|**Income Taxes (Tables)**                                                                  | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|**Income Tax Disclosure [Abstract]**                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Components Of Deferred Tax Assets And Liabilities                                          | The significant components of deferred tax assets and liabilities included in the consolidated balance sheets at December 31 were as follows:(Dollars in millions)At December 31,20152014Deferred tax assets:Loss and loss expense reserves$191$197Unearned premiums151145Investments2516Other4241Total gross deferred tax assets409399Deferred tax liabilities:Unrealized investment gains, net719937Deferred acquisition costs187183Life policy reserves137110Other49Total gross deferred tax liabilities1,0471,239Net deferred income tax liability$638$840
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|(Dollars in millions)                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|Deferred tax assets:                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Loss and loss expense reserves                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Unearned premiums                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Investments                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total gross deferred tax assets                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Deferred tax liabilities:                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Unrealized investment gains, net                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Deferred acquisition costs                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Life policy reserves                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total gross deferred tax liabilities                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Net deferred income tax liability                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|Differences Between The 35 Percent Statutory Income Tax Rate And Effective Income Tax Rate | The differences between the35 percentstatutory federal income tax rate and our effective income tax rate were as follows:(Dollars in millions)Years ended December 31,201520142013Tax at statutory rate:$30835.0%$25235.0%$25035.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(33)(3.7)(33)(4.6)(32)(4.5)Dividend received exclusion(32)(3.6)(29)(4.0)(26)(3.6)Other40.360.850.7Provision for income taxes$24728.0%$19627.2%$19727.6%                                                                                           
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|(Dollars in millions)                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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|Tax at statutory rate:                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Increase (decrease) resulting from:                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Tax-exempt income from municipal bonds                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Dividend received exclusion                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Provision for income taxes                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
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