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Effects of change in tax rates Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Benefits paid Defined Benefit Plan, Benefit Obligation, Benefits Paid Foreign currency exchange rate changes Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss) Purchase of property, plant and equipment Payments to Acquire Property, Plant, and Equipment Other liabilities (Note 8) Other liabilities Other Liabilities, Current Change in unbilled receivables Increase (Decrease) In Contract with Customer, Asset, Gross, Current Increase (Decrease) In Contract with Customer, Asset, Gross, Current Employer contributions Defined Benefit Plan, Plan Assets, Contributions by Employer Other Rest Of Europe And Other Countries [Member] Rest Of Europe And Other Countries [Member] ICON Development Solutions Limited pension plan ICON Development Solutions Limited Pension Plan [Member] ICON Development Solutions Limited Pension Plan Impact of New Accounting Pronouncements New And Recent Accounting Pronouncements Disclosure [Text Block] New And Recent Accounting Pronouncements Disclosure Total Liabilities  Liabilities Net income Temporary Equity, Net Income Deferred tax assets: Deferred Tax Assets, Net [Abstract] 2022 Finite-Lived Intangible Asset, Expected Amortization, Year One Granted (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Vested and exercisable at end of period (USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price Income from operations, excluding restructuring, transaction and integration costs Income (Loss) From Operations, Excluding Restructuring, Transaction And Integration Costs Income (Loss) From Operations, Excluding Restructuring, Transaction And Integration Costs U.S. federal net operating loss carry forwards Operating Loss Carryforwards Ordinary shares, shares issued (in shares) Beginning balance (in shares) Ending balance (in shares) Common Stock, Shares, Issued Range 1 Range 1 [Member] Range 1 [Member] Summary of Assumptions Used in Calculating Pension Benefit Obligations Defined Benefit Plan, Assumptions [Table Text Block] Entity Address, City or Town Entity Address, City or Town Loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Schedule of Annual Benefit Payments which Reflect Expected Future Service Schedule of Expected Benefit Payments [Table Text Block] Schedule of Accumulated Other Comprehensive Income Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Share price of equity instruments other than options, vested in period (in USD per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Per Share Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Per Share Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Measurement Frequency [Domain] Measurement Frequency [Domain] Interest expense (Note 24) Total financing costs Interest Expense Increase related to acquired tax positions Unrecognized Tax Benefits, Increase Resulting from Acquisition Deferred tax (benefit)/expense: Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] 44561 Expected rate of return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Net assets acquired Net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year One Retained earnings Retained Earnings (Accumulated Deficit) Shares expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Equity Method Investments Equity Method Investments [Table Text Block] Redeemable Noncontrolling Interest Redeemable Noncontrolling Interest [Member] Redeemable Noncontrolling Interest [Member] Entity Filler Category Entity Filer Category Risk free interest rate (in percent) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Order backlog Order or Production Backlog [Member] Percentage of outstanding shares authorized Share-Based Compensation Arrangement By Share-Based Payment Award, Percentage Of Outstanding Shares Authorized Share-Based Compensation Arrangement By Share-Based Payment Award, Percentage Of Outstanding Shares Authorized Number of employees Entity Number of Employees Repayment of credit lines and facilities Repayments of Lines of Credit Proceeds from the exercise of equity compensation Total proceeds from exercise of stock options by employees Proceeds from Stock Options Exercised Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Utilization Restructuring Reserve Utilizations Restructuring Reserve Utilizations Amendment Flag Amendment Flag Proceeds from issuance of long-term debt Proceeds from Issuance of Long-term Debt Equity Components [Axis] Equity Components [Axis] Fair Value Disclosures [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Actual plan asset allocations Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage Carrying value of inventory Inventory, Net 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] Canceled/expired (USD per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price Thereafter Lessee, Operating Lease, Liability, to be Paid, after Year Five Series B notes Senior B Notes [Member] Senior B Notes Net change in the total valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount  Total Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax