Variable Interest Entities and Unconsolidated Investments (VIE Texuals) (Details) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2013 USD ($) MW	Mar. 31, 2012 USD ($)	Dec. 31, 2012 USD ($) MW	Mar. 31, 2013 Greenfield [Member] USD ($)	Mar. 31, 2012 Greenfield [Member] USD ($)	Mar. 31, 2013 Greenfield [Member] CAD MW	Mar. 31, 2013 Whitby [Member] USD ($) MW	Mar. 31, 2012 Whitby [Member] USD ($)	Mar. 31, 2013 Inland Empire Energy Center [Member] Y MW	Mar. 31, 2013 Inland Empire Energy Center [Member] Minimum [Member] Y	Mar. 31, 2013 Inland Empire Energy Center [Member] Maximum [Member] Y
Variable Interest Entity [Line Items]
Power generation capacity	8,255		8,255			1,038	50		775
Variable Interest Entity, Financial or Other Support, Amount	$ 0	$ 0
Equity Method Investment, Ownership Percentage						50.00%	50.00%
Equity Method Investment, Summarized Financial Information, Debt	432		448
Prorata Share of Equity Method Investment, Summarized Financial Information, Debt	216		224
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%
Distribution from equity method investee				$ 0	$ 0		$ 0	$ 0
Call Option Exercise Period										2,017	2,024
Put Option Exercise Period									2,025