|**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:(In millions)At December 31,20132012Deferred tax assets:Loss and loss expense reserves$206$202Unearned premiums137124Investments1931Other4639Total408396Deferred tax liabilities:Unrealized investment gains, net and other807605Deferred acquisition costs178163Life policy reserves8665Other1016Total1,081849Net deferred tax liability$673$453
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|(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 and other                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Deferred acquisition costs                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Life policy reserves                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Total                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Net deferred 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 income tax rate and our effective income tax rate were as follows:(Dollars in millions)Years ended December 31,201320122011Tax at statutory rate:$25035.0%$19835.0%$6135.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(32)(4.5)(33)(5.9)(35)(20.0)Dividend received exclusion(26)(3.6)(24)(4.2)(20)(11.7)Other50.740.731.9Provision for income taxes$19727.6%$14525.6%$95.2%                                     
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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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