Operating Lease - Future Minimum Lease Payments [Line Items] Operating Lease, Future Minimum Lease Payments [Line Items] Operating Lease, Future Minimum Lease Payments Customer [Domain] Customer [Domain] Percentage change in unbilled services (unbilled revenue) Increase (Decrease) In Contract with Customer, Asset, Gross, Current, Percent Increase (Decrease) In Contract with Customer, Asset, Gross, Current, Percent Shares vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period PSUs Based on Service and EPS Targets Performance Based Grants [Member] Performance Based Grants [Member] Net income attributable to the Group Net income Net income attributable to the Group Net Income (Loss) Attributable to Parent Operating right-of-use assets Operating Lease, Right-of-Use Asset Available for sale investments Debt Securities, Available-for-sale, Current Income Statement Location [Axis] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs, by Report Line Income Statement Location [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Goodwill Goodwill Disclosure [Text Block] Thereafter Lessee, Operating Lease, Liability, Payments, Due after Rolling Year Five Decrease in unbilled revenue Increase (Decrease) in Contract with Customer, Asset 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 Maximum number of shares per employee (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares Per Employee Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Schedule of Contracts with Customers, Asset and Liabilities Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] Additional paid-in capital Additional Paid in Capital, Common Stock Other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Restructuring Type [Axis] Restructuring Type [Axis] Weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term 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 Other non-current assets Other Assets, Noncurrent 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 Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Stock units to be granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Onerous Lease Onerous Lease [Member] Onerous Lease [Member] Net increase in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Items generating unrecognized tax benefits Unrecognized Tax Benefits, Current Year Tax Positions Unrecognized Tax Benefits, Current Year Tax Positions Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Non-cash stock compensation expense APIC, Share-based Payment Arrangement, Increase for Cost Recognition Entity number of employees Entity Number of Employees Decrease in income taxes payable Increase (Decrease) in Income Taxes Payable Intrinsic value of option exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Prior period acquisitions Goodwill, Purchase Accounting Adjustments Canceled/expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Employee Stock Plan, 2003 Plan Employee Stock Plan, 2003 Plan [Member] Employee Stock Plan, 2003 Plan [Member] Amortization of interest rate hedge Amortization of Deferred Hedge Gains Document fiscal year focus Document Fiscal Year Focus Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Subsequent event type [Domain] Subsequent Event Type [Domain] Non-current deferred tax liability Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Comprehensive income: Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Amortization of intangibles Amortization of Intangible Assets Details of movement in restructuring charge Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Additional intangible assets recorded from acquisition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Weighted average discount rate Operating Lease, Weighted Average Discount Rate, Percent Schedule of Future Minimum Lease Payments [Table] Schedule Of Future Minimum Lease Payments [Table] Schedule Of Future Minimum Lease Payments MMD MolecularMD Corp [Member] MolecularMD Corp [Member] 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 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 Business Acquisition [Axis] Business Acquisition [Axis] Title of Individual [Domain] Title of Individual [Domain] Award Type [Domain] Award Type [Domain] Share Repurchase Program [Domain] Share Repurchase Program [Domain] Realized gain on sale of short term investments Gain (Loss) on Sale of Investments ASSETS Assets [Abstract] Exercisable - number of shares (in shares) Exercisable at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Increase in other non-current liabilities Increase (Decrease) in Other Noncurrent Liabilities Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Table] Accounting Standards Update and Change in Accounting Principle [Table] Outstanding at beginning of period (in shares) Outstanding at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Retained Earnings Retained Earnings [Member] Decrease in prepayments and other current assets Increase (Decrease) in Prepaid Expense and Other Assets 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: Liabilities, Noncurrent [Abstract] Settlement of liability Payments for Previous Acquisition Options Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Shares redeemed, value Stock Repurchased and Retired During Period, Value Distribution of Long-lived Assets, Net, by Geographical Area Long-lived Assets by Geographic Areas [Table Text Block] Decrease in accounts receivable Increase (Decrease) in Accounts Receivable Right-of-use assets obtained in exchange for lease obligations Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Percentage of interest acquired Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Tax impact of restructuring Other Tax Expense (Benefit) Working capital adjustment paid Business Combination, Consideration Transferred, Working Capital Adjustments Business Combination, Consideration Transferred, Working Capital Adjustments Number of operating segments Number of Operating Segments MeDiNova MeDiNova Research [Member] MeDiNova Research [Member] Basic (USD per share) Earnings Per Share, Basic 2024 Lessee, Operating Lease, Liability, to be Paid, Year Three Revenue Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) Revenue from Contract with Customer [Text Block] Income from sub-leases Sublease Income Cash acquired with subsidiary undertaking Cash Acquired from Acquisition Outstanding at beginning of period (in shares) Outstanding at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Foreign exchange movement Restructuring Reserve, Foreign Currency Translation Gain (Loss) Document type Document Type Interest expense Interest Expense, Debt Schedule of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Percentage change in net balance Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Change in Advance Payments Netted Against Unbilled Contracts Receivable, Percent Outstanding Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Customer relationships/ Order backlog Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Compensation Related Costs Share Based Payments Disclosure [Table] Schedule of Movement in Non-vested Share Options Schedule of Nonvested Share Activity [Table Text Block] Cumulative effect, period of adoption, adjustment Cumulative Effect, Period of Adoption, Adjustment [Member] Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Non-current government grants Capital Government Grants Noncurrent Noncurrent capital government grants, which are amortized and recognized in earnings over the grant period. Total consideration Business Combination, Consideration Transferred Goodwill [Roll Forward] Goodwill [Roll Forward] Share repurchase costs Payments for Share Repurchase Costs Payments for Share Repurchase Costs Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Equity method investment, majority ownership percentage Equity Method Investment, Majority Ownership Percentage Equity Method Investment, Majority Ownership Percentage Total Liabilities and Shareholders' Equity Liabilities and Equity Unearned revenue Unearned revenue (payments on account) Contract with Customer, Liability, Current Scenario [Axis] Scenario [Axis] Exercised (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Weighted Average Remaining Contractual Life Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract] Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Accounts receivable and unbilled revenue, net Contracts Receivable, Net Contracts Receivable, Net LIABILITIES AND SHAREHOLDERS’ EQUITY Liabilities and Equity [Abstract] (Decrease)/increase in accounts payable Increase (Decrease) in Accounts Payable Restructuring Reserve [Roll Forward] Restructuring Reserve [Roll Forward] Clients 6-10 Clients 6-10 [Member] Clients 6-10 [Member] Medpass Medpass Group Limited [Member] Medpass Group Limited [Member] Other Undenominated Capital Other Undenominated Capital [Member] Other undenominated capital. Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract] Net income per ordinary share Earnings Per Share [Text Block] 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 Impairment of right-of-use assets Operating Lease, Impairment Loss Equity method investment, ownership percentage Equity Method Investment, Ownership Percentage 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] Accounting Changes and Error Corrections [Abstract] Accounting Changes and Error Corrections [Abstract] Operating income (loss) excluding restructuring Operating Income (Loss) Excluding Restructuring Operating Income (Loss) Excluding Restructuring Exercise of share options Stock Issued During Period, Value, Stock Options Exercised Minimum Minimum [Member] Basis of presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Income taxes receivable Income Taxes Receivable, Current Distribution of Total Assets by Geographical Area Reconciliation of Assets from Segment to Consolidated [Table Text Block] 2023 Lessee, Operating Lease, Liability, to be Paid, Year Two Effect of exchange rate movements on cash Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Balance Sheet Location [Domain] Balance Sheet Location [Domain] Resource rationalization, 2020 Resource Rationalization, 2020 [Member] Resource Rationalization, 2020 Selling, general and administrative expense Selling, General and Administrative Expense Canceled/expired (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Liability for unrecognized tax benefit Unrecognized Tax Benefits Cumulative Effect, Period of Adoption [Axis] Cumulative Effect, Period of Adoption [Axis] Provision balance Restructuring Reserve Employee Stock Plan, 2008 Plan Employee Stock Plan, 2008 Plan [Member] Employee Stock Plan, 2008 Plan [Member] Percentage of share capital acquired Business Acquisition, Percentage of Voting Interests Acquired Income before share of earnings from equity method investments Income (Loss) from Continuing Operations before Equity Method Investments, Noncontrolling Interest Income (Loss) from Continuing Operations before Equity Method Investments, Noncontrolling Interest Non-current deferred tax asset Deferred Income Tax Assets, Net Current Liabilities: Liabilities, Current [Abstract] Redeemable noncontrolling interests and equity Stockholders' Equity Note, Redeemable Non-Controlling Interest, Policy [Policy Text Block] Stockholders' Equity Note, Redeemable Non-Controlling Interest, Policy Restructuring Plan [Domain] Restructuring Plan [Domain] 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 Schedule of Components of Income Tax Expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Other restructuring Other Restructuring [Member] Share-based awards Share-based Payment Arrangement [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. Interest and related penalties Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Accounting Policies [Abstract] Accounting Policies [Abstract] 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 Share issue costs Payments of Stock Issuance Costs Costs and expenses: Cost of Revenue [Abstract] Operating lease, liability Total Operating Lease, Liability Ordinary shares, par value 6 euro cents per share; 100,000,000 shares authorized, 52,958,063 shares issued and outstanding at June 30, 2021 and 52,788,093 shares issued and outstanding at December 31, 2020 Common Stock, Value, Issued Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Unrecognized stock-based compensation expense, weighted average period Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition 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, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Current period acquisitions (Note 6) Goodwill, Acquired During Period Commitments and contingencies Commitments and Contingencies Operating leases Lessee, Operating Leases [Text Block] Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Class of Treasury Stock [Table] Class of Treasury Stock [Table] Non-current income taxes payable Accrued Income Taxes, Noncurrent 2021 Lessee, Operating Lease, Liability, to be Paid, Remainder of Fiscal Year Business combinations Business Combination Disclosure [Text Block] Schedule of Weighted Average Fair Values and Assumptions Used Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Net balance Advance Payments Netted Against Unbilled Contracts Receivable Advance Payments Netted Against Unbilled Contracts Receivable Share repurchase program, authorized percentage Stock Repurchase Program Authorized Percentage Stock Repurchase Program Authorized Percentage Document period end date Document Period End Date Other receivables Nontrade Receivables, Current Total costs and expenses Costs and Expenses Distribution of Revenue by Geographical Area Revenue from External Customers by Geographic Areas [Table Text Block] Entity registrant name Entity Registrant Name Lease imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Equity method investment, period majority voting share capital can be sold to the company Equity Method Investment, Period Majority Voting Share Capital Can Be Sold To The Company Equity Method Investment, Period Majority Voting Share Capital Can Be Sold To The Company Prepayments and other current assets Prepaid Expense and Other Assets, Current Gain on re-measurement of financial assets Increase (Decrease) in Equity Securities, FV-NI Selling, general and administration costs Selling, general and administrative Selling, General and Administrative Expenses [Member] Revenue Revenue from Contract with Customer, Excluding Assessed Tax Restricted Stock Units 2013 Restricted Stock Units Twenty Thirteen [Member] Restricted Stock Units Twenty Thirteen [Member] Employee Stock Option Share-based Payment Arrangement, Option [Member] Share issuance costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Equity [Abstract] Equity [Abstract] Ordinary Shares Common Stock [Member] Interest income Investment Income, Interest Business segment information Segment Reporting Disclosure [Text Block] Income from operations Income from operations, including restructuring Operating Income (Loss) Interest on non-current operating lease liability Accrued Interest On Operating Lease Liability Accrued Interest On Operating Lease Liability Purchase of investments in equity - long term Payments to Acquire Long-term Investments Operating lease liabilities Operating Lease, Liability, Current Noncontrolling interest adjustment to redemption amount Temporary Equity, Accretion to Redemption Value, Adjustment Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Stock compensation expense Share-based Payment Arrangement, Noncash Expense Restructuring Restructuring and Related Activities Disclosure [Text Block] Additional Paid-in Capital Additional Paid-in Capital [Member] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Depreciation expense Depreciation Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Due within 1 year Lessee, Operating Lease, Liability, Payments, Due Next Rolling Twelve Months Clients 2-5 Clients 2-5 [Member] Clients 2-5 [Member] Other undenominated capital Other Undenominated Capital Other undenominated capital. Unbilled revenue Unbilled services (unbilled revenue) Contract with Customer, Asset, before Allowance for Credit Loss, Current Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Shares redeemed (in shares) Stock Repurchased and Retired During Period, Shares Purchase of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Other liabilities Other Liabilities, Current 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 Business Combinations [Abstract] Business Combinations [Abstract] Maximum Maximum [Member] Share-based Payment Arrangement [Abstract] Share-based Payment Arrangement [Abstract] Total Liabilities Liabilities 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 Award Type [Axis] Award Type [Axis] Granted (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Non-current income taxes receivable Income Taxes Receivable, Noncurrent Schedule of Operating Lease Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Total Shareholders' Equity Beginning balance Ending balance Opening retained earnings