|**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 table below includes our net written consolidated property casualty insurance premiums on assumed and ceded business:(Dollars in millions)Years ended December 31,201620152014Direct written premiums$4,646$4,461$4,309Assumed written premiums1037711Ceded written premiums(169)(177)(177)Net written premiums$4,580$4,361$4,143                                                                               
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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                 | Our consolidated statements of income include earned consolidated property casualty insurance premiums onassumed and ceded business:(Dollars in millions)Years ended December 31,201620152014Direct earned premiums$4,567$4,396$4,209Assumed earned premiums772912Ceded earned premiums(162)(154)(176)Earned premiums$4,482$4,271$4,045                                                                             
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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 | Our consolidated statements of income include incurred consolidated property casualty insurance loss and loss expenses on assumed and ceded business:(Dollars in millions)Years ended December 31,201620152014Direct incurred loss and loss expenses$2,874$2,596$2,661Assumed incurred loss and loss expenses43126Ceded incurred loss and loss expenses(56)(36)(40)Incurred loss and loss expenses$2,861$2,572$2,627
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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                                                       | Our consolidated statements of income include earned life insurance premiums on ceded business:(Dollars in millions)Years ended December 31,201620152014Direct earned premiums$290$271$259Ceded earned premiums(62)(62)(61)Earned premiums$228$209$198                                                                                                                                                              
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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                                   | Our consolidated statements of income include life insurance contract holders' benefits incurred on ceded business:(Dollars in millions)Years ended December 31,201620152014Direct contract holders' benefits incurred$303$292$299Ceded contract holders' benefits incurred(57)(56)(70)Contract holders' benefits incurred$246$236$229                                                                              
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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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