|**Reinsurance (Tables)**                                                                               | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|**Reinsurance Disclosures [Abstract]**                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Earned Consolidated Property Casualty Insurance Premiums On Assumed And Ceded Business                 | Our condensed consolidated statements of income include earned consolidated property casualty insurance premiums on assumed and ceded business:(Dollars in millions)Three months ended June 30,Six months ended June 30,2016201520162015Direct earned premiums$1,137$1,093$2,256$2,169Assumed earned premiums163315Ceded earned premiums(39)(37)(77)(74)Earned premiums$1,114$1,059$2,210$2,100                                                                        
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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 condensed consolidated statements of income include incurred consolidated property casualty insurance loss and loss expenses on assumed and ceded business:(Dollars in millions)Three months ended June 30,Six months ended June 30,2016201520162015Direct incurred loss and loss expenses$792$675$1,440$1,366Assumed incurred loss and loss expenses111210Ceded incurred loss and loss expenses(44)(22)(41)(23)Incurred loss and loss expenses$759$654$1,420$1,343
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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 condensed consolidated statements of income include earned life insurance premiums on ceded business:(Dollars in millions)Three months ended June 30,Six months ended June 30,2016201520162015Direct earned premiums$75$68$146$133Ceded earned premiums(16)(16)(29)(28)Earned premiums$59$52$117$105                                                                                                                                                               
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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 condensed consolidated statements of income include life insurance contract holders' benefits incurred on ceded business:(Dollars in millions)Three months ended June 30,Six months ended June 30,2016201520162015Direct contract holders' benefits incurred$80$72$156$144Ceded contract holders' benefits incurred(18)(14)(31)(26)Contract holders' benefits incurred$62$58$125$118                                                                               
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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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