Reinsurance Disclosures [Abstract] Liability for unpaid claims and claims adjustment expense amount ceded Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded Risks and Uncertainties [Abstract] Reinsurance [Table] Reinsurance [Table] Reinsurance [Table] Credit Rating, AM Best [Axis] Credit Rating, AM Best [Axis] Credit Rating, AM Best [Domain] Credit Rating, AM Best [Domain] AM Best, A plus Rating AM Best, A+ Rating [Member] AM Best, A Rating AM Best, A Rating [Member] Credit Rating, Standard & Poor's [Axis] Credit Rating, Standard & Poor's [Axis] External Credit Rating, Standard & Poor's [Domain] External Credit Rating, Standard & Poor's [Domain] Standard & Poor's, A plus Rating Standard & Poor's, A+ Rating [Member] Standard & Poor's, AA- Rating Standard & Poor's, AA- Rating [Member] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Munich Re Munich Re [Member] Munich Re [Member] Lloyds Lloyds [Member] Lloyds. Everest Re Everest Re [Member] Everest Re [Member] Reinsurance [Line Items] Reinsurance [Line Items] Reinsurance [Line Items] Unpaid losses Reinsurance Recoverables, Including Reinsurance Premium Paid Reinsurance Recoverables, Uncollateralized Reinsurance Recoverables, Uncollateralized Reinsurance Recoverables, Uncollateralized Concentration Risk, Percentage Concentration Risk, Percentage Underwriting premiums Premiums Receivable, Net Due for settlement Premiums Receivable Non Current Premiums receivable non current. Premiums Receivable, Allowance for Doubtful Accounts Premiums Receivable, Allowance for Doubtful Accounts Allowable holdings of a single issue or issuer, percentage Percentage Of Allowable Holdings Of A Single Issuer Or Issuer, Minimum Percentage Of Allowable Holdings Of A Single Issuer Or Issuer, Minimum Percentage of written premium major broker accounted minimum Percentage Of Written Premium Major Broker Accounted Minimum Percentage of written premium major broker accounted minimum. Derivative Instruments and Hedging Activities Disclosure [Abstract] Fair Value of Derivative Instruments Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Gain/(Loss) Recognized in Income on Derivative Derivative Instruments, Gain (Loss) [Table Text Block] Quarterly Financial Information Disclosure [Abstract] Unaudited Quarterly Financial Data Quarterly Financial Information [Text Block] Equity [Abstract] Schedule of Accumulated Other Comprehensive Income Reclassification Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Date [Axis] Award Date [Axis] Award Date [Domain] Award Date [Domain] March 25, 2015 (1) March Twenty Five Thousand Fifteen Grant Date One [Member] March Twenty Five Thousand Fifteen Grant Date One March 25, 2015 (2) March Twenty Five Thousand Fifteen Grant Date Two [Member] March Twenty Five Thousand Fifteen Grant Date Two March 25, 2016 (1) March 25, 2016 (1) [Member] March 25, 2016 (1) [Member] March 25, 2016 (2) March 25, 2016 (2) [Member] March 25, 2016 [Member] April 28, 2017 April 28, 2017 [Member] April 28, 2017 [Member] April 28, 2017 (2) April 28, 2017 (2) [Member] April 28, 2017 (2) [Member] April 28, 2017 (3) April 28, 2017 (3) [Member] April 28, 2017 (3) [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Employee Stock Purchase Plan Employee Stock Purchase Plan [Member] Employee Stock Purchase Plan [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Per share weighted average fair value (in dollars 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 Risk free interest rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected life (in years) Expected Life Expected life. Share price volatility Share Price Volatility Share price volatility. Premiums written: Premiums Written, Net [Abstract] Direct Direct Premiums Written Assumed Assumed Premiums Written Ceded Ceded Premiums Written Net premiums written Premiums Written, Net Premiums earned: Premiums Earned, Net [Abstract] Direct Direct Premiums Earned Assumed Assumed Premiums Earned Ceded Ceded Premiums Earned Net premiums earned Premiums Earned, Net Insurance losses and loss adjustment expenses: Policyholder Benefits and Claims Incurred, Net [Abstract] Direct Policyholder Benefits and Claims Incurred, Direct Assumed Policyholder Benefits and Claims Incurred, Assumed Ceded Policyholder Benefits and Claims Incurred, Ceded Net insurance losses and loss adjustment expenses Policyholder Benefits and Claims Incurred, Net Insurance [Abstract] Reserves for Losses and Adjustment Expenses Reserves For Loss And Adjustment Expenses [Text Block] Reserves for Loss and Adjustment Expenses. 2013 Share Incentive Plan Two Thousand And Thirteen Share Incentive Plan [Member] Two Thousand And Thirteen Share Incentive Plan [Member] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Performance Shares Performance Shares [Member] Phantom Shares Phantom Share Units (PSUs) [Member] Number of shares outstanding, beginning of period (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Number of shares outstanding, end of period (in shares) Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted, weighted average grant date fair value (in dollars per share) Vested, weighted average grant date fair value (in dollars 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, weighted average grant date fair value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share) Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Australia/Asia Asia Pacific [Member] Caribbean Caribbean [Member] Caribbean [Member] Europe Europe [Member] United Kingdom UNITED KINGDOM United States & Canada United States And Canada [Member] United States & Canada. Worldwide excluding United States Non-US [Member] Worldwide including United States Worldwide Including United States [Member] Worldwide including United States. Others Other Countries [Member] Other Countries. Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Gross written premiums Premiums Written, Gross Deferred Policy Acquisition Costs Deferred Policy Acquisition Costs [Text Block] Earnings Per Share [Abstract] Dividends Payable [Table] Dividends Payable [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] 5.950% Preference Shares Series D Preferred Stock [Member] 5.625% Preference Shares Series E Preferred Stock [Member] Dividends Payable [Line Items] Dividends Payable [Line Items] Dividend, in usd per share Dividends Payable, Amount Per Share Payable Date Dividends Payable, Date to be Paid Record Date Dividends Payable, Date of Record Preference shares, rate Preferred Stock, Dividend Rate, Percentage SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract] Schedule III - Supplementary Insurance Information SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Disclosure [Text Block] Reinsurance Reinsurance [Text Block] Investment Holdings [Table] Investment Holdings [Table] Aon Corporation Aon Corporation [Member] AON Corporation [Member] Marsh & McLennan Companies, Inc. Marsh And Mc Lennan Companies Inc [Member] Marsh & McLennan Companies, Inc. Willis Group Holdings, Ltd. Willis Group Holdings Ltd [Member] Willis Group Holdings, Ltd. Others Others [Member] Others [Member] Investment Holdings [Line Items] Investment Holdings [Line Items] Gross written premiums, percentage Premiums Written Gross Percentage Premiums written gross percentage. Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Summary of Quarterly Financial Data Quarterly Financial Information [Table Text Block] Debt Disclosure [Abstract] Less than 1 year Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 1-3 years Long-term Debt, Maturing in Years Two and Three 3-5 years Long-term Debt, Maturing in Years Four and Five More than 5 years Long-term Debt, Maturities, Repayments of Principal after Year Five Total Long-term Debt and Capital Lease Obligations Document Document And Entity Information [Abstract] Document Document and Entity Information [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Current Reporting Status Entity Current Reporting Status Entity Voluntary Filers Entity Voluntary Filers Entity Filer Category Entity Filer Category Entity Emerging Growth Company Entity Emerging Growth Company Entity Small Business Entity Small Business Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Public Float Entity Public Float Goodwill and Intangible Assets Disclosure [Abstract] Summary of Changes in Intangible Assets Schedule of Indefinite-Lived Intangible Assets [Table Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] Schedule V - Valuation and Qualifying Accounts SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Provision for losses and LAE, beginning balance Liability for Claims and Claims Adjustment Expense Less reinsurance recoverable Reinsurance Recoverables Adjustment Reinsurance Recoverables Adjustment. Net losses and LAE reserves, beginning balance Liability for Unpaid Claims and Claims Adjustment Expense, Net Net loss and LAE expenses (disposed) Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed Of Liability for unpaid claims and claims adjustment expense amount disposed of. Movement in provision for losses and LAE for claims incurred: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Current year Current Year Claims and Claims Adjustment Expense Prior years Prior Year Claims and Claims Adjustment Expense Total incurred Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Losses and LAE payments for claims incurred: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Current year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Foreign exchange losses/(gains) Liability For Unpaid Claims And Claims Adjustment Expense Foreign Exchange Gains (Losses) Liability for unpaid claims and claims adjustment expense foreign exchange gains losses. Net losses and LAE reserves, ending balance Plus reinsurance recoverable on unpaid losses at the end of the year Provision for losses and LAE at the end of the year Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Additional paid-in capital Additional Paid-in Capital [Member] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] Open Markets Open Markets [Member] Open Markets [Member] Repurchase Program 2017 Repurchase Program 2017 [Member] Repurchase Program 2017 [Member] Preference Shares Preferred Stock [Member] 7.401% Preference Shares Series B Preferred Stock [Member] 7.250% Preference Shares Series C Preferred Stock [Member] Class of Stock [Line Items] Class of Stock [Line Items] Additional paid in capital Additional Paid in Capital Aggregate liquidation preferences Aggregate Liquidation Preferences Aggregate liquidation preferences. Preferred stock issuance cost Preferred Stock Issuance Cost Preferred stock issuance cost. Stock repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Stock repurchased during period, shares Stock Repurchased and Retired During Period, Shares Stock repurchased during period, value Stock Repurchased and Retired During Period, Value Stock repurchased during period, average price per share Stock Repurchased And Retired During Period Average Price Per Share Stock repurchased and retired during period average price per share. Stock repurchase program, remaining authorized repurchase amount Stock Repurchase Program, Remaining Authorized Repurchase Amount Preference shares, issued Preferred Stock, Shares Issued Preferred Stock, Redemption Amount Preferred Stock, Redemption Amount Redemption price per share Preferred Stock, Liquidation Preference Per Share Preference shares redeemed and cancelled Stock Redeemed or Called During Period, Value Preference shares liquidation preference, value Preferred Stock, Liquidation Preference, Value Summary of Restricted Share Units by Grant Year Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Summary of Restricted Share Unit Activity under 2013 Share Incentive Plan Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] Summary of Performance and Phantom Shares by Grant Year Share-based Compensation, Stock Options, Activity [Table Text Block] Summary of Performance and Phantom Share Activity under 2013 Share Incentive Plan Share-based Compensation, Performance Shares Award Outstanding Activity [Table Text Block] Summary of Vesting Criteria Schedule of Share-Based Compensation, Vesting Criteria [Table Text Block] Schedule of Share-Based Compensation, Vesting Criteria [Table Text Block] Fair Value of Employee Options Granted under ESPP Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of Non-Employee Director Restricted Share Units by Grant Year Schedule of Share-based Compensation, Nonemployee Director Stock Award Plan, Activity [Table Text Block] Summary of Restricted Share Unit, Non Employee Directors, under 2013 Share Incentive Plan Schedule of Share-based Compensation, Restricted Stock Units Award Activity, Non-Employee Directors [Table Text Block] Schedule of Share-based Compensation, Restricted Stock Units Award Activity, Non-Employee Directors [Table Text Block] Concentrations of Credit Risk Concentration Risk Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Subsequent Event [Table] Subsequent Event [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum Maximum [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Business acquisition, share price Business Acquisition, Share Price Preferred Stock, Dividend Rate, Percentage Loss from Catastrophes Loss from Catastrophes Condensed Financial Information Disclosure [Abstract] Schedule II - Condensed Financial Information of Registrant Condensed Financial Information of Parent Company Only Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Premiums receivable from underwriting activities Premiums Receivable [Member] Reinsurance Reinsurance [Member] Reinsurance. SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] Balance at Beginning of Year SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Charged to Costs and Expenses SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Cost and Expense Charged to Other Accounts SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account Deductions SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction Balance at End of Year Credit Facility and Long-term Debt Debt Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] U.S. government US Treasury Securities [Member] U.S. agency US Government Agencies Debt Securities [Member] Municipal Municipal Bonds [Member] Corporate Corporate Debt Securities [Member] Non-U.S. government-backed corporate Debt Security, Corporate, Non-US [Member] Non-U.S government Debt Security, Government, Non-US [Member] Asset-backed Asset-backed Securities [Member] Agency mortgage-backed Collateralized Mortgage Backed Securities [Member] Fixed Income Securities Fixed Income Securities [Member] Equity securities Equity Securities [Member] 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] Fair Market Value Determined using Prices from Index Providers Fair Market Value Determined Using Prices From Index Providers Fair market value determined using prices from index providers. % of Total Fair Value from Index Providers Fair Value By Security Type Percentage of total fair value by security type. Financial Assets Measured on Recurring Basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Pricing Sources Used in Pricing Fixed Income Investments Fixed Income Investments Pricing Sources [Table Text Block] Fixed Income Investments Pricing Sources Summary of Securities Priced Using Pricing Information from Index Providers Schedule Of Securities Priced Using Pricing Information From Index Providers [Table Text Block] Schedule Of Securities Priced Using Pricing Information From Index Providers Fair Value Inputs, Assets, Quantitative Information Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Investments, Debt and Equity Securities [Abstract] Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Investment, Name [Axis] Investment, Name [Axis] Investment, Name [Domain] Investment, Name [Domain] MVI MVI [Member] MVI Chaspark Chaspark Maritime Holdings Ltd [Member] Chaspark Maritime Holdings Ltd [Member] Bene Bene Assicurazioni [Member] Bene Assicurazioni [Member] Digital Re Digital Risk [Member] Digital Risk Resources LLP[Member] Crop Re Crop Re [Member] Crop Re Services LLC Schedule Of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Equity Method Investment, Aggregate Cost [Roll Forward] Equity Method Investment, Aggregate Cost [Roll Forward] Equity Method Investment, Aggregate Cost [Roll Forward] Opening undistributed value of investment, beginning balance Equity Method Investment, Aggregate Cost Investment in the period Payments for (Proceeds from) Businesses and Interest in Affiliates Goodwill Goodwill, Acquired During Period Proceeds from sale of equity method investments Proceeds from Sale of Equity Method Investments Unrealized gain/(loss) for the period Equity Method Investment, Unrealized Intercompany Profit (Loss) Not Eliminated, Amount Closing value of investment, ending balance Intangible Assets and Goodwill Intangible Assets Disclosure [Text Block] Schedule Of Finite And Indefinite Lived Intangible Assets By Major Class [Table] Schedule Of Finite And Indefinite Lived Intangible Assets By Major Class [Table] Schedule Of Finite And Indefinite Lived Intangible Assets By Major Class [Table] Disposal Group Name [Axis] Disposal Group Name [Axis] Disposal Group Name [Domain] Disposal Group Name [Domain] AgriLogic AgriLogic [Member] AgriLogic [Member] AgriLogic Consulting, LLC AgriLogic Consulting, LLC [Member] AgriLogic Consulting, LLC [Member] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] AAIC, Aspen Specialty, AIUK [Member] AAIC, Aspen Specialty, AIUK [Member] AAIC, Aspen Specialty, AIUK [Member] Liberty Specialty Liberty Specialty [Member] Liberty Specialty Aspen U.K. Aspen United Kingdom [Member] Aspen United Kingdom. Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Trademarks Trademarks [Member] Aspen trademark Aspen Trademark [Member] Aspen Trademark [Member] Insurance Licenses Licensing Agreements [Member] Agency Relationships Customer Lists [Member] Non-compete Agreements Noncompete Agreements [Member] Value of Business Acquired Other Intangible Assets [Member] Consulting Relationships Customer-Related Intangible Assets [Member] Renewal Rights Contractual Rights [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Blue Waters Blue Waters [Member] Blue Waters [Member] Aspen Specialty Aspen Specialty [Member] Aspen specialty. AAIC Aspen American Insurance Company [Member] Aspen American Insurance Company. Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] Percentage of interests acquired Business Acquisition, Percentage of Voting Interests Acquired Initial purchase price Payments to Acquire Businesses, Gross Business combination, contingent consideration, liability Business Combination, Contingent Consideration, Liability Business combination, contingent consideration, maximum payable Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Business combination, ceding commission Business Combination, Consideration Transferred, Liabilities Incurred Aggregate consideration for sale and purchase agreement Business Combination, Consideration Transferred Value of the asset Intangible Assets, Net (Excluding Goodwill) Indefinite-lived and finite-lived intangible assets acquired Indefinite-lived and Finite-lived Intangible Assets Acquired Indefinite-lived and Finite-lived Intangible Assets Acquired Business combination, office properties and equipments Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Business combination, residual assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Residual Assets BusinessCombination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Residual Assets Business combination, net assets acquired Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Goodwill, acquired Intangible assets acquired eligible tax deductible amount Business Acquisition, Intangible Assets, Expected Tax Deductible Amount Business Acquisition, Intangible Assets, Expected Tax Deductible Amount Goodwill acquired eligible tax deductible amount Business Acquisition, Goodwill, Expected Tax Deductible Amount Business acquisition, eligible tax deduction period Business Acquisition, Eligible Tax Deduction Period Business Acquisition, Eligible Tax Deduction Period Asset amortization period Intangible Assets Amortization Period Intangible assets amortization period. Acquired licenses Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Equity method investment, ownership percentage Equity Method Investment, Ownership Percentage Intangible Assets acquired (disposed) Finite-lived Intangible Assets Acquired (Disposed) Finite-lived Intangible Assets Acquired (Disposed) Goodwill, disposed Goodwill, Sale Related to Sale of Business Unit Goodwill, Sale Related to Sale of Business Unit Goodwill and intangible asset impairment Goodwill and Intangible Asset Impairment Sale of interest in consulting business Business Disposal, Percentage Business Disposal, Percentage Capital Structure Stockholders' Equity Note Disclosure [Text Block] Grants 2016 Grants 2016 [Member] Grants 2016 Grants 2017 Grants 2017 [Member] Grants 2017 [Member] Grants 2018 Grants 2018 [Member] Grants 2018 [Member] Restricted Share Units Restricted Stock Units (RSUs) [Member] Vested (in shares) Forfeited (in shares) Outstanding (in shares) Third party Third Party [Member] Third Party [Member] Measurement Input Type [Axis] Measurement Input Type [Axis] Measurement Input Type [Domain] Measurement Input