Risk Management and Hedging Activities Risk Management and Hedging Activities - Not Designated as Hedging Contracts (Details) (Not Designated as Hedging Instrument [Member], Commodity Contract [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
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 217	$ 438	$ 264	$ 487
Changes in fair value recognized in net regulatory assets	(21)	42	27	(24)
Net gains (losses) reclassified to operating revenue	11	(3)	29	7
Net losses reclassifed to energy costs	(77)	(54)	(190)	(47)
Ending balance	$ 130	$ 423	$ 130	$ 423