Accounts Receivable, Allowance for Credit Loss [Roll Forward] Accounts Receivable, Allowance for Credit Loss [Roll Forward] Transferred balances Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan PRA Health Sciences, Inc PRA Health Sciences, Inc [Member] PRA Health Sciences, Inc Issue of restricted share units / performance share units (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Motor vehicles Vehicles [Member] Statement [Line Items] Statement [Line Items] Other net assets Increase (Decrease) in Other Current Liabilities Non-current bank credit lines and loan facilities (Note 24) Long-term Line of Credit, Noncurrent Unearned revenue Increase (Decrease) in Contract with Customer, Liability Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Issue of restricted share units / performance share units Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Fair Value Total Debt Securities, Available-for-sale Other Other Countries [Member] Other Countries [Member] Contingent consideration, range of outcomes, maximum value Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Restructuring Costs Associated with Exit or Disposal Activity or Restructuring [Policy Text Block] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Tax Period [Axis] Tax Period [Axis] Percentage of option price for fair value of ordinary share (in percent) Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Property, Plant and Equipment, Type [Axis] Long-Lived Tangible Asset [Axis] Valuation allowance for deferred tax assets Valuation allowance for deferred tax assets Deferred Tax Assets, Valuation Allowance Customer [Axis] Customer [Axis] Current bank credit lines and loan facilities (Note 24) Line of Credit, Current Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Variable rate component one Variable Rate Component One [Member] Variable Rate Component One Employee Benefits Retirement Benefits [Text Block] Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Change in advance payments netted against unbilled contracts receivable Increase in advance payments netted against unbilled contracts receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Change in Advance Payments Netted Against Unbilled Contracts Receivable Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Operating lease right-of-use assets 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 Document Annual Report Document Annual Report Payments to settle debt obligations that existed prior to acquisition date Business Combination, Consideration Transferred, Payments To Settle Debt Obligations That Existed Prior To Acquisition Date Business Combination, Consideration Transferred, Payments To Settle Debt Obligations That Existed Prior To Acquisition Date Total Assets Assets Amortization of gain on interest rate hedge Accumulated Other Comprehensive Income (Loss), Amortization Of Cash Flow Hedges, Effect Net of Tax Accumulated Other Comprehensive Income (Loss), Amortization Of Cash Flow Hedges, Effect Net of Tax Plan Name [Domain] Plan Name [Domain] 2026 and thereafter Long-Term Debt, Maturity, Year Five And Thereafter Long-Term Debt, Maturity, Year Five And Thereafter Basis spread on variable rate, step down percentage (in percent) Debt Instrument, Basis Spread On Variable Rate, Step Down Percentage Debt Instrument, Basis Spread On Variable Rate, Step Down Percentage Cash paid for interest Interest Paid, Excluding Capitalized Interest, Operating Activities Geographical [Domain] Geographical [Domain] Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Derivative Contract [Domain] Derivative Contract [Domain] Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Table] Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Table] Accounts receivable adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Receivables Assumed through business combination (USD per share) Share-Based Compensation Arrangements By Share-Based Payment Award, Options, Assumed In Business Combination, Weighted Average Exercise Price Share-Based Compensation Arrangements By Share-Based Payment Award, Options, Assumed In Business Combination, Weighted Average Exercise Price 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 Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Tax credit carry forward Tax Credit Carryforward, Amount Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Office furniture and fixtures Furniture and Fixtures [Member] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Subsequent Event [Line Items] Subsequent Event [Line Items] Other liabilities adjustment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Liabilities Currency impact of long-term funding 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 Effect of dilutive share options outstanding (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Service cost Defined Benefit Plan, Service Cost Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Cash paid for income taxes (net of refunds) Income Taxes Paid, Net Stock Option and Award Plans Stock Option And Award Plans [Member] Stock Option And Award Plans [Member] Interest