|**Income Taxes**                       | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Income Tax Disclosure [Abstract]**   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Income Taxes                           | Income TaxesAs ofJune 30, 2015andDecember 31, 2014, 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 June 30,Six months ended June 30,2015201420152014Tax at statutory rate:$8735.0%$3735.0%$14835.0%$7935.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(8)(3.2)(8)(7.5)(16)(3.8)(16)(7.1)Dividend received exclusion(7)(2.8)(7)(6.5)(15)(3.6)(14)(6.2)Other00.010.510.420.9Provision for income taxes$7229.0%$2321.5%$11828.0%$5122.6%The provision for federal income taxes is based upon filing a consolidated income tax return for the company and its subsidiaries. As of June 30, 2015, we had no operating or capital loss carry forwards.
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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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duration X- DefinitionThe entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.
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