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Retirement Benefits and Trusteed Assets (Pension Plan - Benefits related to Qualified and Nonqualified Pension Plans Expected to be paid in the Next Ten Years) (Details) - Pension plan
$ in Millions
Dec. 31, 2017
USD ($)
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]  
2018 $ 299
2019 301
2020 316
2021 317
2022 323
2023-2027 1,692
Total $ 3,248