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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Not Designated as Hedging Instrument [Member] | Commodity derivative [Member]
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]
Beginning balance	$ 172	$ 357	$ 235	$ 400
Changes in fair value recognized in net regulatory assets	18	(31)	12	42
Net gains reclassified to operating revenue	7	10	9	51
Net losses reclassified to cost of sales	(53)	(87)	(112)	(244)
Ending balance	$ 144	$ 249	$ 144	$ 249