|**Income Taxes (Tables)**                                                   | 6 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 June 30,Six months ended June 30,2020201920202019Tax at statutory rate:$24021.0%$11121.0%$(91)21.0%$29321.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(5)(0.4)(4)(0.8)(10)2.3(9)(0.6)Dividend received exclusion(4)(0.3)(4)(0.8)(8)1.9(8)(0.6)Other50.3(1)(0.2)(5)1.3(2)(0.2)Provision (benefit) for income taxes$23620.6%$10219.2%$(114)26.5%$27419.6%
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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 (benefit) for income taxes                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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