Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 17		$ 30		$ 38
Included in net income:
Included in operating revenues	8	[1]	5	[1]	7	[1]
Included in fuel and purchased energy expense	0	[2]	0	[2]	0	[2]
Included in OCI	0		2		2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	3		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issues	(1)		0		0
Purchases, issuances and settlements:
Settlements	(11)		(18)		(20)
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0		0		0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0		0		0
Transfers into level 3	0	[3],[4]	(2)	[3],[4]	0	[3],[4]
Transfers out of level 3	0	[4],[5]	0	[4],[5]	3
Balance, end of period	16		17		30
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Change in Unrealized Gain (Loss) Held At Period End	8	[2]	5	[2]	7	[2]
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]
Cash Equivalents Included In Cash And Cash Equivalents, Fair Value Disclosure	1,274		1,249
Cash Equivalents Included In Restricted Cash, Fair Value Disclosure	$ 228		$ 166

[1] For power contracts and Heat Rate swaps and options, included on our Consolidated Statements of Operations.

[2] For natural gas contracts, swaps and options, included on our Consolidated Statements of Operations.

[3] There were no significant transfers into level 3 for the years ended December 31, 2012 and 2010. We had $2 million in losses transferred out of level 2 into level 3 for the year ended December 31, 2011 due to changes in market liquidity in various power and natural gas markets.

[4] We transfer amounts among levels of the fair value hierarchy as of the end of each period. There were no significant transfers into/out of level 1 during the years ended December 31, 2012, 2011 and 2010.

[5] We had no significant transfers out of level 3 for the years ended December 31, 2012 and 2011. There were $3 million in losses transferred out of level 3 into level 2 for the year ended December 31, 2010 due to changes in market liquidity in various power markets.