Risk Management and Trading Activities (Details 4) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Level 1 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	$ 18	$ 3	$ (8)	$ 28
Fair Value Measurements net transfers out	65	(81)	279	(66)
Fair Value Measurements net transfers into/out of	83	(78)	271	(38)
Level 2 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	(2)	129	6	129
Fair Value Measurements net transfers out	7	(5)	(226)	132
Fair Value Measurements net transfers into/out of	5	124	(220)	261
Level 3 [Member]
Net transfers into and out of each level of the fair value hierarchy
Fair Value Measurements net transfers in	(72)	1	11	(158)
Fair Value Measurements net transfers out	(16)	(47)	(62)	(65)
Fair Value Measurements net transfers into/out of	$ (88)	$ (46)	$ (51)	$ (223)