Type [Domain] Measurement Input, Expected Term [Member] Measurement Input, Expected Term [Member] Lender Name [Axis] Lender Name [Axis] Line of Credit Facility, Lender [Domain] Line of Credit Facility, Lender [Domain] Silverton 2017 Silverton 2017 [Member] Silverton 2017 [Member] Silverton 2016 Silverton 2016 [Member] Silverton 2016 Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Minimum Minimum [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 3 Fair Value, Inputs, Level 3 [Member] Fair value of Loan Notes Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Reserve for losses Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings Contract period Derivative, Term of Contract Initial value of issuance Debt Instrument, Face Amount, Issued By Third Parties Debt Instrument, Face Amount, Issued By Third Parties Amount classified within accrued expenses and other payables Other Accrued Liabilities, Current Non-U.S. government-backed corporate Debt Security Category [Axis] Debt Security Category [Axis] Other than Temporary Impairment, Credit Losses Recognized in Earnings, Categories of Investments [Domain] Other than Temporary Impairment, Credit Losses Recognized in Earnings, Categories of Investments [Domain] Fair Value, Measurements, Fair Value Hierarchy [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Total fixed income securities — Available for sale Fixed income securities — Available for sale Available-for-sale Securities [Member] Short-term investments Short-term Investments [Member] Held for trading financial assets, at fair value Trading Assets, Excluding Debt and Equity Securities [Member] Short-term investments trading at fair value Short Term Investments Trading At Fair Value [Member] Short-term investments trading, at fair value. Equity investments trading Equity Investments Trading At Fair Value [Member] Equity Investments Trading At Fair Value [Member] Real estate fund Real Estate Funds [Member] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Asset Class [Domain] Derivatives at fair value Derivative Financial Instruments, Assets [Member] Liabilities under derivative contracts Derivative Financial Instruments, Liabilities [Member] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Foreign Exchange Contracts Foreign Exchange Contract [Member] Investments, fair value Investments, Fair Value Disclosure Catastrophe bonds trading, at fair value Catastrophe Bonds, Fair Value Disclosure Catastrophe Bonds, Fair Value Disclosure Derivatives at fair value Derivative Asset Liabilities under derivative contracts, fair value Derivative Liability, Fair Value, Gross Liability Loan notes issued by variable interest entities, at fair value Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) Debt Instrument, Accrued Interest and Payables, Fair Value Disclosure Debt Instrument, Accrued Interest and Payables, Fair Value Disclosure Accrued expenses (current liabilities) Total Derivative Assets (Liabilities), at Fair Value, Net Payables and Accruals [Abstract] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Investment, Name [Axis] Investment, Name [Domain] Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Number of investments in VIEs Number Of Investments In Variable Interest Entities Number Of Investments In Variable Interest Entities Summary of Contractual Obligations Under Long-term Debts Schedule of Maturities of Long-term Debt [Table Text Block] Statement [Table] Statement [Table] Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Insurance Insurance [Member] Insurance. Statement [Line Items] Statement [Line Items] Deferred Policy Acquisition Costs SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Deferred Policy Acquisition Cost Net Reserves for Losses and LAE Net Reserves for Unearned Premiums SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Unearned Premium Net earned premium Net Investment Income SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Net Investment Income Losses and LAE Expenses Losses And Loss Adjustment Expenses Losses And Loss Adjustment Expenses Policy Acquisition Expenses Deferred Policy Acquisition Cost Other Expenses Deferred Policy Acquisition Cost Other Expenses Net written premiums General and Administrative Expenses SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information, Other Operating Expense Income Tax Disclosure [Abstract] Income Taxes [Table] Income Taxes [Table] Income Taxes [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Valuation Allowance, Operating Loss Carryforwards SEC Schedule, 12-09, Valuation Allowance, Operating Loss Carryforward [Member] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Forecast Scenario, Forecast [Member] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Axis] Income Tax Authority, Name [Domain] Income Tax Authority, Name [Domain] Internal Revenue Service (IRS) Internal Revenue Service (IRS) [Member] U.K. Her Majesty's Revenue and Customs (HMRC) [Member] Bermuda Office of the Tax Commissioner, Bermuda [Member] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Domain] Other Comprehensive Income Location [Domain] Other Comprehensive Income Other Comprehensive Income (Loss) [Member] Income Taxes [Line Items] Income Taxes [Line Items] Income Taxes [Line Items] United States corporate tax at a rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent U. K. corporate tax rate reduced during the period Corporate Tax Rate Corporate tax rate. Bermuda tax rate Effective Income Tax Rate Reconciliation, Percent Income tax examination, cumulative interest accrued Income Tax Examination, Cumulative Interest Accrued Income Tax Examination, Cumulative Interest Accrued Unrecognized tax benefits Unrecognized Tax Benefits Prior year reductions Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities Accrued interest and penalties Income Tax Examination, Penalties Expense Income tax examination, interest accrued Income Tax Examination, Interest Accrued Net operating loss carryforwards Operating Loss Carryforwards Operating loss carryforwards, subject to expiration date Deferred Tax Assets, Operating Loss Carryforwards, Subject to Expiration Operating loss carryforwards, date of expiration Operating loss carryforwards, date of expiration Operating loss carryforwards, date of expiration Operating loss carryforwards, not subject to expiration Deferred Tax Assets, Operating Loss Carryforwards, Not Subject to Expiration Capital loss carryforwards Deferred Tax Assets, Capital Loss Carryforwards Charitable contribution carryforwards Deferred Tax Assets, Charitable Contribution Carryforwards Less valuation allowance Deferred Tax Assets, Valuation Allowance Increase (decrease) in valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Foreign tax credit carryforwards Deferred Tax Assets, Operating Loss Carryforwards, Foreign US branch profits, subject to tax US branch profits, subject to tax US branch profits, subject to tax Text Block [Abstract] Statutory Requirements and Dividends Restrictions Statutory Requirements And Dividends Restriction [Text Block] Statutory requirements and dividends restriction. 