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Retirement Plans (Schedule of Expected Benefit Payments for OPEB) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
OPEB, Gross Benefits [Member]  
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]  
2018 $ 30
2019 37
2020 44
2021 49
2022 52
2023 - 2027 306
Total 518
Medicare Part D Subsidy [Member]  
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]  
2023 - 2027 2
Total 2
Postretirement Benefits Other Than Pensions (OPEB) [Member]  
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]  
2018 30
2019 37
2020 44
2021 49
2022 52
2023 - 2027 308
Total $ 520