Debt - Addtional Disclosures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Maturities Of Debt Excluding Capital Leases [Abstract]
Maturities of Debt in 2016	$ 0
Maturities of Debt in 2017	250
Maturities of Debt in 2018	131
Maturities of Debt in 2019	0
Maturities of Debt in 2020	550
Maturities of Debt in years after 2020	4,078
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Maturities of debt under capital lease arrangements in 2016	23
Maturities of debt under capital lease arrangements in 2017	12
Maturities of debt under capital lease arrangements in 2018	7
Maturities of debt under capital lease arrangements in 2019	6
Maturities of debt under capital lease arrangements in 2020	0
Maturities of debt under capital lease arrangements in years after2020	0
Interest Expense [Abstract]
Interest expense on long-term and short-term debt	$ 252	$ 265	$ 270