Stockholders' Equity Attributable to Parent Common Stock, Shares, Issued Ordinary shares, shares issued (in shares) Beginning balance (in shares) Ending balance (in shares) Common Stock, Shares, Issued Fully vested total intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Prepayments and other current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Equity method investments Equity Method Investments and Joint Ventures Disclosure [Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Ireland IRELAND Loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Repurchase of ordinary shares Payments for Repurchase of Common Stock Shareholders' Equity: Stockholders' Equity Attributable to Parent [Abstract] Equity, Class of Treasury Stock [Line Items] Equity, Class of Treasury Stock [Line Items] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Non-current operating lease liabilities Operating Lease, Liability, Noncurrent Adjustments for New Accounting Pronouncements [Axis] Accounting Standards Update [Axis] Accounting Standards Update 2016-02 Accounting Standards Update 2016-02 [Member] Other Assets: Assets, Noncurrent [Abstract] Net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net 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 Retained earnings Retained Earnings (Accumulated Deficit) Shares expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Equity method investments Equity Method Investments [Table Text Block] 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] Business Acquisition [Line Items] Business Acquisition [Line Items] Clients 11-25 Clients 11-25 [Member] Clients 11-25 [Member] Risk-free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Share Repurchase Program Share Repurchase Program [Axis] Order backlog Order or Production Backlog [Member] Income Statement [Abstract] Income Statement [Abstract] Equity method investments Oncacare Limited Equity Method Investments Amendment flag Amendment Flag Accounts receivable and unbilled revenue Contracts Receivable, Gross Contracts Receivable 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 Number of countries in which entity operates Number of Countries in which Entity Operates Accounts receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Impact of change in accounting policies Accounting Standards Update and Change in Accounting Principle [Text Block] Proceeds from exercise of equity compensation Proceeds from Stock Options Exercised Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Provision for income taxes Provision for income taxes Income Tax Expense (Benefit) Utilization Restructuring Reserve Utilizations Restructuring Reserve Utilizations Accounts receivable, net of allowance for credit losses Accounts Receivable, after Allowance for Credit Loss, Current Equity Components [Axis] Equity Components [Axis] Non-vested outstanding at beginning of period (in shares) Non-vested outstanding at end of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Total current assets Assets, Current Total future minimum lease payments (including related costs) Total future minimum lease payments Lessee, Operating Lease, Liability, to be Paid Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Amortization of interest rate hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Distribution of income from operations by geographical area Schedule of Segment Reporting Information, by Segment [Table Text Block] Equity Method Investments and Joint Ventures [Abstract] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Award Date, All years excluding 2018 Award Date, All Years Excluding 2018 [Member] Award Date, Prior to 2018 [Member] Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Vested (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value Restructuring Plan [Axis] Restructuring Plan [Axis] Granted (USD per share) Weighted average grant date fair value Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value PRA Health Sciences Inc PRA Health Sciences Inc [Member] PRA Health Sciences Inc Equity Component [Domain] Equity Component [Domain] Issue of restricted share units (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Non-vested outstanding at beginning of period (USD per share) Non-vested outstanding at end of period (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price 2022 Lessee, Operating Lease, Liability, to be Paid, Year One Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Statement [Line Items] Statement [Line Items] Write off of loss on interest rate hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax Decrease in other current liabilities Increase (Decrease) in Other Current Liabilities Non-current bank credit lines and loan facilities Long-term Line of Credit, Noncurrent Increase in unearned revenue Increase (Decrease) in Contract with Customer, Liability Non-vested outstanding at beginning of period (USD per share) Non-vested outstanding at end 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 Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Amount Issue of restricted share units Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Contingent consideration, maximum amount Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High 2025 Lessee, Operating Lease, Liability, to be Paid, Year Four Percentage of option price for fair value of ordinary share (percentage) Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Income before provision for income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Type of Adoption [Domain] Accounting Standards Update [Domain] Segment Reporting [Abstract] Segment Reporting [Abstract] Type of Restructuring [Domain] Type of Restructuring [Domain] 