|**Income Taxes (Tables)**                                                   | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Income Tax Disclosure [Abstract]**                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Differences Between Statutory Income Tax Rate And Effective Income Tax Rate | The differences between the 21% statutory federal income tax rate and our effective income tax rate were as follows:(Dollars in millions)Three months ended September 30,Nine months ended September 30,2020201920202019Tax at statutory rate:$12921.0%$6221.0%$3821.0%$35521.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(5)(0.8)(5)(1.7)(15)(8.2)(14)(0.8)Dividend received exclusion(5)(0.8)(3)(1.0)(13)(7.1)(11)(0.7)Other111.8(8)(2.7)63.0(10)(0.6)Provision for income taxes$13021.2%$4615.6%$168.7%$32018.9%
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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                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|Other                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Provision for income taxes                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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