|**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 31,20142013Deferred tax assets:Loss and loss expense reserves$197$206Unearned premiums145137Investments1619Other4146Total399408Deferred tax liabilities:Unrealized investment gains, net937807Deferred acquisition costs183178Life policy reserves11086Other910Total1,2391,081Net deferred income tax liability$840$673
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|(Dollars in millions)                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
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|Deferred tax assets:                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Loss and loss expense reserves                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Unearned premiums                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Investments                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Total                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deferred tax liabilities:                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Unrealized investment gains, net                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Deferred acquisition costs                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Life policy reserves                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Total                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|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 31,201420132012Tax at statutory rate:$25235.0%$25035.0%$19835.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(33)(4.6)(32)(4.5)(33)(5.9)Dividend received exclusion(29)(4.0)(26)(3.6)(24)(4.2)Other60.850.740.7Provision for income taxes$19627.2%$19727.6%$14525.6%                                  
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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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