Variable Interest Entities and Unconsolidated Investments (VIE Textuals) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 MW	Dec. 31, 2011 MW	Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Power generation capacity	8,255	11,391
Variable Interest Entity, Financial or Other Support, Amount	$ 20	$ 87	$ 46
Equity Method Investment, Summarized Financial Information, Debt	448	462
Prorata Share of Equity Method Investment, Summarized Financial Information, Debt	224	231
OMEC [Member]
Schedule of Equity Method Investments [Line Items]
Power generation capacity	608
Russell City Energy [Member]
Schedule of Equity Method Investments [Line Items]
Minority Interest Ownership Percentage By Noncontrolling Third Party Owners	25.00%
Equity Method Investment, Ownership Percentage	75.00%
Inland Empire Energy Center [Member]
Schedule of Equity Method Investments [Line Items]
Power generation capacity	775
Put Option Exercise Period	2,025
Minimum [Member] | Inland Empire Energy Center [Member]
Schedule of Equity Method Investments [Line Items]
Call Option Exercise Period	2,017
Maximum [Member] | Inland Empire Energy Center [Member]
Schedule of Equity Method Investments [Line Items]
Call Option Exercise Period	2,024
Greenfield [Member]
Schedule of Equity Method Investments [Line Items]
Power generation capacity	1,038
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investment, Summarized Financial Information, Term Loan Period	18 years
Equity Method Investment, Summarized Financial Information, Term Loan	648
Project financing interest rate spread - Canadian LIBOR	1.125%
Project financing interest rate spread - Canadian Prime Rate	0.125%
Whitby [Member]
Schedule of Equity Method Investments [Line Items]
Power generation capacity	50
Equity Method Investment, Ownership Percentage	50.00%
NDH [Member]
Schedule of Equity Method Investments [Line Items]
Variable Interest Entity, Financial or Other Support, Amount			540
York Energy Center [Member]
Schedule of Equity Method Investments [Line Items]
Variable Interest Entity, Financial or Other Support, Amount			$ 110