|**Employee Retirement Benefits (Tables)**             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                   
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|**Retirement Benefits [Abstract]**                    |                                                                                                                                                                                                                                                                                                                                                                                                  
|Schedule of Defined Benefit Plans Disclosures         | The following summarizes the components of net periodic benefit cost for our qualified and supplemental pension plans:(Dollars in millions)Three months ended March 31,20182017Service cost$3$3Non-service costs (benefit):Interest cost33Expected return on plan assets(5)(5)Amortization of actuarial loss and prior service cost—1Total non-service benefit(2)(1)Net periodic benefit cost$1$2
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|(Dollars in millions)                                 |                                                                                                                                                                                                                                                                                                                                                                                                  
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|Service cost                                          |                                                                                                                                                                                                                                                                                                                                                                                                  
|Non-service costs (benefit):                          |                                                                                                                                                                                                                                                                                                                                                                                                  
|Interest cost                                         |                                                                                                                                                                                                                                                                                                                                                                                                  
|Expected return on plan assets                        |                                                                                                                                                                                                                                                                                                                                                                                                  
|Amortization of actuarial loss and prior service cost |                                                                                                                                                                                                                                                                                                                                                                                                  
|Total non-service benefit                             |                                                                                                                                                                                                                                                                                                                                                                                                  
|Net periodic benefit cost                             |                                                                                                                                                                                                                                                                                                                                                                                                  
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duration X- DefinitionTabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans.
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