7.401% Preference Shares Authorized share capital: Authorized Share Capital [Abstract] Authorized share capital. Number of ordinary shares Common Stock, Shares Authorized Number of non-voting shares Non Voting Stock Shares Authorized Non voting stock shares authorized. Number of preference shares Preferred Stock, Shares Authorized Ordinary Shares, authorized Common Stock Value Authorized Common stock value authorized. Non-Voting Shares, authorized Non Voting Stock Value Authorized Non voting stock value authorized. Preference Shares, authorized Preferred Stock Value Authorized Preferred stock shares authorized. Total authorized share capital Authorized Share Capital Authorized Share Capital Issued share capital: Issued Share Capital [Abstract] Issued share capital. Ordinary shares, issued Common Stock, Shares, Issued Ordinary shares, value Common Stock, Value, Issued Preference shares, value Preferred Stock, Value, Subscriptions Total issued share capital Issued Share Capital Issued share capital. Segment Reporting Segment Reporting Disclosure [Text Block] Schedule of Liability for Unpaid Claims and Claims Adjustment Expense Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Short-duration Insurance Contracts, Claims Development Short-duration Insurance Contracts, Claims Development [Table Text Block] Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table Text Block] Short-duration Insurance Contracts, Schedule of Historical Claims Duration Short-duration Insurance Contracts, Schedule of Historical Claims Duration [Table Text Block] Accounting Policies [Abstract] History, Organization and Business Combination Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Aspen Holdings Parent Company [Member] Movement in Silverton Loan Notes [Roll Forward] Movement in Silverton Loan Notes [Roll Forward] Movement in Silverton Loan Notes [Roll Forward] Opening balance Notes Payable, Related Parties Total change in fair value for the period Gain (Loss) on Fair Value of Loan Notes, Issued by Variable Interest Entity Gain (Loss) on Fair Value of Loan Notes, Issued by Variable Interest Entity Total distributed in the period Repayment of Notes Receivable from Related Parties Closing balance as at December 31, 2018 Loan notes (long-term liabilities) Other Accrued Liabilities, Noncurrent SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract] Gross Amount Ceded to Other Companies Assumed From Other Companies Net premiums earned Percentage of Amount Assumed to Net SEC Schedule, 12-17, Insurance Companies, Reinsurance, Premium, Percentage Assumed to Net Share-based payments Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Operating loss carryforwards Deferred Tax Assets, Operating Loss Carryforwards Loss reserves Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Loss Reserves Unrealized losses on investments Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross Accrued expenses Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals Unearned premiums Deferred Tax Assets, Unearned Premiums Deferred Tax Assets, Unearned Premiums Deferred policy acquisition costs Deferred Tax Assets (Liabilities), Tax Deferred Expense Deferred Tax Assets (Liabilities), Tax Deferred Expense [Line Items] Office properties and equipment Deferred Tax Assets, Timing Differences On Fixed Assets Deferred Tax Assets, Timing Differences On Fixed Assets Other temporary differences Deferred Tax Assets, Other Total gross deferred tax assets Deferred Tax Assets, Gross Less valuation allowance Net deferred tax assets Deferred Tax Assets, Net of Valuation Allowance Intangible assets (other) Deferred Tax Liabilities, Intangible Assets Deferred policy acquisition costs Deferred Tax Liabilities, Deferred Expense, Deferred Policy Acquisition Cost Quota share losses Deferred tax liabilities, quota share losses Deferred tax liabilities, quota share losses Loss portfolio transfer costs Deferred Tax Liability, Intra-entity Transfer, Asset Other than Inventory Other temporary differences Deferred Tax Liabilities, Other Total gross deferred tax (liabilities) Deferred Tax Liabilities, Gross Deferred Tax Assets, Net Deferred Tax Assets, Net Summary of Investment Income Investment Income [Table Text Block] Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments Realized Gain (Loss) on Investments [Table Text Block] Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments Available-for-sale Securities [Table Text Block] Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities Debt Securities, Trading, and Equity Securities, FV-NI [Table Text Block] Other Investments Schedule of Variable Interest Entities [Table Text Block] Summary of Fixed Maturities Investments Classified by Contractual Maturity Date [Table Text Block] Aggregate Fair Value and Gross Unrealized Loss by Type of Security Schedule of Unrealized Loss on Investments [Table Text Block] Increase in BVPS Share-based Compensation Arrangement by Share-based Award, Entities That Calculate Net Asset Value Per Share, Percent Share-based Compensation Arrangement by Share-based Award, Entities That Calculate Net Asset Value Per Share, Percent Schedule of Trading Securities and Other Trading Assets [Table] Debt Securities, Trading, and Equity Securities, FV-NI [Table] Fixed income securities - trading Trading Fixed Maturity Investments [Member] Trading fixed maturity investments. Catastrophe Bonds Catastrophe Bonds [Member] Catastrophe Bonds [Member] Total - Trading Total - Trading [Member] Total - Trading [Member] Schedule of Trading Securities and Other Trading Items [Line Items] Debt and Equity Securities, FV-NI [Line Items] Gross Unrealized Gains Trading Securities, Accumulated Gross Unrealized Gain, before Tax Trading Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Trading Securities, Accumulated Gross Unrealized Loss, before Tax Trading Securities, Accumulated Gross Unrealized Loss, before Tax Fair Market Value Debt Securities, Trading, and Equity Securities, FV-NI Debt Securities, Trading, Amortized Cost Debt Securities, Trading, Amortized Cost Line of Credit Facility [Table] Line of Credit Facility [Table] Citibank Europe plc Citibank Europe plc [Member] Citibank Europe plc [Member] Senior Notes Senior Notes [Member] Silverton 2015 Silverton 2015 [Member] Silverton 2015 [Member] Silverton 2015, Third-party Funded Silverton 2015, Third-party Funded [Member] Silverton 2015, Third-party Funded Silverton 2017, Third-party Funded Silverton 2017, Third-party Funded [Member] Silverton 2017, Third-party Funded Silverton 2016, Third-party Funded Silverton 2016, Third-party Funded [Member] Silverton 2016, Third-party Funded Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Citibank Europe Citibank Europe [Member] Citibank Europe. Letters of credit / guarantees Letter of Credit [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Silverton Loan Notes Silverton Loan Notes [Member] Silverton Loan Notes [Member] Aspen Holdings Aspen Holdings [Member] Aspen Holdings [Member] Line Of Credit Facility Covenant Compliance [Line Items] Line Of Credit Facility Covenant Compliance [Line Items] Line Of Credit Facility Covenant Compliance [Line Items] Revolving credit facility, credit