|**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)202120202019Income tax expense at statutory rates$(312)$(172)$(257)Tax benefit from tax exempt income515253Foreign taxes and credits(3)2(1)State income tax expense(13)(6)(14)Other tax expense(5)(7)(4)Income tax expense$(282)$(131)$(223)                                                                                                                                                                                                                                                                           
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions)                                                                                                                              | 2021                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Income tax expense at statutory rates                                                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Tax benefit from tax exempt income                                                                                                         | 51                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Foreign taxes and credits                                                                                                                  | (3)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|State income tax expense                                                                                                                   | (13)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Other tax expense                                                                                                                          | (5)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|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)202120202019Current tax expense$(235)$(180)$(269)Deferred tax (expense) benefit(47)4946Total income tax expense$(282)$(131)$(223)                                                                                                                                                                                                                                                                                                                                                                                                                               
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions)                                                                                                                              | 2021                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Current tax expense                                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Deferred tax (expense) benefit                                                                                                             | (47)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|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)20212020Deferred Tax Assets:Insurance reserves:Property and casualty claim and claim adjustment expense reserves$173$157Unearned premium reserves193174Deferred Revenue6462Employee benefits46122Deferred retroactive reinsurance benefit9083Other assets8892Gross deferred tax assets654690Deferred Tax Liabilities:Investment valuation differences9328Deferred acquisition costs9993Net unrealized gains272453Software and hardware2731Other liabilities2119Gross deferred tax liabilities512624Net deferred tax asset$142$66
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|December 31                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions)                                                                                                                              | 2021                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred Tax Assets:                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Insurance reserves:                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Property and casualty claim and claim adjustment expense reserves                                                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Unearned premium reserves                                                                                                                  | 193                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deferred Revenue                                                                                                                           | 64                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Employee benefits                                                                                                                          | 46                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Deferred retroactive reinsurance benefit                                                                                                   | 90                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Other assets                                                                                                                               | 88                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Gross deferred tax assets                                                                                                                  | 654                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deferred Tax Liabilities:                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Investment valuation differences                                                                                                           | 93                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Deferred acquisition costs                                                                                                                 | 99                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Net unrealized gains                                                                                                                       | 272                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Software and hardware                                                                                                                      | 27                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Other liabilities                                                                                                                          | 21                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Gross deferred tax liabilities                                                                                                             | 512                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Net deferred tax asset                                                                                                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

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