|**Income Taxes (Tables)**                                                                                                                   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|**Income Tax Disclosure [Abstract]**                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Reconcilliation 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)201720162015Income tax expense at statutory rates$(459)$(398)$(192)Tax benefit from tax exempt income131124123Foreign taxes and credits339Net deferred tax asset remeasurement(83)——Other tax expense(3)(7)(10)Income tax expense$(411)$(278)$(70)                                                                                                                                                                                                                                                                                                  
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|Years ended December 31                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|(In millions)                                                                                                                               | 2017                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Income tax expense at statutory rates                                                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Tax benefit from tax exempt income                                                                                                          | 131                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Foreign taxes and credits                                                                                                                   | 3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net deferred tax asset remeasurement                                                                                                        | (83                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Other tax expense                                                                                                                           | (3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|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)201720162015Current tax expense$(243)$(142)$(220)Deferred tax benefit (expense)(168)(136)150Total income tax expense$(411)$(278)$(70)                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|Years ended December 31                                                                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|(In millions)                                                                                                                               | 2017                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Current tax expense                                                                                                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Deferred tax benefit (expense)                                                                                                              | (168                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|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)20172016Deferred Tax Assets:Insurance reserves:Property and casualty claim and claim adjustment expense reserves$74$125Unearned premium reserves142206Receivables1527Employee benefits154272Life settlement contracts—56Deferred retroactive reinsurance benefit68117Investment valuation differences——Other assets105148Gross deferred tax assets558951Deferred Tax Liabilities:Investment valuation differences5557Deferred acquisition costs77120Net unrealized gains233309Other liabilities5686Gross deferred tax liabilities421572Net deferred tax asset$137$379
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|December 31                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|(In millions)                                                                                                                               | 2017                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Deferred Tax Assets:                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Insurance reserves:                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Property and casualty claim and claim adjustment expense reserves                                                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Unearned premium reserves                                                                                                                   | 142                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Receivables                                                                                                                                 | 15                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Employee benefits                                                                                                                           | 154                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Life settlement contracts                                                                                                                   | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Deferred retroactive reinsurance benefit                                                                                                    | 68                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Investment valuation differences                                                                                                            | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Other assets                                                                                                                                | 105                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Gross deferred tax assets                                                                                                                   | 558                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Deferred Tax Liabilities:                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Investment valuation differences                                                                                                            | 55                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Deferred acquisition costs                                                                                                                  | 77                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Net unrealized gains                                                                                                                        | 233                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Other liabilities                                                                                                                           | 56                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Gross deferred tax liabilities                                                                                                              | 421                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Net deferred tax asset                                                                                                                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

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