|**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 31,20172016Deferred tax assets:Loss and loss expense reserves$123$199Unearned premiums100158Investments—5Other2752Total gross deferred tax assets250414Deferred tax liabilities:Unrealized investment gains and other, net740907Deferred acquisition costs123195Life policy reserves111162Investments10—Other1115Total gross deferred tax liabilities9951,279Net deferred income tax liability$745$865
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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 and other, net                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred acquisition costs                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Life policy reserves                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Investments                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|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 31,201720162015Tax at statutory rate:$25635.0%$28435.0%$30835.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(36)(4.9)(34)(4.2)(33)(3.7)Dividend received exclusion(34)(4.7)(33)(4.1)(32)(3.6)Deferred tax benefit due to tax rate change(495)(67.8)————Other(6)(0.8)40.540.3Provision for income taxes$(315)(43.2)%$22127.2%$24728.0%                                               
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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                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deferred tax benefit due to tax rate change                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Provision for income taxes                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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