Debt - Maturities and Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Maturities Of Debt Excluding Capital Leases [Abstract]
Maturities of Debt in 2018	$ 131
Maturities of Debt in 2019	0
Maturities of Debt in 2020	550
Maturities of Debt in 2021	378
Maturities of Debt in 2022	750
Maturities of Debt in years after 2022	3,550
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Maturities of debt under capital lease arrangements in 2018	9
Maturities of debt under capital lease arrangements in 2019	8
Maturities of debt under capital lease arrangements in 2020	1
Maturities of debt under capital lease arrangements in 2021	0
Maturities of debt under capital lease arrangements in 2022	0
Maturities of debt under capital lease arrangements in years after 2022	0
Interest Expense [Abstract]
Interest expense on long-term and short-term debt	$ 243	$ 251	$ 252