|**Income Taxes (Tables)**                                                                                                                  | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|**Income Tax Disclosure [Abstract]**                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Reconciliation between the Company's federal income tax (expense) benefit at statutory rates and the recorded income tax (expense) benefit | The following table presents a reconciliation between the Company's income tax expense at statutory rates and the recorded income tax expense.Years ended December 31(In millions)202420232022Income tax expense at statutory rates$(254)$(319)$(172)Tax benefit from tax exempt income253041Foreign taxes and credits(3)(5)15State income tax expense(12)(13)(10)Other tax expense(8)(6)(6)Income tax expense$(252)$(313)$(132)                                                                                                                                                                                                                                                                                                                                      
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|(In millions)                                                                                                                              | 2024                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Income tax expense at statutory rates                                                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Tax benefit from tax exempt income                                                                                                         | 25                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Foreign taxes and credits                                                                                                                  | (3)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|State income tax expense                                                                                                                   | (12)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Other tax expense                                                                                                                          | (8)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Income tax expense                                                                                                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Current and deferred components of the Company's income tax (expense) benefit                                                              | The following table presents the current and deferred components of the Company's income tax expense.Years ended December 31(In millions)202420232022Current tax expense$(297)$(311)$(221)Deferred tax benefit (expense)45(2)89Total income tax expense$(252)$(313)$(132)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|(In millions)                                                                                                                              | 2024                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Current tax expense                                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred tax benefit (expense)                                                                                                             | 45                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Total income tax expense                                                                                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Significant components of the Company's deferred tax assets and liabilities                                                                | The deferred tax effects of the significant components of the Company's deferred tax assets and liabilities are presented in the following table.December 31(In millions)20242023Deferred Tax Assets:Insurance reserves:Property and casualty claim and claim adjustment expense reserves$234$202Unearned premium reserves225213Policyholder reserves—160Deferred revenue5962Employee benefits1323Deferred retroactive reinsurance benefit8988Net unrealized losses494418Other assets107111Gross deferred tax assets1,2211,277Deferred Tax Liabilities:Investment valuation differences13083Deferred acquisition costs140126Policyholder reserves48—Software and hardware1718Other liabilities3634Gross deferred tax liabilities371261Net deferred tax asset$850$1,016
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|December 31                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|(In millions)                                                                                                                              | 2024                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Deferred Tax Assets:                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Insurance reserves:                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Property and casualty claim and claim adjustment expense reserves                                                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unearned premium reserves                                                                                                                  | 225                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Policyholder reserves                                                                                                                      | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred revenue                                                                                                                           | 59                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Employee benefits                                                                                                                          | 13                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Deferred retroactive reinsurance benefit                                                                                                   | 89                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Net unrealized losses                                                                                                                      | 494                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Other assets                                                                                                                               | 107                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Gross deferred tax assets                                                                                                                  | 1,221                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Deferred Tax Liabilities:                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Investment valuation differences                                                                                                           | 130                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Deferred acquisition costs                                                                                                                 | 140                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Policyholder reserves                                                                                                                      | 48                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Software and hardware                                                                                                                      | 17                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Other liabilities                                                                                                                          | 36                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross deferred tax liabilities                                                                                                             | 371                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net deferred tax asset                                                                                                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

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