Risk Management and Trading Activities (Details 4) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Level 1 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	$ (4)	$ 95	$ (12)	$ 123
Fair Value Measurements net transfers out	39	89	318	23
Fair Value Measurements net transfers into/out of	35	184	306	146
Level 2 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	28	(8)	34	121
Fair Value Measurements net transfers out	198	(403)	(28)	(271)
Fair Value Measurements net transfers into/out of	226	(411)	6	(150)
Level 3 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	(222)	215	(211)	57
Fair Value Measurements net transfers out	(39)	12	(101)	(53)
Fair Value Measurements net transfers into/out of	$ (261)	$ 227	$ (312)	$ 4