Derivative Instruments (Details 3) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	$ (445)	[1]	$ (547)	[1]	$ (335)	[1]	$ (425)	[1]
Additions/Change in value of existing contracts	(92)	[1]	(146)	[1]	(325)	[1]	(405)	[1]
Settled contracts	50	[1]	63	[1]	173	[1]	200	[1]
Outstanding net asset (liability), Ending Balance	(487)	[1]	(630)	[1]	(487)	[1]	(630)	[1]
NUGs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(447)	[1]	(557)	[1]	(345)	[1]	(444)	[1]
Additions/Change in value of existing contracts	(89)	[1]	(146)	[1]	(325)	[1]	(405)	[1]
Settled contracts	53	[1]	63	[1]	187	[1]	209	[1]
Outstanding net asset (liability), Ending Balance	(483)	[1]	(640)	[1]	(483)	[1]	(640)	[1]
Other
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	2	[1]	10	[1]	10	[1]	19	[1]
Additions/Change in value of existing contracts	(3)	[1]	0	[1]	0	[1]	0	[1]
Settled contracts	(3)	[1]	0	[1]	(14)	[1]	(9)	[1]
Outstanding net asset (liability), Ending Balance	$ (4)	[1]	$ 10	[1]	$ (4)	[1]	$ 10	[1]

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