Financial Instruments - Schedule of carrying amounts and estimated fair values of the financial instruments (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Carrying Amount
Carrying amounts and estimated fair values of financial instruments
Long-term debt	$ 4,931	$ 4,598
Derivative instruments	852	1,048
Fair Value
Carrying amounts and estimated fair values of financial instruments
Long-term debt	4,374	4,582
Derivative instruments	$ 852	$ 1,048