Defined Benefit Postretirement Plans and Defined Contribution Plan - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Employer discretionary contribution amount	$ 18	$ 22	$ 20
Forecast [Member]
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Estimated future employer contributions in Year 1				$ 28
UNITED STATES
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Year 1	39
Year 2	35
Year 3	31
Year 4	29
Year 5	27
2025 through 2029	116
Unfunded Plans [Member] | Forecast [Member]
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Estimated future employer contributions in Year 1				6
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Year 1	18
Year 2	10
Year 3	9
Year 4	8
Year 5	7
2025 through 2029	$ 25
Other Postretirement Benefit Plans, Defined Benefit [Member] | Forecast [Member]
Defined Benefit Plan Estimated Future Benefit Payments [Line Items]
Estimated future employer contributions in Year 1				$ 18