| **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,  2015 2014 2015 2014Service cost $3 $3 $9 $8Interest cost 3 4 10 11Expected return on plan assets (4) (5) (13) (13)Amortization of actuarial loss and prior service  cost 1 1 4 2Net periodic benefit cost $3 $3 $10 $8         See 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 $2 million and $3 million to our401(k) and Top Hat savings plans during the third quarter of 2015 and 2014 and contributions of $9 million during both the first nine months of 2015 and 2014, respectively.  We contributed $5 million to 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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| - Definition
The entire disclosure for pension and other postretirement benefits.
+ References
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