Debt Disclosure [Abstract] Condensed Financial Statements [Table] Condensed Financial Statements [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Consolidating Adjustments [Member] Consolidation, Eliminations [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Other Subsidiaries and Eliminations [Member] Other Subsidiaries And Eliminations [Member] Other subsidiaries and eliminations. Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Argo Group International Holdings, Ltd (Parent Guarantor) [Member] Parent Company [Member] Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer) [Member] Subsidiary Issuer [Member] Condensed Financial Statements, Captions [Line Items] Condensed Financial Statements, Captions [Line Items] Net cash flows from operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from sales of investments Proceeds From Sale Of Investments The cash inflow associated with the sale of fixed maturity and equity securities classified as available-for-sale securities. Maturities and mandatory calls of fixed maturity investments Proceeds from Maturities, Prepayments and Calls of Available-for-sale Securities Purchases of investments Payments to Acquire Investments Change in short-term investments and foreign regulatory deposits Payments For Proceeds From Change In Foreign Regulatory Deposits Voluntary Pools Shortterm Investments Payments For Proceeds From Change In Foreign Regulatory Deposits, Voluntary Pools, and Short-Term Investments. Settlements of foreign currency exchange forward contracts Increase Decrease In Settlements Of Foreign Currency Exchange Forward Contracts Settlements of foreign currency exchange forward contracts Cash acquired with acquisition of Ariscom Cash Acquired from Acquisition Purchases of fixed assets and other, net Payments For Proceeds From Purchases Of Fixed Asset And Other Investing Activities The net cash outflow or inflow from purchases/sales of fixed assets and other investing activities. Cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract] Payment on the intercompany note Payment for Intercompany Note Payment for Intercompany Note Activity under stock incentive plans Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Excluding Stock Options Repurchase of Company's common shares Payments for Repurchase of Common Stock Payment of cash dividends to common shareholders Payments of Ordinary Dividends, Common Stock Cash used in financing activities Net Cash Provided by (Used in) Financing Activities Effect of exchange rate changes on cash Effect of Exchange Rate on Cash and Cash Equivalents Change in cash Cash and Cash Equivalents, Period Increase (Decrease) Cash, beginning of year Cash Cash, end of period Segment Reporting [Abstract] Schedule of Revenues from External Customers and Long-Lived Assets [Table] Schedule of Revenues from External Customers and Long-Lived Assets [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] United States [Member] UNITED STATES United Kingdom [Member] UNITED KINGDOM Bermuda [Member] BERMUDA Malta [Member] MALTA Other Jurisdiction [Member] Other Jurisdiction [Member] Other Jurisdiction [Member] Revenues from External Customers and Long-Lived Assets [Line Items] Revenues from External Customers and Long-Lived Assets [Line Items] Total earned premiums Premiums Earned, Net Supplemental Cash Flow Elements [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Senior Unsecured Fixed Rate Notes [Member] Senior Notes [Member] Junior Subordinated Debentures [Member] Junior Subordinated Debt [Member] Other Indebtedness [Member] Other Long Term Debt [Member] Other long term debt. Supplemental Cash Flow [Line Items] Debt Instrument [Line Items] Total interest paid Interest Paid Income taxes paid Income Taxes Paid Income taxes recovered Proceeds from Income Tax Refunds Income taxes paid, net Income Taxes Paid, Net Number of reportable segments Number of Reportable Segments Assets associated with trade capital providers Assets Associated With Trade Capital Providers Assets associated with trade capital providers. 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 Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Stock-Settled SARs Activity [Member] Stock Settled S A Rs Activity [Member] Stock settled SARs activity. Cash Settled S A Rs Activity [Member] Cash Settled S A Rs Activity [Member] Cash settled SARs activity. Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding, beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Expired or forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding, ending balance (in shares) Weighted-Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding, beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Expired or forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Outstanding, ending balance (in dollars per share) Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability [Table Text Block] Schedule Of Unamortized Debt Issuance Costs Deducted From Carrying Value Of Debt Liability [Table Text Block] Condensed Consolidating Balance Sheet Condensed Balance Sheet [Table Text Block] Condensed Consolidating Statement of Income Condensed Income Statement [Table Text Block] Condensed Consolidating Statement of Cash Flows Condensed Cash Flow Statement [Table Text Block] Leases [Abstract] Operating lease right-of-use assets Operating Lease, Right-of-Use Asset Operating lease liabilities Operating Lease, Liability Operating lease weighted-average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Operating lease weighted-average discount rate Operating Lease, Weighted Average Discount Rate, Percent Investments, Debt and Equity Securities [Abstract] Schedule of Investments [Table] Schedule of Investments [Table] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Fixed Maturities [Member] Fixed Maturities [Member] Equity Securities [Member] Equity Securities [Member] Other-Than-Temporary Impairment Losses on Fixed Maturities [Member] Other Than Temporary Impairment Losses On Fixed Maturities [Member] Other than temporary impairment losses on fixed maturities. Schedule of Investments [Line Items] Schedule of Investments [Line Items] Realized gains on fixed maturities and other Gross Realized Investment On Fixed Maturities And Other Gains Gross realized investment on fixed maturities and other gains. Realized losses on fixed maturities and other Gross Realized Investment On Fixed Maturities And Other Losses Gross realized investment on fixed maturities and other losses. Net realized gains on equity securities Equity Securities F V N I Realized Gain Loss Equity securities FV-NI realized gain loss. Change in unrealized gains (losses) on equity securities held at the end of the period Equity Securities F V N I Unrealized Gain Loss Equity securities FV-NI unrealized gain loss. Net realized gains (losses) on equity securities Equity Securities F V N I Gain Loss Equity securities FV-NI gain loss. Net realized investment gains (losses) before income taxes Realized Investment Gains (Losses) Income tax (provision) benefit Income Tax Expense Benefit Related To Net Realized Investment And Other Gains Income tax expense (benefit) related to net realized investment and other gains. Net realized investment gains (losses), net of income taxes Net Realized Investment Gains Losses Net Of Income Taxes Net realized investment gains (losses) net of income taxes. Equity [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Unrealized Gains and Losses on Securities [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Unrealized gains and losses on securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract] Net realized investment loss (gains) Reclassification from AOCI, Current Period, before Tax, Attributable to Parent (Benefit) provision for income taxes Reclassification from AOCI, Current Period, Tax Net of taxes Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Accumulated Other Comprehensive (Loss) Income Comprehensive Income (Loss) Note [Text Block] Statement of Financial Position [Abstract] Fixed maturities available-for-sale, cost Available-for-sale Debt Securities, Amortized Cost Basis Equity securities, cost Available-for-sale Equity Securities, Amortized Cost Basis Other investments, cost Other Investments and Securities, at Cost Short-term investments, cost Short Term Investments Amortized Cost This item represents the cost of Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy. Common shares, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common shares, shares authorized (in shares) Common Stock, Shares Authorized Common shares, shares issued (in shares) Common Stock, Shares, Issued Treasury shares (in shares) Treasury Stock, Shares Income Tax Disclosure [Abstract] Income Tax Examination [Table] Income Tax Examination [Table] Belgium [Member] BELGIUM Brazil [Member] BRAZIL United Arab Emirates [Member] UNITED ARAB EMIRATES Ireland [Member] IRELAND Italy [Member] ITALY Luxembourg [Member] LUXEMBOURG Switzerland [Member] SWITZERLAND Income Tax Examination [Line Items] Income Tax Examination [Line Items] Pre-Tax Income (Loss) Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Foreign Currency Translation Adjustments [Member] Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Unrealized Holding Gains on Securities [Member] Defined Benefit Pension Plans [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) Member] AOCI Attributable to Parent [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] ASU 2016-01 [Member] Accounting Standards Update 2016-01 [Member] ASU 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 AOCI Attributable to Parent, Net of Tax [Roll Forward] AOCI Attributable to Parent, Net of Tax [Roll Forward] Beginning Balance Stockholders' Equity Attributable to Parent Other comprehensive income before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from accumulated other comprehensive income Net current-period other comprehensive income Other Comprehensive Income (Loss), Net of Tax, Including Cumulative Effect of Adoption Other Comprehensive Income (Loss), Net of Tax, Including Cumulative Effect of Adoption Cumulative effect of adoption of ASU Cumulative Effect of New Accounting Principle in Period of Adoption Ending Balance Debt Security [Axis] Debt Security [Axis] Major Types of Debt Securities [Domain] Major Types of Debt Securities [Domain] U.S. Governments [Member] US Government Agencies Debt Securities [Member] Foreign Governments [Member] Foreign Government Debt Securities [Member] Obligations of States and Political Subdivisions [Member] US States and Political Subdivisions Debt Securities [Member] Corporate Bonds [Member] Corporate Bond Securities [Member] Commercial Mortgage-backed Securities [Member] Commercial Mortgage Backed Securities [Member] Residential Mortgage-backed Securities [Member] Residential Mortgage Backed Securities [Member] Asset-backed Securities [Member] Asset-backed Securities [Member] Collateralized Debt Obligations [Member] Collateralized Debt Obligations [Member] Fair Value, Less Than One Year Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Unrealized Losses, Less Than One Year Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Fair Value, One Year or Greater Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Unrealized Losses, One Year or Greater Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Fair Value, Total Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Unrealized Losses, Total Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Fair Value Disclosures [Abstract] Summary of Company's Financial Instruments Whose Carrying Amount Did Not Equal Fair Value Fair Value, by Balance Sheet Grouping [Table Text Block] Accounting Policies [Abstract] Recently Issued Accounting Pronouncements New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Schedule of Gain (Loss) on Investments [Table] Schedule of Gain (Loss) on Investments [Table] Gain (Loss) on Investments [Line Items] Gain (Loss) on Investments [Line Items] Other-than-temporary impairment: Other than Temporary Impairment Losses, Investments [Abstract] Other-than-temporary impairment losses Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities Insurance [Abstract] Reserves for Losses and Loss Adjustment Expenses Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Impact from (Favorable) Unfavorable Development of Prior Accident Years’ Loss and LAE Reserves on Each Reporting Segment Schedule of Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Reclassification Reclassification, Policy [Policy Text Block] Recently Issued Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Leases Leases of Lessee Disclosure [Text Block] Earnings Per Share [Abstract] Net Income (Loss) Per Common Share on Basic and Diluted Basis Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Underwriting Acquisition And Insurance Expenses [Abstract] Underwriting acquisition and insurance expenses. Commissions Noninterest Expense Commission Expense General expenses General and Administrative Expense Premium taxes, boards and bureaus Premium Taxes Boards And Bureaus Costs paid to regulatory agencies or industry boards and bureaus primarily based on a percentage of premiums Underwriting, acquisition and insurance expenses, total Underwriting Acquisition And Insurance Expense Gross Costs incurred during the period, such as those relating to general administration and policy maintenance that do not vary with and are not primarily related to the acquisition or renewal of insurance contracts, excluding amortization of deferred acquisition costs. Net deferral of policy acquisition costs Net Supplementary Insurance Information Amortization Deferral Of Deferred Policy Acquisition Costs Net amount of amortization of deferred policy acquisition costs recognized and/or policy acquisition costs deferred in current period. Total underwriting, acquisition and insurance expenses Underwriting Acquisition And Insurance Expenses Costs incurred during the period related to selling the Company's products and services including the amortization of deferred policy acquisition costs charged to expense in the period, as well as other costs incurred during the period, such as those relating to general administration and policy maintenance. Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Foreign Currency Exchange Forward Contracts [Member] Foreign Exchange Forward [Member] Operational Currency Exposure [Member] Operational Currency Exposure [Member] Operational currency exposure. Asset Manager Investment Exposure [Member] Asset Manager Investment Exposure [Member] Asset manager investment exposure. Total return strategy [Member] Return Strategy [Member] Return strategy. Foreign currency exchange, fair value Derivative Assets (Liabilities), at Fair Value, Net Underwriting, Acquisition and Insurance Expenses Underwriting Acquisition Insurance Restructuring And Other Expenses Disclosure [Text Block] Disclosures pertaining to the costs incurred during the period related to selling the Company's products and services, as well as other costs incurred during the period, such as those relating to general administration and policy maintenance. Reconciliation of Assets from Segment to Consolidated [Table] Reconciliation of Assets from Segment to Consolidated [Table] Operating Segments [Member] Operating Segments [Member] Corporate and Other [Member] Corporate, Non-Segment [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] U.S. Operations [Member] U S Operations [Member] U.S. operations. International Operations [Member] International Operations [Member] International operations. Run Off Lines [Member] Run Off Lines [Member] Run off lines. Segment Reporting, Asset Reconciling Item [Line Items] Segment Reporting, Asset Reconciling Item [Line Items] Identifiable assets Assets Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Common Shares [Member] Common Stock [Member] Additional Paid-In Capital [Member] Additional Paid-in Capital [Member] Treasury Shares [Member] Treasury Stock [Member] Retained Earnings [Member] Retained Earnings [Member] Statement [Line Items] Statement [Line Items] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Net income Net Income (Loss) Attributable to Parent Other comprehensive income (loss)- Change in fair value of investment, net of tax Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax Other comprehensive income (loss)- Other Other Comprehensive Income, Other, Net of Tax Repurchase of common shares Treasury Stock, Value, Acquired, Cost Method Activity under stock incentive plans Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Retirement of common shares (tax payments on equity compensation) Stock Repurchased and Retired During Period, Value Employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan 15% Stock Dividend Stock Issued During Period, Value, Stock Dividend Cash dividends declared - common shares Dividends, Common Stock, Cash Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Other Investments [Member] Other Long-term Investments [Member] Short-Term Investments [Member] Short-term Investments [Member] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Underlying Asset Class [Axis] Underlying Asset Class [Axis] Underlying Asset Class [Domain] Underlying Asset Class [Domain] Collateralized Loan Obligations [Member] Collateralized Loan Obligations [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value Investments Senior Unsecured Fixed Rate Notes Senior Unsecured Fixed Rate Notes Disclosure [Text Block] Senior unsecured fixed rate notes disclosure. Condensed Financial Statements, Captions [Line Items] Assets Assets [Abstract] Investments Cash Accrued investment income Accrued Investment Income Receivable Premiums receivable Premiums Receivable, Net Reinsurance recoverables Reinsurance Recoverables, Including Reinsurance Premium Paid Goodwill and other intangible assets, net Intangible Assets, Net (Including Goodwill) Current income taxes receivable, net Income Taxes Receivable Deferred acquisition costs, net Deferred Policy Acquisition Costs Ceded unearned premiums Prepaid Reinsurance Premiums Other assets Other Assets Intercompany note receivable Due from Affiliates Investments in subsidiaries Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Total assets Liabilities and Shareholders' Equity Liabilities and Equity [Abstract] Reserves for losses and loss adjustment expenses Liability for Claims and Claims Adjustment Expense Unearned premiums Unearned Premiums Funds held and ceded reinsurance payable, net Funds Held And Ceded Reinsurance Payable Net Funds held and ceded reinsurance payable net. Debt Long-term Debt Deferred tax liabilities, net Deferred Tax Liabilities, Net Accrued underwriting expenses and other liabilities Accrued Underwriting Expenses And Other Liabilities Accrued underwriting expenses and other liabilities. Due to (from) affiliates Related Party Transaction, Due from (to) Related Party Intercompany note payable Intercompany Note Payable Intercompany Note Payable Total liabilities Liabilities Total shareholders' equity Total liabilities and shareholders' equity Liabilities and Equity Amortized Cost Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract] Due in one year or less Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount Due after one year through five years Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount Due after five years through ten years Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount Thereafter Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount Structured securities Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Net Carrying Amount Total Held-to-maturity Securities Fair Value Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract] Due in one year or less Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value Due after one year through five years Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value Due after five years through ten years Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value Thereafter Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value Structured securities Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value Total Held-to-maturity Securities, Fair Value Commitments and Contingencies Disclosure [Abstract] Contractual commitments related to its limited partnership investments (up to) Contractual Obligation Contractual commitments period (not to exceed) Contractual Obligation Period Contractual obligation period. Senior Unsecured Fixed Rate Notes [Member] Floating Rate Loan Stock [Member] Floating Rate Loan Stock [Member] Floating Rate Loan Stock. Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Trust Preferred Debentures [Member] Trust Preferred Debentures [Member] Trust preferred debentures. Junior Subordinated Debentures, Maybrooke [Member] Maybrooke Junior Subordinated Debt [Member] Maybrooke junior subordinated debt. Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Fair Value Disclosure Item Amounts [Default] Portion at Fair Value Measurement [Member] Carrying Amount [Member] Reported Value Measurement [Member] Fair Value [Member] Estimate of Fair Value Measurement [Member] Financial Instruments Long-term Debt, Fair Value Segment Information Segment Reporting Disclosure [Text Block] Investments: Investments [Abstract] Fixed maturities available-for-sale, at fair value (cost: 2019 - $3,580.1; 2018 - $3,529.1) Available-for-sale Securities, Debt Securities Equity securities, at fair value (cost: 2019 - $305.2; 2018 - $310.6) Available-for-sale Securities, Equity Securities Other investments (cost: 2019 - $473.7; 2018 - $482.0) Other Investments Short-term investments, at fair value (cost: 2019 - $444.1; 2018 - $482.3) Short-term Investments Total investments Goodwill Goodwill Intangible assets, net of accumulated amortization Intangible Assets, Net (Excluding Goodwill) Ceded reinsurance payable, net Reinsurance Payable Funds held Policyholder Contract Deposits Senior unsecured fixed rate notes Senior Notes Other indebtedness Other Long-term Debt Junior subordinated debentures Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Commitments and contingencies (Note 14) Commitments and Contingencies Shareholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Common shares - $1.00 par, 500,000,000 shares authorized; 45,365,223 and 45,276,999 shares issued at March 31, 2019 and December 31, 2018, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Treasury shares (11,315,889 shares at March 31, 2019 and December 31, 2018, respectively) Treasury Stock, Value Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive loss, net of taxes Accumulated Other Comprehensive Income (Loss), Net of Tax Total shareholders' equity Shareholders' Equity Stockholders' Equity Note Disclosure [Text Block] Income Statement [Abstract] Premiums and other revenue: Revenues [Abstract] Earned premiums Net investment income Net Investment Income Fee and other income Fee And Other Income Fee and other income. Net realized investment gains (losses): Realized Investment Gains (Losses) [Abstract] Net realized investment (losses) gains Gain (Loss) on Sale of Investments Change in fair value of equity securities Net realized investment gains (losses) Total revenue Revenues Expenses: Benefits, Losses and Expenses [Abstract] Losses and loss adjustment expenses Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Underwriting, acquisition and insurance expenses Interest expense Interest Expense Fee and other expense Fee And Other Expense Fee and other expense. Foreign currency exchange losses Foreign Currency Transaction Gain (Loss), before Tax Total expenses Benefits, Losses and Expenses Income before income taxes Income tax provision Income Tax Expense (Benefit) Net income Net income per common share: Basic (in dollars per share) Earnings Per Share, Basic Diluted (in dollars per share) Earnings Per Share, Diluted Dividend declared per common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Weighted average common shares: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Net realized investment losses before other-than-temporary impairment losses Gain (Loss) on Investments, Excluding Other than Temporary Impairments Other-than-temporary impairment losses recognized in earnings: Other than Temporary Impairment Losses, Investments, Available-for-sale Securities [Abstract] Other-than-temporary impairment losses on fixed maturities Other Than Temporary Impairment Losses Fixed Maturities The amount by which the fair value of an investment in debt securities categorized as available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. Impairment losses recognized in earnings Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Underwriting, Acquisition and Insurance Expenses Underwriting Acquisition And Insurance Expense [Table Text Block] Underwriting Acquisition And Insurance Expense [Table Text Block] Income tax provision (benefit) at expected rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Tax effect of: Effective Income Tax Rate Reconciliation, Amount [Abstract] Nontaxable investment income Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount Foreign exchange adjustments Income Tax Reconciliation Foreign Exchange Adjustments Income tax reconciliation foreign exchange adjustments. Withholding taxes Income Tax Reconciliation Withholding Taxes Income tax reconciliation withholding taxes. Change in valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Other Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Income tax provision Schedule of Interest Paid and Income Taxes Paid (Recovered) Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Hedge funds [Member] Hedge Funds [Member] Private equity [Member] Private Equity [Member] Private equity. Long only funds [Member] Hedge Funds Long Position Only [Member] Hedge funds long position only. Other [Member] Other Investment [Member] Other investment. Other Investments [Member] Carrying Value Unfunded Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Risk-free rate of return Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected dividend yields Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Expected award life (years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Revenue and Income (Loss) Before Income Taxes for Each Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Earned Premiums by Geographic Location Schedule of Revenue from External Customers Attributed to Foreign Countries by Geographic Area [Table Text Block] Identifiable Assets Reconciliation of Assets from Segment to Consolidated [Table Text Block] Statement of Cash Flows [Abstract] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Ariscom [Member] Ariscom Compagnia Di Assicurazioni S P A [Member] Ariscom Compagnia di Assicurazioni S.p.A. Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by (used in) operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Amortization and depreciation Depreciation, Depletion and Amortization, Nonproduction Share-based payments expense Share-based Compensation Deferred income tax provision (benefit), net Deferred Income Tax Expense (Benefit) Net realized investment (gains) loss Undistributed earnings from alternative investment portfolio Undistributed Earnings From Alternative Investment Portfolio Undistributed earnings from alternative investment portfolio. Change in: Increase (Decrease) in Operating Capital [Abstract] Accrued investment income Increase (Decrease) in Accrued Interest Receivable, Net Receivables Increase (Decrease) in Premiums Receivable Deferred acquisition costs Increase (Decrease) in Deferred Policy Acquisition Costs Ceded unearned premiums Increase (Decrease) in Prepaid Reinsurance Premiums Reserves for losses and loss adjustment expenses Increase (Decrease) in Future Policy Benefit Reserves Unearned premiums Increase (Decrease) in Unearned Premiums Ceded reinsurance payable and funds held Increase Decrease In Reinsurance Payables And Funds Held The increase (decrease) during the reporting period in the amount due to other insurance companies when the reporting entity has assumed a portion of the cedant's insurance risk which has resulted in insurance losses and/or the liability to policyholders for deposits held under the terms of insurance contracts. Income taxes Increase (Decrease) in Income Taxes Payable Accrued underwriting expenses and other liabilities Increase Decrease In Accrued Underwriting Expenses The increase (decrease) during the reporting period in underwriting expenses incurred but not yet paid. Other, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Cash provided by operating activities Sales of fixed maturity investments Proceeds from Sale of Available-for-sale Securities, Debt Sales of equity securities Proceeds from Sale of Available-for-sale Securities, Equity Sales of other investments Proceeds from Sale of Other Investments Purchases of fixed maturity investments Payments to Acquire Available-for-sale Securities, Debt Purchases of equity securities Payments to Acquire Available-for-sale Securities, Equity Purchases of other investments Payments to Acquire Other Investments Change in foreign regulatory deposits and voluntary pools Payments For Proceeds From Change In Foreign Regulatory Deposits And Voluntary Pools Payments for Proceeds from Change in Foreign Regulatory Deposits Change in short-term investments Payments for (Proceeds from) Short-term Investments Purchases of fixed assets Payments to Acquire Property, Plant, and Equipment Other, net Payments for (Proceeds from) Other Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Net Income Per Common Share Earnings Per Share [Text Block] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Net reserves beginning of the year Liability for Unpaid Claims and Claims Adjustment Expense, Net Net Ariscom reserves acquired Liability For Unpaid Claims And Claims Adjustment Expense Net Reserves Acquired Liability for unpaid claims and claims adjustment expense net reserves acquired. Losses and LAE incurred during current calendar year, net of reinsurance: Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract] Current accident year Current Year Claims and Claims Adjustment Expense Prior accident years Prior Year Claims and Claims Adjustment Expense Losses and LAE incurred during calendar year, net of reinsurance Losses and LAE payments made during current calendar year, net of reinsurance: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract] Current accident year Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior accident years Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Losses and LAE payments made during current calendar year, net of reinsurance: Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Change in participation interest Liability For Unpaid Claims And Claims Adjustment Expense Change In Participation Interest Liability for unpaid claims and claims adjustment expense change in participation interest. Foreign exchange adjustments Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss) Net reserves - end of period Reinsurance recoverables on unpaid losses and LAE, end of period Reinsurance Recoverables, Incurred but Not Reported Claims Gross reserves - end of period Schedule Of Invested Assets [Table] Schedule Of Invested Assets [Table] Schedule of invested assets. Schedule of Invested Assets [Line Items] Schedule Of Invested Assets [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Fixed maturities Available-for-sale Securities, Debt Maturities [Abstract] Amortized Cost Gross Unrealized Gains Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Table] Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Principal amount of senior note Debt Instrument, Face Amount Interest rate stated percentage Debt Instrument, Interest Rate, Stated Percentage Redemption price, percentage Debt Instrument, Redemption Price, Percentage Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Credit-Financial [Member] Credit Financial [Member] Credit-Financial. Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Transfers into Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Transfers out of Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Included in net income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Included in other comprehensive income (loss) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Issuances Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Ending balance Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period Amount of gains or losses for period included in net income attributable to change in unrealized gains or losses relating to assets held at end of period. Reserves for Losses and Loss Adjustment Expenses Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block] Operating leases costs Operating Lease, Cost Variable lease costs Variable Lease, Cost Sublease income Sublease Income Total lease costs Lease, Cost Operating cash flows from operating lease (fixed payments) Operating Lease, Payments Operating cash flows from operating lease (liability reduction) Operating Lease, Payments, Liability Reduction Operating Lease, Payments, Liability Reduction Statement of Comprehensive Income [Abstract] Net income Other comprehensive income (loss): Other Comprehensive Income (Loss), before Tax [Abstract] Foreign currency translation adjustments Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax Unrealized gains (losses) on securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, [Abstract] Gains (losses) arising during the year Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax Reclassification adjustment for losses (gains) included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Other comprehensive income (loss) before tax Other Comprehensive Income (Loss), before Tax Income tax provision related to other comprehensive income (loss): Other Comprehensive Income (Loss), Tax [Abstract] Unrealized gains (losses) on securities: Marketable Securities, Unrealized Gain (Loss) [Abstract] Gains (losses) arising during the year Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Reclassification adjustment for losses (gains) included in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Income tax provision (benefit) related to other comprehensive income Other Comprehensive Income (Loss), Tax Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Attributable to Parent Weighted average common shares outstanding - basic (in shares) Effect of dilutive securities: Equity compensation awards (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Weighted average common shares outstanding - diluted (in shares) Net income per common share: Earnings Per Share, Diluted [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Dividends [Axis] Dividends [Axis] Dividends [Domain] Dividends [Domain] Stock Dividend [Member] Stock Dividend [Member] Stock dividend. Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] 2016 Repurchase Authorization [Member] Two Thousand Sixteen Repurchase Authorization [Member] Two thousand sixteen repurchase authorization. Class of Stock [Line Items] Class of Stock [Line Items] Cash dividends paid - common shares, total Stock dividend percent Common Stock Dividend Rate Percentage Common stock dividend rate percentage. Additional stock issued as dividend (in shares) Common Stock Dividends, Shares Total number of shares authorized for purchase Stock Repurchase Program, Authorized Amount Remaining number of shares available under repurchase authorization Stock Repurchase Program, Remaining Authorized Repurchase Amount Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Lease term Lessee, Operating Lease, Term of Contract Lease cost Number of leased properties Number of Leased Properties, Short-term Number of Leased Properties, Short-term Lease income Operating Lease, Lease Income Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Share-based Compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Table] Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Table] Schedule of prior accident years' loss and loss adjustment expense reserves development. Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] Schedule of prior accident years' loss and loss adjustment expense reserves development. Total unfavorable (favorable) prior-year development Supplemental Cash Flow Information Cash Flow, Supplemental Disclosures [Text Block] Net (loss) income before equity in earnings of subsidiaries Net Income Loss Before Equity In Earnings Of Subsidiaries Net income loss before equity in earnings of subsidiaries. Equity in undistributed earnings of subsidiaries Equity In Undistributed Net Income Loss Of Subsidiaries Equity in undistributed net income loss of subsidiaries. Gross realized investment gains Foreign Currency Transaction Gain (Loss), Realized Gross realized investment losses Foreign Currency Transaction Loss Realized Foreign currency transaction loss, realized. Net realized investment gains on foreign currency exchange forward contracts Gain (Loss) on Derivative Instruments, Net, Pretax Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Net investment income Income (loss) before income taxes Equity compensation awards with anti-dilutive effect (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Changes in Accumulated Other Comprehensive (Loss) Income Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Restricted Stock Units (RSUs) [Member] Restricted Stock Units (RSUs) [Member] Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Outstanding, beginning balance (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 and issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Expired or forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Outstanding, ending balance (in shares) Weighted-Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Outstanding, beginning balance (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 (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 Vested and issued (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 Expired or forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Outstanding, ending balance (in dollars per share) Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 2014 Plan [Member] Two Thousand Seven Plan [Member] Two thousand seven plan. 2007 Plan [Member] Two Thousand Fourteen Plan [Member] Two thousand fourteen plan. Share Options and Stock Appreciation Rights [Member] Share Options and Stock Appreciation Rights [Member] Share Options and Stock Appreciation Rights [Member] Restricted Shares, RSUs, Performance Units, Performance Shares, and Other [Member] Restricted Shares, RSUs, Performance Units, Performance Shares, and Other [Member] Restricted Shares, RSUs, Performance Units, Performance Shares, and Other [Member] Employee Stock Option [Member] Employee Stock Option [Member] Share-based compensation arrangement by share-based payment award, award vesting period, years Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Share-based compensation arrangement by share-based payment award, number of shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Share-based compensation arrangement by share-based payment award, number of shares available for grant (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Share-based compensation arrangement by share-based payment award, additional number of shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized Count of restricted shares, settle in common shares (in shares) Count Of Other Than Stock Option Shares Settle In Common Shares Count of other than stock option shares settle in common shares. Expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Share-based compensation arrangement by share-based payment award, compensation expense Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Employee service share-based compensation, unrecognized compensation costs on non-vested awards Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Document And Entity Information [Abstract] Document and entity information. 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 Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Fair value of investments, assets managed on behalf of the trade capital providers Fair Value Of Investments Assets Managed On Behalf Of Trade Capital Providers Fair Value of Investments, Assets Managed on Behalf of the Trade Capital Provider Number of securities in an unrealized loss position, total Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Number of securities in an unrealized loss position for less than one year Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year Number of securities in an unrealized loss position for a period of one year or greater Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Common shares repurchased (in shares) Treasury Stock, Shares, Acquired Repurchase of common shares, weighted average price (in dollars per share) Treasury Stock Acquired, Average Cost Per Share Lessee, Assets and Liabilities Lessee, Assets and Liabilities [Table Text Block] Lessee, Assets and Liabilities [Table Text Block] Lease, Cost Lease, Cost [Table Text Block] Lessee, Operating Lease, Liability, Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Disclosures about Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Fair Value Assumptions Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Summary of Restricted Share Activity Schedule of Nonvested Restricted Stock Units Activity [Table Text Block] Stock Settled Share Appreciation Rights Share Based Compensation Arrangement By Share Based Payment Award Stock Settled Share Appreciation Rights [Table Text Block] Share Based Compensation Arrangement By Share Based Payment Award Stock Settled Share Appreciation Rights [Table Text Block] Summary of Cash-Settled SARs Activity Share Based Compensation Arrangement By Share Based Payment Award Cash Settled Share Appreciation Rights [Table Text Block] Share Based Compensation Arrangement By Share Based Payment Award Cash Settled Share Appreciation Rights [Table Text Block] Principal Less: unamortized debt issuance costs Unamortized Debt Issuance Expense Senior unsecured fixed rate notes, less unamortized debt issuance costs Schedule of Pre-Tax Income (Loss) and Effective Income Tax Rates Schedule Of Pre Tax Loss Income And Effective Income Tax Rate [Table Text Block] Schedule Of Pre Tax Loss Income And Effective Income Tax Rate [Table Text Block] Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Operating Lease Liabilities, Payments Due [Abstract] Operating Lease Liabilities, Payments Due [Abstract] 2019 Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year 2020 Lessee, Operating Lease, Liability, Payments, Due Year Two 2021 Lessee, Operating Lease, Liability, Payments, Due Year Three 2022 Lessee, Operating Lease, Liability, Payments, Due Year Four 2023 Lessee, Operating Lease, Liability, Payments, Due Year Five Thereafter Lessee, Operating Lease, Liability, Payments, Due after Year Five Total future minimum lease payments Lessee, Operating Lease, Liability, Payments, Due Less imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Total operating lease liability Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] 2019 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total future minimum lease payments Operating Leases, Future Minimum Payments Due Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity Investments Classified by Contractual Maturity Date [Table Text Block] Schedule of Carrying Value And Unfunded Investment Commitments Of Other Invested Assets Portfolio Schedule Of Carrying Value Redemption Characteristics And Unfunded Investment Commitments Of Other Invested Assets Portfolio [Table Text Block] Schedule Of Carrying Value Redemption Characteristics And Unfunded Investment Commitments Of Other Invested Assets Portfolio [Table Text Block] Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments Schedule of Unrealized Loss on Investments [Table Text Block] Schedule of Recognized Other-than-temporary Losses on Fixed Maturities and Equity Portfolios Schedule of Temporary Impairment Losses, Investments [Table Text Block] Schedule of Company's Gross Realized Investment Gains (Losses) Schedule of Realized Gain (Loss) [Table Text Block] Schedule Of Changes In Unrealized Appreciation (Depreciation) Schedule Of Changes In Unrealized Appreciation (Depreciation) Table [Table Text Block] Schedule Of Changes In Unrealized Appreciation (Depreciation) Table [Table Text Block] Schedule of Fair Value of Foreign Currency Exchange Forward Contracts Schedule of Foreign Exchange Contracts, Statement of Financial Position [Table Text Block] Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts Derivative Instruments, Gain (Loss) [Table Text Block] Schedule of Restricted Assets Schedule Of Components Of Restricted Investments Other Than Temporary Losses [Table Text Block] Schedule Of Components Of Restricted Investments Other Than Temporary Losses [Table Text Block] Financial Assets Measured at Fair Value on Recurring Basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Income Taxes Income Tax Disclosure [Text Block] Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Schedule of Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table] Securities Deposits For Regulatory and Other Purposes [Member] Securities Deposit For Regulatory And Other Purpose [Member] Securities deposit for regulatory and other purpose. Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] Securities on deposit for regulatory and other purposes Investments, Fair Value Disclosure Securities pledged as collateral for letters of credit Pledged Assets Separately Reported, Securities Pledged under Letter of Credit Facilities, at Fair Value Securities and cash on deposit supporting Lloyd’s business Securities And Letters Of Credit Pledged Securities and letters of credit pledged. Total restricted investments Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value Net unrealized investment gains (losses) before income taxes Changes In Investment Unrealized Appreciation Depreciation Net Before Tax Changes in investment unrealized appreciation depreciation net before tax. Income tax (provision) benefit Changes In Investment Unrealized Appreciation Depreciation Net Tax Effects Changes in investment unrealized appreciation depreciation net tax effects. Net unrealized investment gains (losses), net of income taxes Changes In Investment Unrealized Appreciation Depreciation Net Of Tax Effect Changes in investment unrealized appreciation depreciation net of tax effect. Income Taxes Disclosure [Table] Income Taxes Disclosure [Table] Income taxes disclosure. Income Taxes Disclosure [Line Items] Income Taxes Disclosure [Line Items] Income taxes disclosure. Number of subsidiaries subject to corporate tax Number of Subsidiaries Subject to Corporate Tax Number of Subsidiaries Subject to Corporate Tax Net operating losses carryback period Net Operating Losses Carry Back Period Net operating losses carry back period. Capital losses carryback period Capital Losses Carry Back Period Capital losses carry back period. Unrecognized tax benefits Unrecognized Tax Benefits