|**Income Taxes**                       | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
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|**Income Tax Disclosure [Abstract]**   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Income Taxes                           | Income TaxesAs ofSeptember 30, 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 September 30,Nine months ended September 30,2018201720182017Tax at statutory rate:$13021.0%$4635.0%$17521.0%$18735.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(5)(0.8)(9)(7.0)(15)(1.8)(27)(5.1)Dividend received exclusion(3)(0.5)(8)(6.2)(11)(1.3)(25)(4.7)Tax accounting method changes(50)(8.1)00.0(50)(6.0)00.0Other(7)(1.1)(2)(0.9)(6)(0.7)(5)(0.8)Provision for income taxes$6510.5%$2720.9%$9311.2%$13024.4%The provision for federal income taxes is based upon filing a consolidated income tax return for the company and its subsidiaries.See our 2017 Annual Report on Form 10-K, Item 8, Note 11, Income Taxes, Page 153, which discusses enactment of the Tax Cuts and Jobs Act (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 will update our estimates and our disclosure on a quarterly basis as interpretative guidance is received within each quarter that it is received. During the three months ended September 30, 2018, the Internal Revenue Service (IRS) issued guidance on executive compensation which had an immaterial impact on our financial statements. Additional clarification or guidance may be issued by the U.S. Treasury and the IRS in the fourth quarter which will allow us to update our estimates for items for which our accounting for the Tax Act is incomplete. We will complete our accounting for the effects of the Tax Act under the SAB 118 guidance in the fourth quarter of 2018.During the third quarter of 2018, we received approval from the IRS to change our method of tax accounting for certain items applicable for the 2017 tax year and tax return, primarily related to the valuation of our tax base unpaid losses. Accounting guidance does not allow recognition of the impact of certain tax accounting method changes until approved by the IRS. As a result, we recognized a benefit in the third quarter provision for income taxes for the difference between the current tax rate and the 2017 tax rate for the related items. This reduced our effective tax rate by8.1percent and6.0percent for the three months and nine months ended September 30, 2018, respectively.As ofSeptember 30, 2018, we had no operating or capital loss carryforwards.
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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            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Tax accounting method changes          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|Provision for income taxes             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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X- ReferencesNo definition available.
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+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -URI http://asc.fasb.org/topic&trid=2144680
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