|**Employee Retirement Benefits**                     | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|**Retirement Benefits [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 30,2018201720182017Service cost$2$2$5$5Non-service costs (benefit):Interest cost3467Expected return on plan assets(6)(5)(11)(10)Amortization of actuarial loss and prior servicecost1—11Total non-service benefit(2)(1)(4)(2)Net periodic benefit cost$—$1$1$3See our 2017 Annual Report on Form 10-K, Item 8, Note 13, Employee Retirement Benefits, Page 156, for information on our retirement benefits. Service costs and non-service costs (benefit) are allocated in the same proportion primarily to the underwriting, acquisition and insurance expenses line item with the remainder allocated to the insurance losses and contract holders' benefits line item on the condensed consolidated statements of income for both 2018 and 2017.We made matching contributions totaling$4 millionand$3 millionto our 401(k) and Top Hat savings plans during thesecond quarterof 2018 and 2017 and contributions of$10 millionand$9 millionfor the first half of 2018 and 2017, respectively.We contributed$5 millionto our qualified pension plan during the first six months of 2018.
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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 servicecost |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total non-service benefit                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net periodic benefit cost                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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