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 Customer [Axis] Customer [Axis] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Granted (in shares) Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Change in net balance Increase in net balance Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable 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 Business acquisition, number of shares issued (in shares) Business Combination, Consideration Transferred, Shares Issued To Shareholders, Per Share Held Business Combination, Consideration Transferred, Shares Issued, Per Current Share Held To Shareholders Forecast Forecast [Member] Total Assets Assets Assets Symphony CRN Holdings LLC, Symphony [Member] CRN Holdings LLC, Symphony [Member] Deferred taxes Deferred Income Taxes and Tax Credits Amortization of government grants Recovery of Amortization of Government Grants Amortization of Government Grants. Ordinary shares, shares authorized (in shares) Common Stock, Shares Authorized Investments in equity-long term Other Long-term Investments Geographical [Domain] Geographical [Domain] Title of Individual [Axis] Title of Individual [Axis] Net income attributable to the Group (including NCI redemption adjustment) Net Income (Loss) Available to Common Stockholders, Basic Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Contingent consideration Business Combination, Contingent Consideration, Liability Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Significant accounting policies Significant Accounting Policies [Text Block] Revenue, remaining performance obligation, expected timing of satisfaction, period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Cumulative Effect, Period of Adoption [Domain] Cumulative Effect, Period of Adoption [Domain] Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Schedule of Lease Costs Lease, Cost [Table Text Block] Geographical [Axis] Geographical [Axis] 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. 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 Diluted (USD per share) Earnings Per Share, Diluted Reduction in carrying value of operating right-of-use assets Operating Lease, Right-of-Use Asset, Amortization Expense Effect of dilutive share options outstanding (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Accounting Standards Update [Extensible List] Accounting Standards Update [Extensible Enumeration] Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Total comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Goodwill expected to be tax deductible Business Acquisition, Goodwill, Expected Tax Deductible Amount Restructuring reserve, current Restructuring Reserve, Current Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Weighted average number of Ordinary Shares outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Non-current deferred tax liability Deferred Income Tax Liabilities, Net 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 Shares vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Non-current other liabilities Other Noncurrent Liabilities [Member] Share of equity method investments Loss on equity method investments Loss from equity method investments Income (Loss) from Equity Method Investments Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Property, plant and equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Consultants Restricted Stock Units 2019 Consultants Restricted Stock Units Twenty Nineteen [Member] Consultants Restricted Stock Units Twenty Nineteen [Member] Restructuring Restructuring Charges Restructuring Charges Accounts payable Accounts Payable, Current Performance Share Unit (PSUs) Performance Share Unit [Member] Performance Share Unit [Member] Accumulated Other Comprehensive Income AOCI Attributable to Parent [Member] Ordinary shares, shares outstanding (in shares) Common Stock, Shares, Outstanding Accounts payable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable Award Date, 2018 Award Date, 2018 [Member] Award Date, 2018 [Member] Provision for income taxes (excluding restructuring) Income Tax Expense (Benefit), Excluding Other Tax Expense (Benefit) Income Tax Expense (Benefit), Excluding Other Tax Expense (Benefit) Restructuring and Related Activities [Abstract] Restructuring and Related Activities [Abstract] Share repurchase program, number of shares authorized to be repurchased (in shares) Stock Repurchase Program, Number of Shares Authorized to be Repurchased Schedule of Non-cash Stock Compensation Expense Schedule of Non Cash Stock Compensation Expense [Table Text Block] Schedule of Non Cash Stock Compensation Expense. Options Outstanding Number of Shares  Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract] Award Date [Domain] Award Date [Domain] Net operating lease costs Total lease cost Lease, Cost Income Statement Location [Domain] Income Statement Location [Domain] Ordinary shares which have been reserved for issuance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Customer Relationships Customer Relationships [Member] 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] Property, plant and equipment, net Property, Plant and Equipment, Net Non-current other liabilities Other Liabilities, Noncurrent 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 Net income per Ordinary Share attributable to the Group (note 11): Earnings Per Share [Abstract] Allowance for credit losses Accounts Receivable, Allowance for Credit Loss 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] Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Percentage Other liabilities Other Current Liabilities [Member] Sale of available for sale investments Proceeds from Sale of Debt Securities, Available-for-sale