agreement maximum limit Line of Credit Facility, Maximum Borrowing Capacity Line of credit facility Increasable capacity Line Of Credit Facility Increasable Capacity Line of credit facility Increasable capacity. Line of credit facility borrowings outstanding Long-term Line of Credit Minimum consolidated tangible net worth under credit facility Line Of Credit Facility Tangible Net Worth Restrictions Minimum Line of credit facility tangible net worth restrictions minimum. Percentage of consolidated net income Line Of Credit Facility Tangible Net Worth Net Income Restriction Line of credit facility tangible net worth net income restriction. Percentage of aggregate net cash proceeds from the issuance of capital stock Line Of Credit Facility Tangible Net Worth Cash Proceeds From Issuance Of Capital Stock Restriction Line of credit facility tangible net worth cash proceeds from the issuance of capital stock restriction. Percentage of consolidated leverage ratio permitted Percentage Of Consolidated Leverage Ratio Permitted Percentage of consolidated leverage ratio permitted. Line of Credit Facility, Interest Rate During Period Line of Credit Facility, Interest Rate During Period Outstanding collateralized letters of credit facility Line Of Credit Facility Increase Additional Borrowing Capacity Line of credit facility increase additional borrowing capacity. Loans issued Debt Instrument, Face Amount Debt stated interest rate Debt Instrument, Interest Rate, Stated Percentage Payment for extinguishment of debt Payment for Extinguishment of Debt Payment for Extinguishment of Debt Net proceeds from senior notes Proceeds from Issuance of Senior Long-term Debt Long-term debt repayment Repayments of Long-term Debt Amount of indebtedness subject to default in payment Debt Default Minimum Debt default minimum. Loan notes issued by third parties Notes Issued Maximum exposure to loss Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Annual interest payable Line of Credit Facility, Periodic Payment, Interest Provision for losses and LAE for claims incurred Liability for unpaid claims and claims adjustment expense amount ceded Unpaid claims and claims adjustment expense assumed Unpaid Claims And Claims Adjustment Expense Specialty Reinsurance Contracts Unpaid claims and claims adjustment expense specialty reinsurance contracts. Summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of Company's business segments Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of gross written premiums based on geographical areas Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas [Table Text Block] Condensed Financial Statements [Table] Condensed Financial Statements [Table] Parent [Member] Parent [Member] Parent Company Condensed Financial Statements, Captions [Line Items] Condensed Financial Statements, Captions [Line Items] Cash flows (used in)/from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income (1) (excluding equity in net earnings of subsidiaries) Net Income (Loss) Before Equity In Net Earnings Of Subsidiaries Net Income (Loss) Before Equity In Net Earnings Of Subsidiaries Share-based compensation Share-based Compensation Realized and unrealized losses/(gains) Net Realized And Unrealized Gains Losses Net realized and unrealized (gains) losses Loss on derivative contracts Derivative, Loss on Derivative Increase (Decrease) in Accounts and Other Receivables Increase (Decrease) in Accounts and Other Receivables Change in other assets Change In Other Assets Change In Other Assets Change in accrued expenses and other payables Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Change in intercompany activities Increase Decrease In Intercompany Activities Increase decrease in intercompany activities. Net cash (used in)/generated by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from/(used in) investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds/(purchases) of short term investments Payments to Acquire Short-term Investments Payments to Acquire Marketable Securities Payments to Acquire Marketable Securities Investment in subsidiaries Payments to Acquire Other Investments Investment in long-term debt issued by Silverton Investments In Long-term Debt Issued By Subsidiary Investments In Long-term Debt Issued By Subsidiary Repayment of loan notes issued by Silverton Repayments of Other Long-term Debt Payments for acquisitions and investments, net of cash acquired Net (purchases) of investments, equity method Payments for (Proceeds from) Other Investing Activities Net cash from/(used in) investing activities Net Cash Provided by (Used in) Investing Activities Cash flows (used in)/from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from the issuance of ordinary shares, net of issuance costs Proceeds from Issuance of Common Stock Proceeds from the issuance of preference shares, net of issuance costs Proceeds from Issuance of Preferred Stock and Preference Stock Preference share redemption Payments for Repurchase of Preferred Stock and Preference Stock Ordinary share repurchase Payments for Repurchase of Common Stock Ordinary and preference share dividends paid Payments of Dividends Proceeds from maturity of Eurobond Proceeds From Maturity Of Eurobond Proceeds From Maturity Of Eurobond Eurobond purchased from subsidiary Eurobond Related To Affiliates Eurobond related to affiliates. Realized loss on debt extinguishment Gain (Loss) on Extinguishment of Debt Cash paid for withholdings purposes Payments Related to Tax Withholding for Share-based Compensation Net cash (used in)/from financing activities Net Cash Provided by (Used in) Financing Activities Increase/(decrease) in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents at beginning of period Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents at end of period Schedule Of Gross Written Premium From Major Brokers Schedule Of Gross Written Premium From Major Brokers [Table Text Block] Tabular disclosure of gross written premium from major brokers. Schedule of Gain (Loss) on Securities [Table] Schedule of Gain (Loss) on Securities [Table] Partner Type [Axis] Partner Type [Axis] Partner Type of Partners' Capital Account, Name [Domain] Partner Type of Partners' Capital Account, Name [Domain] Limited Partner Limited Partner [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] CGB Insurance Diversified Services, Inc. CGB Insurance Diversified Services, Inc. [Member] CGB Insurance Diversified Services, Inc. BBB Emerging Market Debt BBB Emerging Market Debt [Member] BBB Emerging Market Debt [Member] Managed portfolio Managed Portfolio [Member] Managed Portfolio Gain (Loss) on Securities [Line Items] Gain (Loss) on Securities [Line Items] Percentage of investment Investment Owned, Percentage Investment Owned, Percentage Catastrophe bonds, trading, cost Catastrophe Bond Catastrophe Bond Restricted assets Restricted Assets Restricted assets. Other investments Other Investments Percentage of ownership acquired Noncontrolling Interest, Ownership Percentage by Parent Gross realized and unrealized gains (loss) Income (Loss) from Equity Method Investments Equity method investment, aggregate cost