|**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 the21% 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 30,2019201820192018Tax at statutory rate:$11121.0%$5621.0%$29321.0%$4521.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(4)(0.8)(5)(1.9)(9)(0.6)(10)(4.7)Dividend received exclusion(4)(0.8)(5)(1.5)(8)(0.6)(8)(3.7)Other(1)(0.2)10.2(2)(0.2)10.5Provision for income taxes$10219.2%$4717.8%$27419.6%$2813.1%
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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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