Fair Value Measurements - Level 3 (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 17		$ (233)		$ 23		$ (331)
Changes included in earnings	(2)	[1]	6	[1]	7	[1]	10	[1]
Changes in fair value recognized in other comprehensive income	0		0		3		0
Changes in fair value recognized in net regulatory assets	(3)		4		0		87
Sales	0		0		0		0
Settlements	0		15		(21)		26
Transfers from level 2			1				1
Ending balance	12		(207)		12		(207)
Net unrealized (losses) gains included in earnings	(2)		4		3		5
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	36		37		35		50
Changes included in earnings	0	[1]	0	[1]	0	[1]	0	[1]
Changes in fair value recognized in other comprehensive income	2		(2)		4		0
Changes in fair value recognized in net regulatory assets	0		0		0		0
Sales	0		0		(1)		(15)
Settlements	0		0		0		0
Transfers from level 2			0				0
Ending balance	$ 38		$ 35		$ 38		$ 35

[1] Changes included in earnings are reported as operating revenue on the Consolidated Statements of Operations. For commodity derivatives held as of September 30, 2012 and 2011, net unrealized (losses) gains included in earnings for the three-month periods ended September 30, 2012 and 2011 totaled $(2) million and $4 million, respectively, and for the nine-month periods ended September 30, 2012 and 2011, totaled $3 million and $5 million, respectively.