|**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 31,20162015Deferred tax assets:Loss and loss expense reserves$199$191Unearned premiums158151Investments525Other5242Total gross deferred tax assets414409Deferred tax liabilities:Unrealized investment gains, net907719Deferred acquisition costs195187Life policy reserves162137Other154Total gross deferred tax liabilities1,2791,047Net deferred income tax liability$865$638
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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 31,201620152014Tax at statutory rate:$28435.0%$30835.0%$25235.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(34)(4.2)(33)(3.7)(33)(4.6)Dividend received exclusion(33)(4.1)(32)(3.6)(29)(4.0)Other40.540.360.8Provision for income taxes$22127.2%$24728.0%$19627.2%                                                                                           
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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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