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Pension and Other Postretirement Benefits - Funding and Cash Flows and Defined Contribution Plans (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Nov. 01, 2016 | Apr. 30, 2019 | Mar. 31, 2018 | Apr. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Payment for pension benefits |  |  |  |  | $ 92 |  | $ 268 | $ 298 | $ 310
Payment for pension benefit plan in excess of minimum required |  |  |  |  |  |  |  | 53 |
Contribution next fiscal year |  |  |  |  |  |  | 475 |  |
Cash contribution term to pension plan | 30 months |  |  |  |  |  |  |  |
Aggregate cash contribution to pension plan | $ 150 |  |  |  |  |  |  |  |
Payment for pension and other postretirement benefits |  | $ 34 | $ 66 | $ 50 |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |  |
Expenses related to defined contribution plan |  |  |  |  |  |  | 125 | $ 123 | $ 89
Pension benefits paid |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  | 470 |  |
2021 |  |  |  |  |  |  | 465 |  |
2022 |  |  |  |  |  |  | 460 |  |
2023 |  |  |  |  |  |  | 455 |  |
2024 |  |  |  |  |  |  | 450 |  |
Thereafter |  |  |  |  |  |  | 2,120 |  |
Total benefit payments |  |  |  |  |  |  | 4,420 |  |
Gross Other post- retirement benefits |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  | 80 |  |
2021 |  |  |  |  |  |  | 80 |  |
2022 |  |  |  |  |  |  | 80 |  |
2023 |  |  |  |  |  |  | 80 |  |
2024 |  |  |  |  |  |  | 75 |  |
Thereafter |  |  |  |  |  |  | 260 |  |
Total benefit payments |  |  |  |  |  |  | 655 |  |
Medicare Part D subsidy receipts |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  | 5 |  |
2021 |  |  |  |  |  |  | 5 |  |
2022 |  |  |  |  |  |  | 5 |  |
2023 |  |  |  |  |  |  | 5 |  |
2024 |  |  |  |  |  |  | 5 |  |
Thereafter |  |  |  |  |  |  | 25 |  |
Medicare Part D subsidy receipts |  |  |  |  |  |  | 50 |  |
Net Other post- retirement benefits |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |  |
2020 |  |  |  |  |  |  | 75 |  |
2021 |  |  |  |  |  |  | 75 |  |
2022 |  |  |  |  |  |  | 75 |  |
2023 |  |  |  |  |  |  | 75 |  |
2024 |  |  |  |  |  |  | 70 |  |
Thereafter |  |  |  |  |  |  | 235 |  |
Total benefit payments |  |  |  |  |  |  | 605 |  |
United States |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Contribution next fiscal year |  |  |  |  |  |  | $ 403 |  |
Scenario, Forecast |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Payment for pension benefits |  |  |  |  |  | $ 60 |  |  |
Minimum required cash contribution to pension plan |  |  |  |  |  | $ 415 |  |  |

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