|**Reinsurance (Tables)**                               | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Reinsurance Disclosures [Abstract]**                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Property Casualty Insurance Assumed And Ceded Business | The table below summarizes our consolidated property casualty insurance net written premiums, earned premiums and incurred loss and loss expenses:(Dollars in millions)Years ended December 31,201820172016Direct written premiums$5,018$4,854$4,646Assumed written premiums173125103Ceded written premiums(161)(139)(169)Net written premiums$5,030$4,840$4,580Direct earned premiums$4,931$4,752$4,567Assumed earned premiums14913277Ceded earned premiums(160)(162)(162)Earned premiums$4,920$4,722$4,482Direct incurred loss and loss expenses$3,188$2,961$2,874Assumed incurred loss and loss expenses12511343Ceded incurred loss and loss expenses(90)64(56)Incurred loss and loss expenses$3,223$3,138$2,861
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|(Dollars in millions)                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Direct written premiums                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Assumed written premiums                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Ceded written premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Net written premiums                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Direct earned premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Assumed earned premiums                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Ceded earned premiums                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Earned premiums                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Direct incurred loss and loss expenses                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Assumed incurred loss and loss expenses                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Ceded incurred loss and loss expenses                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Incurred loss and loss expenses                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Life Insurance Assumed And Ceded Business              | The table below summarizes our consolidated life insurance earned premiums and contract holders' benefits incurred:(Dollars in millions)Years ended December 31,201820172016Direct earned premiums$320$300$290Ceded earned premiums(70)(68)(62)Earned premiums$250$232$228Direct contract holders' benefits incurred$328$319$303Ceded contract holders' benefits incurred(61)(67)(57)Contract holders' benefits incurred$267$252$246                                                                                                                                                                                                                                                                               
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|(Dollars in millions)                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Direct earned premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Ceded earned premiums                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Earned premiums                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Direct contract holders' benefits incurred             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Ceded contract holders' benefits incurred              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Contract holders' benefits incurred                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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