|**Employee Retirement Benefits**                      | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Compensation and Retirement Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Employee Retirement Benefits                          | Employee Retirement BenefitsThe following summarizes the components of net periodic benefit cost for our qualified and supplemental pension plans:(Dollars in millions)Three months ended June 30,Six months ended June 30,2017201620172016Service cost$2$2$5$5Interest cost4477Expected return on plan assets(5)(4)(10)(9)Amortization of actuarial loss and prior service cost—112Net periodic benefit cost$1$3$3$5See our 2016 Annual Report on Form 10-K, Item 8, Note 13, Employee Retirement Benefits, Page 155, for information on our retirement benefits. We made matching contributions totaling$3 millionand$4 millionto our  401(k) and Top Hat savings plans during thesecond quarterof 2017 and 2016 and contributions of$9 millionand$8 millionfor the first half of 2017 and 2016, respectively.We contributed$5 millionto our qualified pension plan during the first six months of 2017.
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|(Dollars in millions)                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Service cost                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Interest cost                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Expected return on plan assets                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Amortization of actuarial loss and prior service cost |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Net periodic benefit cost                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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