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Items] Finite-Lived Intangible Assets [Line Items] Forfeited (in USD per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Standard & Poor's, BB Rating Standard & Poor's, BB Rating [Member] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Indefinite-Lived Intangible Assets [Line Items] Indefinite-Lived Intangible Assets [Line Items] Rest of World Rest of World [Member] Rest of World [Member] Non-current bank credit lines, loan facilities and notes Long-Term Line of Credit, Noncurrent Acquisition-related gain Business Combination, Bargain Purchase, Gain Recognized, Amount Options Outstanding Number of Shares  Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding [Roll Forward] Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Basic (in USD per share) Earnings Per Share, Basic Cash acquired from acquisition Cash Acquired from Acquisition Standard & Poor's, BBB Rating Standard & Poor's, BBB Rating [Member] Total derivatives designated as hedging instruments Designated as Hedging Instrument [Member] 2028 and thereafter Long-Term Debt, Maturity, Year Four And Thereafter Long-Term Debt, Maturity, Year Four And Thereafter Personnel related liabilities Personnel Related Liabilities [Member] Personnel Related Liabilities Senior Secured Notes (the "2029 Notes")* Senior Secured Notes Due May 2029 [Member] Senior Secured Notes Due May 2029 Total current liabilities Liabilities, Current Weighted average number of Ordinary Shares outstanding (note 13): Weighted Average Number of Shares Outstanding, Diluted [Abstract] Share Repurchase Program Share Repurchase Program [Axis] Total Assets Assets Senior Secured Notes (the "2034 Notes")* Senior Secured Notes Due May 2034 [Member] Senior Secured Notes - 2034 Granted (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period Income taxes receivable Income Taxes Receivable, Current Facility liability Facility Liability [Member] Facility Liability Fair Value, Assets Measured on Recurring Basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Purchase of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Non-current other liabilities Other Liabilities, Noncurrent Patient database Database Rights [Member] Direct costs (excluding depreciation and amortization) Cost of Goods and Service, Excluding Depreciation, Depletion, and Amortization Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Commitments and contingencies Commitments and Contingencies 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] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Year 2 Lessee, Operating Lease, Liability, to be Paid, Year Two 2025 Long-Term Debt, Maturity, Year One Share-based awards Share-Based Payment Arrangement [Text Block] Derivative Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Derivative Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Schedule of Distribution of Revenue by Geographical Area Revenue from External Customers by Geographic Areas [Table Text Block] Buyback Program Buyback Program [Member] Buyback Program [Member] Debt Instrument [Axis] Debt Instrument [Axis] Summary of Stock Option Activity Share-Based Payment Arrangement, Option, Activity [Table Text Block] Total Share-Based Payment Arrangement, Expense Net income Net Income (Loss) Attributable to Parent Granted (in USD per share) Share-Based Compensation Arrangements by Share-Based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Forfeited (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period Business Acquisition [Axis] Business Acquisition [Axis] Schedule of Finite-Lived Intangible Assets Schedule of Finite-Lived Intangible Assets [Table Text Block] 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] Liabilities: Liabilities, Fair Value Disclosure [Abstract] Revenue Accounts receivable, unbilled revenue (contract assets) and unearned revenue or payments on account (contract liabilities) Revenue from Contract with Customer [Text Block] Document Fiscal Year Focus Document Fiscal Year Focus Income Statement [Abstract] Income Statement [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Effect of exchange rate movements on cash Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Continuing Operations Severance costs Severance Costs Total assets Total assets Assets, Fair Value Disclosure Weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Document Period End Date Document Period End Date Onerous contract provision Loss Contingency Accrual, Provision Other non-cash items Other Noncash Income (Expense) Base rate Base Rate [Member] Senior Secured Notes (the "2027 Notes")* Senior Secured Notes Due May 2027 [Member] Senior Secured Notes Due May 2027 Total future minimum lease payments Lessee, Operating Lease, Liability, to be Paid 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 Variable Rate Component [Domain] Variable Rate Component [Domain] Variable Rate Component [Domain] Goodwill at fair value Goodwill, Fair Value Disclosure Year 3 Lessee, Operating Lease, Liability, to be Paid, Year Three 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 Prepayments and other current assets Prepaid Expense and Other Assets, Current Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Customer [Domain] Customer [Domain] Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings [Member] Amortization of financing costs and debt discount Amortization of Debt Issuance Costs 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 Schedule of Segment Reporting Information, by Segment [Table] Schedule of Revenues from External Customers and Long-Lived Assets [Table] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Change in unbilled services (unbilled revenue) Increase in unbilled services (unbilled revenue) Increase (Decrease) In Contract With Customer, Asset, Gross, Current Increase (Decrease) In Contract With Customer, Asset, Gross, Current Schedule of Components of Income Tax Expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Restructuring reserve Opening provision Closing provision Restructuring Reserve Revenue, remaining performance obligation Revenue, Remaining Performance Obligation, Amount Income Statement Location [Domain] Statement of Income Location, Balance [Domain] Costs and expenses: Cost of Revenue [Abstract] Other Liabilities