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Basis of Presentation and Significant Accounting Policies (Details) - USD ($)
 12 Months Ended
Dec. 31, 2027
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Participating dividends to policyholders     $ 0 $ 0 $ 0
Derivative Liability, Statement of Financial Position [Extensible Enumeration]     Other investments, Other liabilities Other investments, Other liabilities  
Deposit Contracts, Liabilities     $ 0 $ 0  
Accumulated depreciation     2,500,000,000 2,400,000,000  
Depreciation expense     177,000,000 204,000,000 213,000,000
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization     108,000,000 85,000,000 78,000,000
Policyholder Dividends, Expense     $ 39,000,000 $ 39,000,000 29,000,000
Equity Security, FV-NI, Contractual Sale Restriction, Nature of Restriction and Circumstance for Lapse     Level 3 includes investments that have contractual sales restrictions that require consent to sell and are in place for the duration that the securities are held by the Company. Level 3 includes investments that have contractual sales restrictions that require consent to sell and are in place for the duration that the securities are held by the Company.  
Change in Accounting Principle, Accounting Standards Update, Adoption Date     Dec. 31, 2024    
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]     true    
Accounting Standards Update [Extensible Enumeration]     Accounting Standards Update 2023-07 [Member]    
Subsequent Event [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Change in Accounting Principle, Accounting Standards Update, Adoption Date Dec. 31, 2027 Dec. 31, 2025      
Accounting Standards Update [Extensible Enumeration] Accounting Standards Update 2024-03 [Member] Accounting Standards Update 2023-09 [Member]      
Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible Enumeration] Accounting Standards Updates 2024-03 Prospective [Member]        
Minimum [Member] | Other Intangible Assets [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Finite-Lived Intangible Asset, Useful Life     1 year    
Maximum | Other Intangible Assets [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Finite-Lived Intangible Asset, Useful Life     15 years    
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Deferred Revenue     $ 64,000,000 $ 209,000,000  
Policyholder Benefits and Claims Incurred, Ceded     300,000,000    
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group | Remaining [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Reinsurance, Excess Retention, Amount Reinsured, Per Policy     0    
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group | Maximum          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Reinsurance, Excess Retention, Amount Reinsured, Per Policy     300,000,000    
Reinsurance Policy, Type [Axis]: Retroactive Reinsurance [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Paid loss and loss adjustment expenses     1,600,000,000 1,700,000,000  
Deferred Revenue, Additions     (83,000,000) (194,000,000) $ (229,000,000)
Reinsurance Policy, Type [Axis]: Retroactive Reinsurance [Member] | Other liabilities          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Deferred Revenue     $ 914,000,000 $ 997,000,000  
Property and Casualty Insurance Subsidiaries          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Participating Insurance, Percentage of Premium Income     6.00% 6.00% 7.00%
Policyholder Benefits and Claims Incurred, Ceded     $ 1,241,000,000 $ 1,043,000,000 $ 1,338,000,000