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
	Mar. 31, 2013	Mar. 31, 2012
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 121	$ 264
Changes in fair value recognized in regulatory assets	(19)	52
Net gains reclassified to operating revenue	1	14
Net losses reclassifed to energy costs	(21)	(46)
Ending balance	$ 82	$ 284