Derivative Instruments (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	$ (335)	$ (425)
Additions/Change in value of existing contracts	(207)	(181)
Settled contracts	241	271
Outstanding net asset (liability), Ending Balance	(301)	(335)
NUGs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(345)	(444)
Additions/Change in value of existing contracts	(202)	(181)
Settled contracts	254	280
Outstanding net asset (liability), Ending Balance	(293)	(345)
Other
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	10	19
Additions/Change in value of existing contracts	(5)	0
Settled contracts	(13)	(9)
Outstanding net asset (liability), Ending Balance	$ (8)	$ 10