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Benefit Plans - Estimated future benefit payments (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Pension benefits  
Estimated future benefit payments  
2018 $ 426
2019 463
2020 485
2021 514
2022 533
2023-2027 2,477
Total benefit payments 4,898
Postretirement benefits  
Estimated future benefit payments  
2018 22
2019 24
2020 24
2021 25
2022 26
2023-2027 136
Total benefit payments $ 257