|**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 30,2015201420152014Service cost$3$3$6$5Interest cost4377Expected return on plan assets(5)(4)(9)(8)Amortization of actuarial loss and prior servicecost2031Net periodic benefit cost$4$2$7$5See our 2014 Annual Report on Form 10-K, Item 8, Note 13, Employee Retirement Benefits, Page 148, for information on our retirement benefits. We made matching contributions totaling$3 millionand$2 millionto our401(k) and Top Hat savings plans during the second quarter of 2015 and 2014 and contributions of$7 millionand$6 millionfor the first half of 2015 and 2014, respectively.We contributed$5 millionto our qualified pension plan during the first quarter of 2015. We do not anticipate further contributions to our qualified pension plan during the remainder of 2015.
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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 servicecost  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net periodic benefit cost                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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