Derivative Instruments (Details 3) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives	$ 230	[1]	$ (50)	[1]	$ 83	[1]
Unrealized Gain (Loss) on Derivatives	72	[2]	30	[2]	(56)	[2]
Gain (Loss) on Derivative Instruments, Net, Pretax	302		(20)		27
Power contracts included in operating revenues	187		(20)		(19)
Natural gas contracts included in fuel and purchased energy expense	118		138		276
Interest rate swaps included in interest expense	(11)		(7)		7
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	(14)		(145)		(223)
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives	(157)	[1]	(193)	[1]	(31)	[1]
Unrealized Gain (Loss) on Derivatives	154	[2]	55	[2]	(199)	[2]
Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Sale of Derivatives	387	[1]	143	[1]	114	[1]
Unrealized Gain (Loss) on Derivatives	$ (82)	[2]	$ (25)	[2]	$ 143	[2]

[1] Does not include the realized value associated with derivative instruments that settle through physical delivery.

[2] 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 earnings, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.