|**Reinsurance (Tables)**                                                                               | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                
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|**Reinsurance Disclosures [Abstract]**                                                                 |                                                                                                                                                                                                                                                                                                                                                                
|Property Casualty Insurance Premiums Written On Assumed And Ceded Business [Table Text Block]          | The tables below summarize our consolidated property casualty insurance net written premiums, earned premiums and incurred loss and loss expenses:(Dollars in millions)Years ended December 31,201720162015Direct written premiums$4,854$4,646$4,461Assumed written premiums12510377Ceded written premiums(139)(169)(177)Net written premiums$4,840$4,580$4,361
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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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|Earned Consolidated Property Casualty Insurance Premiums On Assumed And Ceded Business                 | (Dollars in millions)Years ended December 31,201720162015Direct earned premiums$4,752$4,567$4,396Assumed earned premiums1327729Ceded earned premiums(162)(162)(154)Earned premiums$4,722$4,482$4,271                                                                                                                                                           
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|(Dollars in millions)                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
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|Direct earned premiums                                                                                 |                                                                                                                                                                                                                                                                                                                                                                
|Assumed earned premiums                                                                                |                                                                                                                                                                                                                                                                                                                                                                
|Ceded earned premiums                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
|Earned premiums                                                                                        |                                                                                                                                                                                                                                                                                                                                                                
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|Incurred Consolidated Property Casualty Insurance Loss And Loss Expenses On Assumed And Ceded Business | (Dollars in millions)Years ended December 31,201720162015Direct incurred loss and loss expenses$2,961$2,874$2,596Assumed incurred loss and loss expenses1134312Ceded incurred loss and loss expenses64(56)(36)Incurred loss and loss expenses$3,138$2,861$2,572                                                                                                
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|(Dollars in millions)                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
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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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|Earned Life Insurance Premiums On Ceded Business                                                       | consolidated life insurance earned premiums and contract holders' benefits incurred:(Dollars in millions)Years ended December 31,201720162015Direct earned premiums$300$290$271Ceded earned premiums(68)(62)(62)Earned premiums$232$228$209                                                                                                                    
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|(Dollars in millions)                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
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|Direct earned premiums                                                                                 |                                                                                                                                                                                                                                                                                                                                                                
|Ceded earned premiums                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
|Earned premiums                                                                                        |                                                                                                                                                                                                                                                                                                                                                                
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|Life Insurance Contract Holders' Benefits Incurred On Ceded Business                                   | (Dollars in millions)Years ended December 31,201720162015Direct contract holders' benefits incurred$319$303$292Ceded contract holders' benefits incurred(67)(57)(56)Contract holders' benefits incurred$252$246$236                                                                                                                                            
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|(Dollars in millions)                                                                                  |                                                                                                                                                                                                                                                                                                                                                                
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|Direct contract holders' benefits incurred                                                             |                                                                                                                                                                                                                                                                                                                                                                
|Ceded contract holders' benefits incurred                                                              |                                                                                                                                                                                                                                                                                                                                                                
|Contract holders' benefits incurred                                                                    |                                                                                                                                                                                                                                                                                                                                                                
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