Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fair value of commodity derivatives
Derivative Assets	$ 297	$ 237
Derivative Liabilities	(274)	(281)
Current Assets
Fair value of commodity derivatives
Derivative Assets	207	157
Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	90	80
Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(117)	(106)
Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(157)	(175)
Commodity contracts | Current Assets
Fair value of commodity derivatives
Derivative Assets	205	150
Commodity contracts | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	88	78
Commodity contracts | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(109)	(94)
Commodity contracts | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(20)	(37)
FTRs | Current Assets
Fair value of commodity derivatives
Derivative Assets	2	7
FTRs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	1	1
FTRs | Current Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(8)	(12)
FTRs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	(1)	(1)
NUGs | Noncurrent Assets
Fair value of commodity derivatives
Derivative Assets	1	1
NUGs | Noncurrent Liabilities
Fair value of commodity derivatives
Derivative Liabilities	$ (136)	$ (137)