Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015
Assets:
Investments	$ 56	$ 7
Foreign Exchange Contracts	30	15
Total Assets at Fair Value	86	22
Liabilities:
Foreign Exchange Contracts	4	11
Total Liabilities at Fair Value	4	11
Fair Value of Long Term Fixed Rate and Variable Rate Debt, excluding Capital Leases
Carrying amount — liability	6,305	5,659
Prior period reclassification adjustment between fixed rate debt and variable debt	452
Fixed Rate Debt, Excluding Capital Leases
Fair Value of Long Term Fixed Rate and Variable Rate Debt, excluding Capital Leases
Carrying amount — liability	3,680	3,890
Fair value — liability	3,773	4,065
Variable Rate Debt, Excluding Capital Leases
Fair Value of Long Term Fixed Rate and Variable Rate Debt, excluding Capital Leases
Carrying amount — liability	2,625	1,769
Fair value — liability	2,622	1,767
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1)
Assets:
Investments	56	7
Foreign Exchange Contracts	0	0
Total Assets at Fair Value	56	7
Liabilities:
Foreign Exchange Contracts	0	0
Total Liabilities at Fair Value	0	0
Fair Value of Long Term Fixed Rate and Variable Rate Debt, excluding Capital Leases
Fair value — liability	4,603	4,337
Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Foreign Exchange Contracts	30	15
Total Assets at Fair Value	30	15
Liabilities:
Foreign Exchange Contracts	4	11
Total Liabilities at Fair Value	4	11
Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Foreign Exchange Contracts	0	0
Total Assets at Fair Value	0	0
Liabilities:
Foreign Exchange Contracts	0	0
Total Liabilities at Fair Value	$ 0	$ 0