|**Income Taxes**                       | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Income Tax Disclosure [Abstract]**   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Income Taxes                           | Income TaxesAs ofMarch 31, 2018, andDecember 31, 2017, we had no liability for unrecognized tax benefits.The differences between the21percent and35percent statutory federal income tax rates and our effective income tax rate were as follows:(Dollars in millions)Three months ended March 31,20182017Tax at statutory rate:$(11)21.0%$9735.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(5)10.0(9)(3.3)Dividend received exclusion(3)6.0(8)(2.9)Other—1.0(5)(1.6)Provision for income taxes$(19)38.0%$7527.2%The provision for federal income taxes is based upon filing a consolidated income tax return for the company and its subsidiaries.Our 2017 10-K discusses enactment of the Tax Cuts and Jobs Act (the “Tax Act”) on December 22, 2017, and its impact on our financial results for that period. Interpretive guidance of the Tax Act will be received throughout 2018, and we expect to update our estimates and our disclosure on a quarterly basis as interpretative guidance is received within each quarter that it is received. During the period ended March 31, 2018, the U.S. Treasury Department (the “Treasury”) and the Internal Revenue Service (the ”IRS”) have not issued further clarification or guidance for the items for which our accounting for the Tax Act is incomplete. We expect to complete determination of the effects of the Tax Act on our deferred tax assets and liabilities as part of the annual income tax return filing process.As ofMarch 31, 2018, we had no operating or capital loss carryforwards.
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|(Dollars in millions)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Increase (decrease) resulting from:    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Tax-exempt income from municipal bonds |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Provision for income taxes             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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X- ReferencesNo definition available.
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