Debt - Maturities and Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Maturities Of Debt Excluding Capital Leases [Abstract]
Scheduled maturities, long-term debt, 2019	$ 1,337
Scheduled maturities, long-term debt, 2020	4,700
Scheduled maturities, long-term debt, 2021	7,378
Scheduled maturities, long-term debt, 2022	2,250
Scheduled maturities, long-term debt, 2023	4,917
Scheduled maturities, long-term debt, after 2023	20,582
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Scheduled maturities, capital leases, 2019	17
Scheduled maturities, capital leases, 2020	14
Scheduled maturities, capital leases, 2021	4
Scheduled maturities, capital leases, 2022	4
Scheduled maturities, capital leases, 2023	4
Scheduled maturities, capital leases, after 2023	7
Interest Expense [Abstract]
Interest expense on long-term and short-term debt	$ 507	$ 243	$ 251