| **Employee Retirement Benefits**                      | 9 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 September 30, Nine months ended September 30,  2016 2015 2016 2015Service cost $3 $3 $8 $9Interest cost 4 3 11 10Expected return on plan assets (5) (4) (14) (13)Amortization of actuarial loss and prior service cost 0 1 2 4Net periodic benefit cost $2 $3 $7 $10         See our 2015 Annual Report on Form 10-K, Item 8, Note 13, Employee Retirement Benefits, Page 146, for information on our retirement benefits. We made matching contributions totaling $3 million and $2 million to our 401(k) and Top Hat savings plans during the third quarter of 2016 and 2015, respectively, and contributions of$11 million and $9 million for the first nine months of 2016 and 2015, respectively.  We contributed $13 million to our qualified pension plan during the first nine months of 2016. We do not anticipate further contributions during the remainder of 2016. |

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| - Definition
The entire disclosure for pension and other postretirement benefits.
+ References
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