Risk Management and Trading Activities (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Level 1 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	$ (17)	$ 14
Fair Value Measurements net transfers out	297	28
Fair Value Measurements net transfers into/out of	280	42
Level 2 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	0	312
Fair Value Measurements net transfers out	(97)	(329)
Fair Value Measurements net transfers into/out of	(97)	(17)
Level 3 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	(114)	30
Fair Value Measurements net transfers out	(69)	(55)
Fair Value Measurements net transfers into/out of	$ (183)	$ (25)