Derivative Instruments (Details 3) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues	$ (272)		$ 48		$ (180)		$ (9)
Natural gas contracts included in fuel and purchased energy expense	20		20		124		22
Interest rate swaps included in interest expense	3		1		6		2
Gain (Loss) on interest rate derivatives, net	0		37		14		146
Gain (Loss) on Derivative Instruments, Net, Pretax	(249)	[1]	32	[1]	(64)	[1]	(131)	[1]
Gain (Loss) on Sale of Derivatives	94		(18)		55		(54)
Unrealized gain (loss)	(343)	[1]	50	[1]	(119)	[1]	(77)	[1]
Interest Rate Swap [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	0		(60)		(157)		(106)
Unrealized gain (loss)	3	[1]	24	[1]	149	[1]	(38)	[1]
Commodity Option [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	94		42		212		52
Unrealized gain (loss)	$ (346)	[1]	$ 26	[1]	$ (268)	[1]	$ (39)	[1]

[1] In addition to changes in market value on derivatives not designated as hedges, changes in unrealized gain (loss) also includes de-designation of interest rate swap cash flow hedges and related reclassification from AOCI into income, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure