|**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 federal income tax expense at statutory rates and the recorded income tax expense.Years ended December 31(In millions)201920182017Income tax expense at statutory rates$(257)$(203)$(459)Tax benefit from tax exempt income5363131Foreign taxes and credits(1)(1)3State income taxes(14)(13)(7)Net deferred tax asset remeasurement—6(83)Other tax expense(4)(3)4Income tax expense$(223)$(151)$(411)                                                                                                                                                                                                                              
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(In millions)                                                                                                                              | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Income tax expense at statutory rates                                                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Tax benefit from tax exempt income                                                                                                         | 53                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Foreign taxes and credits                                                                                                                  | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|State income taxes                                                                                                                         | (14                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Net deferred tax asset remeasurement                                                                                                       | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Other tax expense                                                                                                                          | (4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|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)201920182017Current tax expense$(269)$(171)$(243)Deferred tax benefit (expense)4620(168)Total income tax expense$(223)$(151)$(411)                                                                                                                                                                                                                                                                                                                                                                                                                            
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|Years ended December 31                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(In millions)                                                                                                                              | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Current tax expense                                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deferred tax benefit (expense)                                                                                                             | 46                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|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)20192018Deferred Tax Assets:Insurance reserves:Property and casualty claim and claim adjustment expense reserves$129$108Unearned premium reserves153108Receivables1115Employee benefits127143Deferred retroactive reinsurance benefit8279Other assets132131Gross deferred tax assets634584Deferred Tax Liabilities:Investment valuation differences4044Deferred acquisition costs8378Net unrealized gains26414Software and hardware3444Other liabilities1412Gross deferred tax liabilities435192Net deferred tax asset$199$392
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|December 31                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(In millions)                                                                                                                              | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Deferred Tax Assets:                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Insurance reserves:                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Property and casualty claim and claim adjustment expense reserves                                                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Unearned premium reserves                                                                                                                  | 153                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Receivables                                                                                                                                | 11                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Employee benefits                                                                                                                          | 127                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Deferred retroactive reinsurance benefit                                                                                                   | 82                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Other assets                                                                                                                               | 132                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross deferred tax assets                                                                                                                  | 634                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Deferred Tax Liabilities:                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Investment valuation differences                                                                                                           | 40                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred acquisition costs                                                                                                                 | 83                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net unrealized gains                                                                                                                       | 264                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Software and hardware                                                                                                                      | 34                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Other liabilities                                                                                                                          | 14                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Gross deferred tax liabilities                                                                                                             | 435                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Net deferred tax asset                                                                                                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

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