|**Income Taxes**                       | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|-------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Income Tax Disclosure [Abstract]**   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Income Taxes                           | Income TaxesAs ofMarch 31, 2016, andDecember 31, 2015, we had no liability for unrecognized tax benefits.The differences between the35percent statutory federal income tax rate and our effective income tax rate were as follows:(Dollars in millions)Three months ended March 31,20162015Tax at statutory rate:$9335.0%$6135.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(9)(3.4)(8)(4.6)Dividend received exclusion(8)(3.0)(8)(4.6)Other10.510.6Provision for income taxes$7729.1%$4626.4%The provision for federal income taxes is based upon filing a consolidated income tax return for the company and its subsidiaries. As of March 31, 2016, we had no operating or capital loss carry forwards.
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|(Dollars in millions)                  | Three months ended March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|                                       | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Tax at statutory rate:                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Increase (decrease) resulting from:    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Tax-exempt income from municipal bonds | (9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Dividend received exclusion            | (8                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Other                                  | 1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Provision for income taxes             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
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