receivable, related party loans Interest Receivable, Related Party Loans Interest Receivable, Related Party Loans Retirement period used as a basis to estimate expected cash flows Expected Retirement period Expected Retirement period Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Total comprehensive income attributable to the Group Total comprehensive income attributable to the Group Comprehensive Income (Loss), Net of Tax, Attributable to Parent Workforce reductions Employee Severance [Member] Contract with customer, liability Contract with Customer, Liability Other comprehensive income, net of tax Other comprehensive income   Other Comprehensive Income (Loss), Net of Tax [Abstract] Replacement share-based awards issued to acquiree employees APIC, Share-based Payment Arrangement, Other, Increase for Cost Recognition Increase (decrease) in revenue from contracts with customers Increase (Decrease) Revenue From Contracts With Customers Increase (Decrease) Revenue From Contracts With Customers Debt Issuance Costs Debt, Policy [Policy Text Block] Deferred tax liability (Note 14) Deferred Income Tax Liabilities, Net Other Noncurrent Liabilities Other Noncurrent Liabilities [Member] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Fixed assets Property, plant and equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Corporate Bonds (including 50% high yield bonds) Corporate Debt Securities [Member] Schedule of Reconciliation of Consolidated Reported Provision for Income Taxes and Statutory Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Exercise of call option on noncontrolling interest shares Redeemable Noncontrolling Interest, Accretion To Redemption Value Redeemable Noncontrolling Interest, Accretion To Redemption Value Accounts payable Accounts Payable, Current Provision for income taxes Income Tax Benefit [Member] Income Tax Benefit [Member] Fixed asset impairment Impairment, Long-Lived Asset, Held-for-Use Depreciation and amortization expense Depreciation, Depletion and Amortization Accumulated Other Comprehensive Income AOCI Attributable to Parent [Member] Schedule of Restructuring Charges Restructuring and Related Costs [Table Text Block] Ordinary shares, shares outstanding (in shares) Common Stock, Shares, Outstanding Financial Instruments [Domain] Financial Instruments [Domain] Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Award date, 2018 and thereafter Award Date, 2018 [Member] Award Date, 2018 [Member] Direct costs (excluding depreciation and amortization) Cost of Goods and Services Sold Other non-current liabilities Other Sundry Liabilities, Noncurrent Restructuring and Related Activities [Abstract] Restructuring and Related Activities [Abstract] Ordinary shares issued in respect of certain RSUs previously awarded by the Company (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures Entity Central Index Key Entity Central Index Key Settlement Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement Net operating lease costs Net operating lease costs Lease, Cost Maximum Range Exercise Price (USD per share) Share-based Payment Arrangement, Option, Exercise Price Range, Upper Range Limit Decrease related to prior year tax positions Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Customer relationships Customer Relationships [Member] Other Other Customers [Member] Other Customers [Member] Actuarial (gain)/loss - benefit obligation Defined Benefit Plan, Actuarial Losses to be Amortized and Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Pension Cost Defined Benefit Plan, Actuarial Losses to be Amortized and Reclassified from Accumulated Other Comprehensive Income (Loss) to Net Pension Cost Office furniture and fixtures Furniture and Fixtures, Gross Accounts Receivable, after Allowance for Credit Loss [Abstract] Accounts Receivable, after Allowance for Credit Loss [Abstract] Property, Plant and Equipment, net Property, Plant and Equipment Disclosure [Text Block] Non-current other liabilities (Note 9) Non-current other liabilities (Note 9) Other Liabilities, Noncurrent Benefit obligation Benefit obligation at beginning of year   Benefit obligation at end of year Defined Benefit Plan, Benefit Obligation Amortization of loss on interest rate hedge Accumulated Other Comprehensive Loss, Amortization Of Cash Flow Hedges, Effect Net of Tax Accumulated Other Comprehensive Loss, Amortization Of Cash Flow Hedges, Effect Net of Tax Computer equipment and software Capitalized Computer Software, Gross Schedule of Long-term Debt Instruments Schedule of Long-term Debt Instruments [Table Text Block] Debt Instrument [Line Items] Debt Instrument [Line Items] Trading Symbol Trading Symbol Business Acquisition, Pro Forma Revenue Business Acquisition, Pro Forma Revenue Short term government grants Capital Government Grants Current Capital Government Grants Current Summary 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] Related Party Transaction [Domain] Related Party Transaction [Domain] Other Deferred Tax Liabilities, Other Realized gain on interest rate hedge AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax