|**Employee Retirement Benefits (Tables)**                     | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
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|**Retirement Benefits [Abstract]**                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Schedule of Defined Benefit Plans Disclosures                 | The following summarizes the components of net periodic benefit for our qualified and supplemental pension plans:(Dollars in millions)Three months ended June 30,Six months ended June 30,2024202320242023Service cost$2$2$3$3Non-service (benefit) costs:Interest cost3366Expected return on plan assets(6)(5)(11)(10)Amortization of actuarial loss (gain) and prior  service cost1(1)1(2)Other———(5)Total non-service benefit(2)(3)(4)(11)Net periodic benefit$—$(1)$(1)$(8)
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|(Dollars in millions)                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|Service cost                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Non-service (benefit) costs:                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Interest cost                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Expected return on plan assets                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Amortization of actuarial loss (gain) and prior  service cost |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Other                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Total non-service benefit                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Net periodic benefit                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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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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