Rest of World Rest of World [Member] Rest of World [Member] 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, Outstanding, Weighted Average Exercise Price 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] Exercisable - weighted average remaining contractual life Exercisable at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Thereafter Lessee, Operating Lease, Liability, To Be Paid, After Year Four Lessee, Operating Lease, Liability, To Be Paid, After Year Four Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment): Earnings Per Share, Basic and Diluted [Abstract] 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] Document fiscal period focus Document Fiscal Period Focus Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Billed services (accounts receivable) Accounts Receivable, before Allowance for Credit Loss, Current Amortization of financing costs Amortization of Debt Issuance Costs (Gain)/loss on disposal of property, plant and equipment Gain (Loss) on Disposition of Property Plant Equipment 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 U.S. UNITED STATES Ordinary shares, par value (in EUR cents per share) Common Stock, Par or Stated Value Per Share Current Assets: Assets, Current [Abstract] 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] Award Date [Axis] Award Date [Axis] Leases [Abstract] Leases [Abstract] Entity central index key Entity Central Index Key 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 Exercise of share options (in shares) Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Resource rationalizations 2018 Resource Rationalizations 2018 [Member] Resource Rationalizations 2018 [Member] Diluted (in shares) Weighted average number of ordinary shares outstanding for diluted net income per ordinary share (in shares) Weighted Average Number of Shares Outstanding, Diluted Income taxes Income Tax Disclosure [Text Block] Buyback Program Buyback Program [Member] Buyback Program [Member] Business acquisition, share price (in USD per share) Business Combination, Consideration Transferred, Cash Payment To Shareholders, Per Share Held Business Combination, Consideration Transferred, Cash Payment To Shareholders, Per Share Held Current fiscal year end date Current Fiscal Year End Date Schedule of Share Option Activity Share-based Payment Arrangement, Option, Activity [Table Text Block] Share capital Stockholders' Equity Note Disclosure [Text Block] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Purchase of equity method investments Payments to Acquire Equity Method Investments Payments to Acquire Equity Method Investments 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 Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Restructuring reserve, noncurrent Restructuring Reserve, Noncurrent Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities 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] Consultants Stock Plan, 2008 Plan Consultants Stock Plan, 2008 Plan [Member] Consultants Stock Plan, 2008 Plan [Member] Changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] 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, 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 Expected life Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Total current liabilities Liabilities, Current 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 Purchase of subsidiary undertakings Cash outflows Payments to Acquire Businesses, Gross Foreign exchange movement Goodwill, Foreign Currency Translation Gain (Loss) Cash & cash equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Maximum number of award as percentage of shares outstanding (percentage) Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum Long-lived assets, net Long-Lived Assets 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 Statement [Table] Statement [Table] 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 New accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] (Increase)/decrease in other non-current assets Increase (Decrease) in Other Noncurrent Assets Assumptions: Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Non-cash stock compensation expense not yet recognized Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Direct costs (excluding depreciation and amortization) Cost of Goods and Service, Excluding Depreciation, Depletion, and Amortization Statistical Measurement [Axis] Statistical Measurement [Axis] Decrease in other receivables Increase (Decrease) in Other Receivables Subsequent event Subsequent Event [Member] Cover [Abstract] Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares Non-executive directors Non-executive Director [Member] Non-executive Director [Member] Intangible assets Finite-Lived Intangible Assets, Net Goodwill Opening balance Closing balance Goodwill Direct costs Cost of Sales [Member] Income taxes payable Accrued Income Taxes, Current Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares Scenario [Domain] Scenario [Domain] Schedule of Goodwill Schedule of Goodwill [Table Text Block] 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] 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 Subsequent event type [Axis] Subsequent Event Type [Axis] Operating lease costs Operating Lease, Cost Forfeited (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value 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 Change in accounting principle, Accounting Standards Update, transition option elected [Extensible List] Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible Enumeration] 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 Accounting Standards Update 2016-02 Cumulative Effect, Period of Adoption [Member]