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Benefit Plans (Estimated Future Benefit Payments) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Pension [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 480
2019 451
2020 456
2021 459
2022 463
2023-2027 2,372
Postretirement Benefit Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2018 142
2019 140
2020 138
2021 136
2022 133
2023-2027 $ 620