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Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares Clients 11-25 Clients 11-25 [Member] Clients 11-25 [Member] Equity Method Investments and Joint Ventures [Abstract] Accounts receivable and unbilled revenue Contracts Receivable, Gross Contracts Receivable Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible List] Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible List] 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 Schedule of Weighted Average Fair Values and Assumptions Used Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Current lease liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current 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Restructuring Reserve, Foreign Currency Translation Gain (Loss) 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 Business Acquisition [Line Items] Business Acquisition [Line Items] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Weighted average discount rate (percentage) Operating Lease, Weighted Average Discount Rate, Percent Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Award Date, All years excluding 2018 Award Date, All Years Excluding 2018 [Member] Award Date, Prior to 2018 [Member] Accounts receivable, net of allowance for credit losses Accounts Receivable, after Allowance for Credit Loss, Current Total future minimum lease payments Lessee, Operating Lease, 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liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Change in net balance Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Basic (USD per share) Earnings Per Share, Basic 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 Other receivables Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Other Receivables Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Other Receivables Number of operating segments Number of Operating Segments Expected life Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Other liabilities Other Current Liabilities [Member] Weighted average remaining lease term (in years) Operating Lease, Weighted Average Remaining Lease Term Gain on re-measurement of financial assets Increase (Decrease) in Equity Securities, FV-NI Symphony CRN Holdings LLC, Symphony [Member] CRN Holdings LLC, Symphony [Member] Amortization of government grants Recovery of Amortization of Government Grants Amortization of Government Grants. 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, 52,508,751 shares issued and outstanding at March 31, 2020 and 53,622,206 shares issued and outstanding at December 31, 2019 Common Stock, Value, Issued Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Deferred tax asset Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets 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 Business combination fair value increase to accounts receivable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Purchase of available for sale investments Payments to Acquire Debt Securities, Available-for-sale Business combination fair value adjustment increase to other liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Liabilities 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. 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 Total comprehensive income Temporary Equity, Comprehensive Income (Loss) Temporary Equity, Comprehensive Income (Loss) 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 and Redeemable Noncontrolling Interest 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 Noncontrolling interest adjustment to redemption amount Redeemable Noncontrolling Interest, Accretion To Redemption Value Redeemable Noncontrolling Interest, Accretion To Redemption Value 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] Business combination fair value decrease to unbilled revenue Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Unbilled Revenue Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Unbilled Revenue 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 in other non-current assets Increase (Decrease) in Other Noncurrent Assets Property, plant and equipment, net Property, Plant and Equipment, Net Provision for income taxes Income Tax Expense (Benefit), Excluding Other Tax Expense (Benefit) Income Tax Expense (Benefit), Excluding Other Tax Expense (Benefit) Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Share-based awards Share-based Payment Arrangement [Text Block] Interest Rate Hedge Interest Rate Contract [Member] 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 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 Goodwill Goodwill Disclosure [Text Block] Net income per Ordinary Share attributable to the Group (note 11): Earnings Per Share [Abstract] Restructuring Restructuring charge 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) 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] 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] 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] 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 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] Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Restructuring reserve, current Restructuring Reserve, Current Buyback Program Buyback Program [Member] Buyback Program [Member] Equity method investment, ownership percentage (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 Loss on settlement of derivative Gain (Loss) on Sale of Derivatives 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] 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 Change in unearned revenue (payments on account) (percentage) 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 (in years) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Allowance for credit losses/doubtful debts 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] Redeemable noncontrolling interest Beginning balance Ending balance Redeemable Noncontrolling Interest, Equity, Carrying Amount 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