|**Quarterly Financial Data (Unaudited) (Tables)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
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|**Quarterly Financial Data [Abstract]**           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Schedule of quarterly financial information       | The following tables present unaudited quarterly financial data.2019(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,695$2,610$2,685$2,777$10,767Net income (loss)(1)3422781072731,000Basic earnings (loss) per share(3)1.261.030.391.003.68Diluted earnings (loss) per share(3)$1.25$1.02$0.39$1.00$3.672018(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,535$2,574$2,622$2,403$10,134Net income (loss)(2)291270336(84)813Basic earnings (loss) per share(3)1.070.991.24(0.31)2.99Diluted earnings (loss) per share(3)$1.07$0.99$1.23$(0.31)$2.98(1)Net income (loss) in the third quarter of 2019 included a$170 millioncharge related to recognition of an active life reserve premium deficiency as a result of the third quarter 2019 GPV.(2)Net income (loss) in the fourth quarter of 2018 included a loss on limited partnership and common stock investments of$109 millionand catastrophe losses, net of reinsurance, of$107 millionrelated to Hurricane Michael and the California wildfires.(3)Earnings (loss) per share (EPS) in each quarter is computed using the weighted average number of shares outstanding during that quarter, while EPS for the full year is computed using the weighted average number of shares outstanding during the year. Thus, the sum of the four quarters EPS may not equal the full year EPS.
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|2019                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions, except per share data)              | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Revenues                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Net income (loss)(1)                              | 342                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Basic earnings (loss) per share(3)                | 1.26                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted earnings (loss) per share(3)              | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|2018                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|(In millions, except per share data)              | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Revenues                                          | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Net income (loss)(2)                              | 291                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Basic earnings (loss) per share(3)                | 1.07                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted earnings (loss) per share(3)              | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|(1)                                               | Net income (loss) in the third quarter of 2019 included a$170 millioncharge related to recognition of an active life reserve premium deficiency as a result of the third quarter 2019 GPV.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|(2)                                               | Net income (loss) in the fourth quarter of 2018 included a loss on limited partnership and common stock investments of$109 millionand catastrophe losses, net of reinsurance, of$107 millionrelated to Hurricane Michael and the California wildfires.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

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