Derivatives (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	$ (28)	$ (1)	$ (7)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(15)	(24)	(46)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Unrealized Gain (Loss) Recognized in Income	(6)	(2)	(5)
Fuel [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)
Interest expense, net of amounts capitalized [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(14)	(25)	(46)
Other operations and maintenance [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)		1
Other income (expense), net [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1)
Wholesale revenues [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Unrealized Gain (Loss) Recognized in Income	2
Energy-related derivatives [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		1	(2)
Energy-related derivatives [Member] | Fuel [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Unrealized Gain (Loss) Recognized in Income	(9)	1	(6)
Energy-related derivatives [Member] | Wholesale revenues [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Unrealized Gain (Loss) Recognized in Income		(2)	5
Energy-related derivatives [Member] | Purchased power [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Unrealized Gain (Loss) Recognized in Income	1	(1)	(4)
Interest rate derivatives [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)	(28)	(3)	(5)
Foreign currency derivatives [Member]
Pre-tax effect of derivatives designated as cash flow hedging instruments
Gain (Loss) Recognized in OCI on Derivative (Effective Portion)		$ 1