Derivative Instruments (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	$ (301)	[1]	$ (335)	[1]
Additions/Change in value of existing contracts	(80)	[1]	(89)	[1]
Settled contracts	76	[1]	62	[1]
Outstanding net asset (liability), Ending Balance	(305)	[1]	(362)	[1]
NUGs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(293)	[1]	(345)	[1]
Additions/Change in value of existing contracts	(79)	[1]	(89)	[1]
Settled contracts	72	[1]	72	[1]
Outstanding net asset (liability), Ending Balance	(300)	[1]	(362)	[1]
Other
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(8)	[1]	10	[1]
Additions/Change in value of existing contracts	(1)	[1]	0	[1]
Settled contracts	4	[1]	(10)	[1]
Outstanding net asset (liability), Ending Balance	$ (5)	[1]	$ 0	[1]

[1] Changes in the fair value of certain contracts are deferred for future recovery from (or credited to) customers.