Derivative Instruments (Details 3) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Summary of Derivative Instruments by Risk Exposure [Abstract]
Power contracts included in operating revenues	$ (74)		$ 92
Natural gas contracts included in fuel and purchased energy expense	45		104
Interest rate swaps included in interest expense	2		3
Gain (Loss) on interest rate derivatives, net	0		(14)
Gain (Loss) on Derivative Instruments, Net, Pretax	(27)		185
Gain (Loss) on Sale of Derivatives	28	[1]	(39)	[1]
Unrealized gain (loss)	(55)	[2]	224	[2]
Interest Rate Swap [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	0	[1]	(157)	[1]
Unrealized gain (loss)	2	[2]	146	[2]
Commodity Option [Member]
Summary of Derivative Instruments by Risk Exposure [Abstract]
Gain (Loss) on Sale of Derivatives	28	[1]	118	[1]
Unrealized gain (loss)	$ (57)	[2]	$ 78	[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 hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.