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Retirement Benefits (Benefit Payments) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Other Postretirement Benefits (Gross, Before Medicare Part D) [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2017 $ 24
2018 23
2019 21
2020 19
2021 17
2022-2026 37
Unfunded Plans [Member] | Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2017 8
2018 6
2019 6
2020 5
2021 4
2022-2026 $ 15