|**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, 2018and2017:(Dollars in millions, except per share data)Quarter1st2nd3rd4thFull year2018Revenues$1,224$1,558$1,915$710$5,407Income (loss) before income taxes(50)264618(581)251Net income (loss)(31)217553(452)287Net income (loss) per common share—basic(0.19)1.333.40(2.78)1.76Net income (loss) per common share—diluted(0.19)1.323.38(2.78)1.752017Revenues$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—diluted1.210.600.613.886.29
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|(Dollars in millions, except per share data)              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|2018                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Revenues                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Income (loss) before income taxes                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net income (loss)                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net income (loss) per common share—basic                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net income (loss) per common share—diluted                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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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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