Risk Management and Hedging Activities Risk Management and Hedging Activities - Not Designated as Hedging Contracts (Details) (Not Designated as Hedging Instrument [Member], Commodity derivative [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Not Designated as Hedging Instrument [Member] | Commodity derivative [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 223	$ 182
Changes in fair value recognized in net regulatory assets	60	4
Net gains (losses) reclassified to operating revenue	9	(30)
Net (losses) gains reclassified to cost of sales	(37)	3
Ending balance	$ 255	$ 159