|**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 31,20182017Deferred tax assets:Loss and loss expense reserves$60$123Unearned premiums105100Other3327Total gross deferred tax assets198250Deferred tax liabilities:Investment gains and other, net542740Deferred acquisition costs131123Life policy reserves117111Investments1810Other1711Total gross deferred tax liabilities825995Net deferred income tax liability$627$745                                         
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|(Dollars in millions)                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Deferred tax assets:                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Loss and loss expense reserves                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Unearned premiums                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Total gross deferred tax assets                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Deferred tax liabilities:                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|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 the21 percentand35 percentstatutory federal income tax rate and our effective income tax rate were as follows:(Dollars in millions)Years ended December 31,201820172016Tax at statutory rate:$5321.0%$25635.0%$28435.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(20)(8.0)(36)(4.9)(34)(4.2)Dividend received exclusion(15)(6.0)(34)(4.7)(33)(4.1)Tax accounting method changes(50)(19.9)————Deferred tax benefit due to tax rate change——(495)(67.8)——Other(4)(1.4)(6)(0.8)40.5Provision for income taxes$(36)(14.3)%$(315)(43.2)%$22127.2%
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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                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Tax accounting method changes                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Deferred tax benefit due to tax rate change                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Other                                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Provision for income taxes                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Reconciliation of Unrecognized Tax Benefits                                                | The following is a tabular reconciliation of the total amounts of unrecognized tax benefits.(Dollars in millions)Years ended December 31,201820172016Gross unrecognized tax benefits at January 1$—$—$—Gross increase in prior year positions———Gross decrease in prior year positions———Gross increase in current year positions34——Settlements with tax authorities———Lapse of statute of limitations———Gross unrecognized tax benefits at December 31$34$—$—                                                                                                                                    
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|(Dollars in millions)                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Gross unrecognized tax benefits at January 1                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross increase in prior year positions                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross decrease in prior year positions                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross increase in current year positions                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Settlements with tax authorities                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Lapse of statute of limitations                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Gross unrecognized tax benefits at December 31                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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