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