Equity method investment, cash payment Payments to Acquire Equity Method Investments Equity method investment, counterparty ownership percentage Equity Method Investment, Counterparty Ownership Percentage Equity Method Investment, Counterparty Ownership Percentage Investment funded Investment Funds Investment Funds Other-than-temporary impairment charge Other than Temporary Impairment Losses, Investments Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Non-Employee Directors Non Employee Directors [Member] Non Employee Directors [Member] 2003 Share Incentive Plan Two Thousand And Three Share Incentive Plan [Member] Two Thousand And Three Share Incentive Plan [Member] International Employee Share Purchase Plan International Employee Share Purchase Plan [Member] International employee share purchase plan. 2008 Share Save Scheme Share Save Scheme [Member] Share save scheme. ESPP Employee Share Purchase Plan [Member] Employee share purchase plan. Director Stock Option Plan Non Employee Equity Incentive Plan [Member] Non Employee Equity Incentive Plan. Restricted Share Units Number of arrangements under options and equity incentive plans Share Based Compensation Arrangement By Share Based Payment Award Number Of Arrangements Share based compensation arrangement by share based payment award number of arrangements. Number of shares available for grant Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Vesting period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Compensation cost Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Total tax credit Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Unrecognized share compensation costs, RSUs Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted average period of compensation cost recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Percentages for vesting Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Fair value adjustment on compensation cost Share Based Compensation Arrangement By Share Based Payment Award Fair Value Adjustments Share based compensation arrangement by share based payment award fair value adjustments. Tax credit recognized by the company Income Tax Credits and Adjustments Contract period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Options exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Intrinsic value of options Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Employee savings under ESPP Employee Share Purchase Plan Maximum Saving By Employee Employee share purchase plan maximum saving by employee. Employee contribution period (in years) Share-based Compensation Arrangement by Share-based Payment Award, Employee Contribution, Period Share-based Compensation Arrangement by Share-based Payment Award, Employee Contribution, Period Purchase price percentage of fair market value Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Employee Stock Ownership Plan, shares issued during period (in shares) Stock Issued During Period, Shares, Employee Stock Ownership Plan Options granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Compensation cost, non employee directors Allocated Share-based Compensation Expense Tax charge, non employee directors Employee Service Share Based Compensation Tax Expenses Recognized For Stock Options Employee service share based compensation tax expenses recognized for stock options. Statement of Financial Position [Abstract] Fixed maturities Fixed Maturities [Member] Fixed income securities, available for sale amortized cost Debt Securities, Available-for-sale, Amortized Cost Fixed income securities, trading, amortized cost Equity securities, trading, cost Trading Securities, Cost Short term investments available for sale, amortized cost Short term investments available for sale, amortized cost Short term investments available for sale, amortized cost Short-term investments, trading, amortized cost Short Term Investments Trading Amortized Cost Short-term investments, trading, amortized cost. Cash and cash equivalents, within VIEs Cash and Cash Equivalents, within VIEs Cash and Cash Equivalents, within VIEs Ordinary shares, par value Common Stock, Par or Stated Value Per Share Ordinary shares, issued Preference shares, par value Preferred Stock, Par or Stated Value Per Share Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Expected tax (benefit)/expense Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Overseas statutory tax rates differential Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount Base erosion and anti-abuse tax expense Effective Income Tax Reconcilation, Base Erosion Anti-Abuse Tax expense U.S Base Erosion Anti-Abuse Tax (BEAT) on premiums ceded from U.S. subsidiaries to non-U.S. related parties. Prior year adjustments Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount Valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Impact of unrecognized tax benefits Effective Income Tax Rate Reconciliation, Tax Contingency, Amount Restricted foreign tax credits Effective Income Tax Rate Reconciliation, Tax Settlement, Foreign, Amount Australian non-resident withholding tax Effective Income Tax Rate Reconciliation, Tax Settlement, Non-Resident Withholding Tax, Amount Effective Income Tax Rate Reconciliation, Tax Settlement, Non-Resident Withholding Tax, Amount Share-based payments Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount Foreign exchange Effective Income Tax Rate Reconciliation, Tax Contingency, Foreign, Amount Non-deductible expenses Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount Non-taxable items Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Impact of changes in statutory tax rates Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Total income tax expense/(benefit) Income Tax Expense (Benefit) Income tax examination, tax settlement Effective Income Rate Tax Reconciliation, Tax Settlement, Foreign Credit, Amount Effective Income Rate Tax Reconciliation, Tax Settlement, Foreign Credit, Amount Income tax examination, interest accrued Income Taxes Income Tax Disclosure [Text Block] Summary of Total Income Tax Schedule Of Income Tax Expense Benefit Intraperiod Tax Allocation [Table Text Block] Schedule of income tax expense benefit intraperiod tax allocation. Schedule of Income Tax by Taxing Authority Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block] Income Tax Reconciliation Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Schedule of Unrecognized Tax Benefits Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block] Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward] Balance at the beginning of the period Deferred Policy Acquisition Costs Acquisition costs deferred Deferred Policy Acquisition Costs, Additions Amortization of deferred policy acquisition costs Deferred Policy Acquisition Costs, Amortization Expense Balance at the end of the period Schedule of Investment Income, Reported Amounts, by Category [Table] Investment Income [Table] Fixed income securities Debt Securities [Member] Short-term investment, Trading Short-term Investment, Trading [Member] Short-term Investment, Trading [Member] Fixed term deposits (included in cash and cash