|**Quarterly Supplementary Data (Tables)**                 | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Quarterly Financial Information Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Unaudited Quarterly Financial Information                 | This table includes unaudited quarterly financial information for the years endedDecember 31, 2017and2016:(Dollars in millions, except per share data)Quarter1st2nd3rd4thFull year2017Revenues$1,523$1,386$1,412$1,411$5,732Income before income taxes276128129197730Net income2011001026421,045Net income per common share—basic1.220.610.623.926.36Net income per common share—diluted1.210.600.613.886.292016Revenues$1,364$1,371$1,402$1,312$5,449Income before income taxes265166253128812Net income188123180100591Net income per common share—basic1.140.751.090.613.59Net income per common share—diluted1.130.741.080.603.55
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|(Dollars in millions, except per share data)              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|2017                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Revenues                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Income before income taxes                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income per common share—basic                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income per common share—diluted                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|2016                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Revenues                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Income before income taxes                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income per common share—basic                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income per common share—diluted                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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