Laboratory equipment   Equipment [Member] Other current liabilities Other Current Liabilities [Member] Sale of available for sale investments Sales and maturities Proceeds from Sale of Debt Securities, Available-for-sale Plan participants' contributions Defined Benefit Plan, Plan Assets, Contributions by Plan Participant Top client Top Client [Member] Top Client [Member] Total long-term debt, net Long-term Debt, Excluding Current Maturities Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Axis] Repayment of term loan obligations and accrued interest Business Combination, Consideration Transferred, Liabilities Incurred Net income per Ordinary Share attributable to the Group (including NCI redemption adjustment): Earnings Per Share, Basic and Diluted [Abstract] Settlement Defined Benefit Plan, Plan Assets, Payment for Settlement Net Income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Payables and Accruals [Abstract] Payables and Accruals [Abstract] Cash and cash equivalents at beginning of year Cash and cash equivalents at end of year Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Ordinary shares, par value (in EUR per share) Common Stock, Par or Stated Value Per Share Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Current Assets: Assets, Current [Abstract] Total cost Finite-Lived Intangible Assets, Gross Settlement of interest rate hedge Payments for Hedge, Financing Activities Schedule of Finite-Lived Intangible Assets Schedule of Finite-Lived Intangible Assets [Table Text Block] Impairment of Long-Lived Assets Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Federal NOL's Internal Revenue Service (IRS) [Member] Exercise of share options (in shares) Exercised (in shares) Options exercised by employees (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Exercise Price Range [Domain] Exercise Price Range [Domain] Buyback program Buyback Program [Member] Buyback Program [Member] Amounts used during the year Accounts Receivable, Allowance for Credit Loss, Writeoff Share Capital Stockholders' Equity Note Disclosure [Text Block] Lapse of statute of limitations Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations 2026 Finite-Lived Intangible Asset, Expected Amortization, Year Five Disclosure of Fair Value of Financial Instruments Fair Value of Financial Instruments, Policy [Policy Text Block] Auditor firm ID Auditor Firm ID Investment in equity method investments Payments to Acquire Equity Method Investments Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Changes in operating assets and liabilities, net of acquired assets and assumed liabilities: Increase (Decrease) in Operating Capital [Abstract] Effective interest rate (in percent) Debt Instrument, Interest Rate During Period 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 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 (net of cash acquired) Fair value of cash consideration Payments to Acquire Businesses, Gross Rest of World Other Geographic Locations [Member] Other Geographic Locations [Member] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Foreign exchange movement Goodwill, Foreign Currency Translation Gain (Loss) Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination [Table Text Block] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Building Buildings and Improvements, Gross Long-lived assets Long-Lived Assets Change in unearned revenue (payments on account) (in percent) Increase (Decrease) In Contract With Customer, Liability, Gross, Current, Percent Increase (Decrease) In Contract with Customer, Liability, Gross, Current, Percent Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 2013 Legacy PRA Plan 2013 Legacy PRA Plan [Member] 2013 Legacy PRA Plan Non-current other liabilities (note 9) Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position Goodwill Deferred Tax Liabilities, Goodwill Tax Year 2036-2040 2036-2040 Tax Year 2036-2040 [Member] Tax Year 2037 [Member] Taxes at Irish statutory rate of 12.5% (2021:12.5%; 2020:12.5%) Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Research and Development Credits Research and Development Credits [Policy Text Block] Research and Development Credits [Policy Text Block] Number of Shares (in shares) Share-based Payment Arrangement, Option, Exercise Price Range, Shares Outstanding Direct costs Cost of Sales [Member] Income taxes payable (Note 14) Accrued Income Taxes, Current Inventory Inventory, Policy [Policy Text Block] Finite lived intangible assets, amortization expense, net, total Finite Lived Intangible Assets, Amortization Expense, Net Finite Lived Intangible Assets, Amortization Expense, Net Description of business Nature of Operations [Text Block] Less than 1 year Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Debt discount paid Debt Instrument, Unamortized Discount Option Plans 2008 Option Plans 2008 [Member] Option Plans 2008 [Member] Non-current bank credit lines and loan facilities Long-term Debt [Text Block] Transaction and one time financing costs Transaction And One Time Financing Costs Transaction And One Time Financing Costs Other Liabilities