Debt - Maturities and Interest Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Maturities Of Debt Excluding Capital Leases [Abstract]
Maturities of Debt in 2017	[1]	$ 255
Maturities of Debt in 2018	[1]	131
Maturities of Debt in 2019	[1]	0
Maturities of Debt in 2020	[1]	549
Maturities of Debt in 2021	[1]	378
Maturities of Debt in years after 2021	[1]	3,694
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Maturities of debt under capital lease arrangements in 2017		21
Maturities of debt under capital lease arrangements in 2018		10
Maturities of debt under capital lease arrangements in 2019		10
Maturities of debt under capital lease arrangements in 2020		0
Maturities of debt under capital lease arrangements in 2021		0
Maturities of debt under capital lease arrangements in years after2021		0
Interest Expense [Abstract]
Interest expense on long-term and short-term debt		$ 251	$ 252	$ 265

[1] Long-term debt maturity amounts exclude capital leases.