equivalents) Cash and Cash Equivalents [Member] Equity securities — Trading Catastrophe bonds — Trading Other investments, at fair value Other Security Investments [Member] Schedule of Investment Income, Reported Amounts, by Category [Line Items] Net Investment Income [Line Items] Investment income Interest and Dividend Income, Operating Investment expenses Investment Income, Investment Expense Net investment income Net Investment Income Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments Operating Segments [Member] Segments [Domain] Gross earned premiums Premiums Earned, Gross Premiums Earned, Gross Losses and loss adjustment expenses Amortization of deferred policy acquisition costs General and administrative expenses General and Administrative Expense Underwriting income (loss) Underwriting Income (Loss) Corporate expenses Corporate Expenses Corporate expenses. Non-operating expenses Nonoperating Income (Expense) Net investment income Realized and unrealized investment gains Gain (Loss) on Investments Realized and unrealized investment losses Net Realized And Unrealized Loss Net Realized And Unrealized Loss Change in fair value of loan notes issued by variable interest entities Change in fair value of derivatives Derivative, Gain (Loss) on Derivative, Net Interest expense on long term debt Interest Expense, Debt Net realized and unrealized foreign exchange (losses) Foreign Currency Transaction Gain (Loss), before Tax Other income Other Income Other expenses Other Expenses (Loss)/income from operations before income tax Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax benefit/(expense) Net (loss)/income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Losses and loss adjustment expenses Ratios Insurance Ratios [Abstract] Loss ratio Loss Ratio Policy acquisition expense ratio Acquisition Cost Ratio General and administrative expense ratio General and Administrative Expense Ratio Expense ratio Underwriting Expense Ratio Combined ratio Combined Ratio Non-operating expenses related to Effectiveness And Efficiency Program Nonoperating Expenses Related To Effectiveness And Efficiency Program Nonoperating Expenses Related To Effectiveness And Efficiency Program Non-operating expenses related to Merger Agreement Non-operating expenses related to Merger Agreement Non-operating expenses related to Merger Agreement Non-operating expenses related to retention costs Non-operating expenses related to retention costs Non-operating expenses related to retention costs Write back of buy out provision Loss Contingency Accrual, Provision Summary of Statutory Requirements and Dividends Restrictions Dividend Payment Restrictions [Text Block] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Table] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Table] Affiliated transactions Affiliated Entity [Member] Third party Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] Total restricted assets Other investments (equity method) Total as percent of investable assets Collateral Funds Held As Percentage Of Cash And Short Term Investments Collateral funds held as percentage of cash and short term investments. Securities and cash as collateral secured letters of credit Deposit Securities And Cash As Collateral Secured Letters Of Credit Deposit securities and cash as collateral secured letters of credit. Total revenues Revenues Amortization of deferred policy acquisition costs General, administrative and corporate expenses Selling, General and Administrative Expense Interest on long-term debt Change in fair value of derivatives Change in fair value of loan notes issued by variable interest entities Realized and unrealized investment losses Net realized and unrealized foreign exchange (gains)/losses Other (expense) Other Nonoperating Income (Expense) Total expenses Expense Expense. Ordinary shares: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic weighted average ordinary shares (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted weighted average ordinary shares (in shares) Weighted Average Number of Shares Outstanding, Diluted Weighted average number of ordinary share and share equivalents Basic, in usd per share Earnings Per Share, Basic Diluted, in usd per share Earnings Per Share, Diluted ASSETS Assets [Abstract] Short-term investments (available for sale) Short-term Investments Fixed income maturities, trading at fair value (amortized cost — $1,205.0 and $1,634.9) Cash and cash equivalents (including cash within consolidated variable interest entities of — $26.9 and $166.6) Investments in subsidiaries (1) Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Eurobond issued by subsidiary Bond Issued By Subsidiary Bond issued by subsidiary. Long-term debt issued by Silverton Long-term Debt Issued By Subsidiary Long-term Debt Issued By Subsidiary Intercompany funds due from affiliates Due from Affiliates Other assets Other Assets Total assets Assets LIABILITIES Liabilities [Abstract] Accrued expenses and other payables Accrued Liabilities Intercompany funds due to affiliates Due to Affiliate Long-term debt Long-term Debt Total liabilities Liabilities SHAREHOLDERS’ EQUITY Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] 59,743,156 shares of par value 0.15144558¢ each (December 31, 2017 — 59,474,085) Preference shares, value Preferred Stock, Value, Issued Retained earnings Retained Earnings (Accumulated Deficit) Non-controlling interest Stockholders' Equity Attributable to Noncontrolling Interest Gain/(loss) on derivatives Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) on Derivatives, Effect Net of Tax Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) on Derivatives, Effect Net of Tax Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) on Derivatives, Effect Net of Tax Gains on foreign currency translation Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax Total accumulated other comprehensive (loss)/income Accumulated Other Comprehensive Income (Loss), Net of Tax Total shareholders’ equity Stockholders' Equity Attributable to Parent Total liabilities and shareholders’ equity Liabilities and Equity Preference shares, rate Preferred Stock Dividends Rate Percentage Preferred Stock Dividends Rate Percentage Disclosure Statutory Requirements And Dividends Restrictions Summary Of Statutory Requirements And Dividends Restrictions [Abstract] Disclosure - Statutory Requirements and Dividends Restrictions - Summary of Statutory Requirements and Dividends Restrictions [Abstract] Jurisdiction [Axis] Jurisdiction [Axis] Statutory Accounting Practices, Jurisdiction [Domain] Statutory Accounting Practices, Jurisdiction [Domain] U.S. UNITED STATES Bermuda BERMUDA U.K. Required statutory capital and surplus Dividend Payments Restrictions Schedule Required Statutory Capital And Surplus Dividend payments restrictions schedule required statutory capital and surplus. Actual statutory capital and surplus Dividend Payments Restrictions Schedule, Statutory Capital and Surplus Restricted Cash And Collateral [Table] Restricted Cash And Collateral [Table] Restricted Cash And Collateral Table Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain]