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Expense Derivative Financial Instruments Derivatives and Fair Value [Text Block] Commercial mortgage-backed Commercial mortgage-backed Commercial Mortgage-Backed Securities [Member] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Credit Concentration Risk Credit Concentration Risk [Member] Purchase of fixed assets Payments to Acquire Productive Assets Statement of Cash Flows [Abstract] Underwriting ratio Underwriting Expense Ratio Hedging Designation [Axis] Hedging Designation [Axis] 6.625% Fixed Rate Reset Junior Subordinated notes due 2041 Six Point Six Two Five Fixed Rate Reset Junior Subordinated Notes [Member] Six Point Six Two Five Fixed Rate Reset Junior Subordinated Notes Schedule of Common Shares Information Schedule of Stock by Class [Table Text Block] Credit Facility [Axis] Credit Facility [Axis] Uncommitted Tranche Uncommitted Tranche [Member] Uncommitted Tranche Total assets measured at fair value Assets, Fair Value Disclosure Schedule of Common Shares Repurchased Schedule of Repurchase Agreements [Table Text Block] Amounts due to The Fidelis Partnership Accounts Payable Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Income Taxes Income Tax Disclosure [Text Block] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Debt Instrument [Line Items] Debt Instrument [Line Items] Investment Strategy [Domain] Investment Strategy [Domain] Investment Strategy [Domain] Reinsurance balances recoverable on reserves for losses and loss adjustment expenses Increase (Decrease) in Liability for Claims and Claims Adjustment Expense Reserve Additional paid-in capital Additional Paid-in Capital [Member] Fair value Debt Securities, Available-for-Sale, Unrealized Loss Position Fixed income funds Multi-Asset Credit Funds [Member] Multi-Asset Credit Funds Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table Text Block] Total other liabilities Other Liabilities, Fair Value Disclosure Cash and cash equivalents Cash and cash equivalents Cash and cash equivalents Cash and Cash Equivalent Recovery of commissions Recovery Of Commissions Recovery Of Commissions Loss ratio - total Loss Ratio Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Schedule of Group Segment Disclosures Segment Reporting, Reconciliation of Profit (Loss) by Segment to Consolidated [Table Text Block] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Investments, Debt and Equity Securities [Abstract] Investment Strategy [Axis] Investment Strategy [Axis] Investment Strategy Total Cash, Cash Equivalents, Restricted Cash and Restricted Investments Cash and Cash Equivalents Disclosure [Text Block] A A [Member] A Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Short-Duration Insurance Contracts, Historical Claims Duration [Table] Short-Duration Insurance Contracts, Historical Claims Duration [Table] Securities sold/redeemed/matured Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold Derivative liabilities Derivative Liability Senior Notes Senior Notes [Member] Combined ratio Combined Ratio Total expenses Benefits, Losses and Expenses Reinsurance, profit commission, hurdle rate of return, percentage Reinsurance, Profit Commission, Hurdle Rate Of Return, Percentage Reinsurance, Profit Commission, Hurdle Rate Of Return, Percentage Income tax expense Income tax expense Income tax (expense)/benefit Income Tax Expense (Benefit) Net foreign exchange (gains)/losses Net foreign exchange gains (losses) Gain (Loss), Foreign Currency Transaction, before Tax Net increase/(decrease) in cash, restricted cash, and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Property management fee, percent fee Property Management Fee, Percent Fee Premiums and other receivables (net of allowance for credit losses of $14.9, 2025: $15.8) Premiums Receivable, Net Notional amounts Derivative Liability, Notional Amount Net income Net Income (Loss) Attributable to Parent Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] The Fidelis Partnership commissions ratio Commissions Expense Ratio Commissions Expense Ratio Statement [Table] Statement [Table] Policy acquisition expense ratio Acquisition Cost Ratio Derivative assets at fair value Derivative Asset, Subject to Master Netting Arrangement, before Offset Derivative, basis spread on variable rate Derivative, Basis Spread on Variable Rate Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items] Debt securities, available-for-sale, continuous unrealized loss position, largest single amount Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Largest Single Amount Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Largest Single Amount Fidelis MGU Fidelis MGU [Member] Fidelis MGU Revenues Revenues [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]