Risk Management and Hedging Activities Risk Management and Hedging Activities - Not Designated as Hedging Activities (Details) (Not Designated as Hedging Instrument [Member], Commodity Contract [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 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	45	15
Net (losses) gains reclassified to operating revenue	(4)	9
Net losses reclassified to energy costs	(11)	(90)
Ending balance	$ 85	$ 55