] Non-executive Director [Member] Details of movement in restructuring charge Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Exercisable - weighted average exercise price (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 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 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 Net income per ordinary share Earnings Per Share [Text Block] Current Liabilities: Liabilities, Current [Abstract] Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Option Plans 2008 Option Plans 2008 [Member] Option Plans 2008 [Member] Decrease in operating lease liabilities Increase (Decrease) In Operating Lease Liabilities Increase (Decrease) in Operating Lease Liabilities New accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] 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 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