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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Not Designated as Hedging Instrument [Member] | Commodity derivative [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 174	$ 415	$ 235	$ 400
Changes in fair value recognized in net regulatory assets	13	3	(6)	73
Net (losses) gains reclassified to operating revenue	(2)	12	2	41
Net losses reclassified to cost of sales	(13)	(73)	(59)	(157)
Ending balance	$ 172	$ 357	$ 172	$ 357