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Pension and Other Postretirement Benefits - Funding and Cash Flows and Defined Contribution Plans (Details) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Nov. 01, 2016 USD ($) installment | Mar. 31, 2018 USD ($) | Apr. 30, 2017 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Payment for pension benefits |  |  |  | $ 92 |  | $ 298 | $ 310 | $ 290
Payment for pension benefit plan in excess of minimum required |  |  |  |  |  | 72 |  |
Contribution next fiscal year |  |  |  | 255 |  | 255 |  |
Minimum required cash contribution to pension plan | $ 50 |  |  |  |  |  |  |
Cash contribution term to pension plan | 30 months |  |  |  |  |  |  |
Number of installment made for cash contribution to pension plan | installment | 3 |  |  |  |  |  |  |
Aggregate cash contribution to pension plan | $ 150 |  |  |  |  |  |  |
Payment for pension and other postretirement benefits |  | $ 50 | $ 50 |  | $ 16 | 116 |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |
Expenses related to defined contribution plan |  |  |  |  |  | 123 | $ 89 | $ 71
Pension benefits paid |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |
2019 |  |  |  | 460 |  | 460 |  |
2020 |  |  |  | 460 |  | 460 |  |
2021 |  |  |  | 455 |  | 455 |  |
2022 |  |  |  | 455 |  | 455 |  |
2023 |  |  |  | 450 |  | 450 |  |
Thereafter |  |  |  | 2,170 |  | 2,170 |  |
Total benefit payments |  |  |  | 4,450 |  | 4,450 |  |
Gross Other post- retirement benefits |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |
2019 |  |  |  | 90 |  | 90 |  |
2020 |  |  |  | 85 |  | 85 |  |
2021 |  |  |  | 85 |  | 85 |  |
2022 |  |  |  | 85 |  | 85 |  |
2023 |  |  |  | 85 |  | 85 |  |
Thereafter |  |  |  | 300 |  | 300 |  |
Total benefit payments |  |  |  | 730 |  | 730 |  |
Medicare Part D subsidy receipts |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |
2019 |  |  |  | 5 |  | 5 |  |
2020 |  |  |  | 5 |  | 5 |  |
2021 |  |  |  | 5 |  | 5 |  |
2022 |  |  |  | 5 |  | 5 |  |
2023 |  |  |  | 5 |  | 5 |  |
Thereafter |  |  |  | 25 |  | 25 |  |
Total benefit payments |  |  |  | 50 |  | 50 |  |
Net Other post- retirement benefits |  |  |  |  |  |  |  |
Defined Benefit Plan, Expected Future Benefit Payment [Abstract] |  |  |  |  |  |  |  |
2019 |  |  |  | 85 |  | 85 |  |
2020 |  |  |  | 80 |  | 80 |  |
2021 |  |  |  | 80 |  | 80 |  |
2022 |  |  |  | 80 |  | 80 |  |
2023 |  |  |  | 80 |  | 80 |  |
Thereafter |  |  |  | 275 |  | 275 |  |
Total benefit payments |  |  |  | 680 |  | 680 |  |
United States |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Contribution next fiscal year |  |  |  | 230 |  | 230 |  |
Pension Benefit Guaranty Corporation |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Contribution next fiscal year |  |  |  | $ 34 |  | $ 34 |  |

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