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, 2015	Mar. 31, 2014
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 85	$ 55
Changes in fair value recognized in regulatory assets	48	(22)
Net gains (losses) reclassified to operating revenue	25	(11)
Net (losses) gains reclassified to energy costs	(28)	5
Ending balance	$ 130	$ 27