Derivative Instruments (Details 4) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Loss) Recognized in OCI (Effective Portion)	$ 13		$ 0
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion)	(9)	[1]	10	[1]
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Loss) Recognized in OCI (Effective Portion)	13	[2]	1	[2]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion)	(9)	[1],[2]	(7)	[1],[2]
Power Derivative Instruments [Member] | Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Loss) Recognized in OCI (Effective Portion)	0	[3]	(15)	[3]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion)	0	[1],[3]	38	[1],[3]
Natural Gas Derivative Instruments [Member] | Commodity Option [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Loss) Recognized in OCI (Effective Portion)	0	[3]	14	[3]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion)	$ 0	[1],[3]	$ (21)	[1],[3]

[1] Cumulative cash flow hedge losses, net of tax, remaining in AOCI were $228 million and $242 million at March 31, 2013 and December 31, 2012, respectively.

[2] We did not record any gain (loss) on hedge ineffectiveness related to our interest rate swaps designated as cash flow hedges during the three months ended March 31, 2013 and 2012.

[3] There were no commodity derivative instruments designated as cash flow hedges during the three months ended March 31, 2013. We recorded a gain on hedge ineffectiveness of $2 million related to our commodity derivative instruments designated as cash flow hedges during the three months ended March 31, 2012.