Fair Value Measurements - Level 3 (Details) (Commodity derivative [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 2	$ (240)	$ 1	$ (345)
Changes in fair value recognized in net regulatory assets	0	5	1	84
Settlements	(1)	18	(1)	44
Ending balance	$ 1	$ (217)	$ 1	$ (217)