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, 2014	Mar. 31, 2013
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 55	$ 121
Changes in fair value recognized in regulatory assets	(22)	(19)
Net (losses) gains reclassified to operating revenue	(11)	1
Net gains (losses) reclassified to energy costs	5	(21)
Ending balance	$ 27	$ 82