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