|**Reinsurance (Tables)**                                                                               | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|**Reinsurance Disclosures [Abstract]**                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Property Casualty Insurance Premiums Written On Assumed And Ceded Business                             | The table below includes our net written consolidated property casualty insurance premiums on assumed and ceded business:(Dollars in millions)Three months ended June 30,Six months ended June 30,2017201620172016Direct written premiums$1,265$1,211$2,491$2,372Assumed written premiums42427563Ceded written premiums(36)(59)(64)(94)Net written premiums$1,271$1,194$2,502$2,341                                                                                   
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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 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 30,2017201620172016Direct earned premiums$1,189$1,137$2,352$2,256Assumed earned premiums33166031Ceded earned premiums(41)(39)(80)(77)Earned premiums$1,181$1,114$2,332$2,210                                                                     
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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 30,2017201620172016Direct incurred loss and loss expenses$770$792$1,558$1,440Assumed incurred loss and loss expenses20113521Ceded incurred loss and loss expenses4(44)(11)(41)Incurred loss and loss expenses$794$759$1,582$1,420
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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 30,2017201620172016Direct earned premiums$77$75$151$146Ceded earned premiums(17)(16)(34)(29)Earned premiums$60$59$117$117                                                                                                                                                              
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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 30,2017201620172016Direct contract holders' benefits incurred$87$80$163$156Ceded contract holders' benefits incurred(27)(18)(38)(31)Contract holders' benefits incurred$60$62$125$125                                                                              
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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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