Variable Interest Entities and Unconsolidated Investments (VIE Texuals) (Details)	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2013 USD ($) MW	Jun. 30, 2012 USD ($)	Jun. 30, 2013 USD ($) MW	Jun. 30, 2012 USD ($)	Dec. 31, 2012 USD ($) MW	Jun. 30, 2013 Greenfield [Member] USD ($)	Jun. 30, 2012 Greenfield [Member] USD ($)	Jun. 30, 2013 Greenfield [Member] USD ($)	Jun. 30, 2012 Greenfield [Member] USD ($)	Jun. 30, 2013 Greenfield [Member] CAD MW	Jun. 30, 2013 Whitby [Member] USD ($) MW	Jun. 30, 2012 Whitby [Member] USD ($)	Jun. 30, 2013 Whitby [Member] USD ($) MW	Jun. 30, 2012 Whitby [Member] USD ($)	Jun. 30, 2013 Inland Empire Energy Center [Member] Y MW	Jun. 30, 2013 Inland Empire Energy Center [Member] Minimum [Member] Y	Jun. 30, 2013 Inland Empire Energy Center [Member] Maximum [Member] Y
Variable Interest Entity [Line Items]
Power generation capacity	9,027		9,027		8,255					1,038	50		50		775
Variable Interest Entity, Financial or Other Support, Amount	$ 0	$ 0	$ 0	$ 0
Equity Method Investment, Ownership Percentage										50.00%	50.00%		50.00%
Equity Method Investment, Summarized Financial Information, Debt	412,000,000		412,000,000		448,000,000
Prorata Share of Equity Method Investment, Summarized Financial Information, Debt	206,000,000		206,000,000		224,000,000
Equity Method Investment, Summarized Financial Information, Term Loan Period										18 years
Equity Method Investment, Summarized Financial Information, Term Loan										648,000,000
Distribution from equity method investee						$ 7,000,000	$ 9,000,000	$ 7,000,000	$ 9,000,000		$ 9,000,000	$ 7,000,000	$ 9,000,000	$ 7,000,000
Call Option Exercise Period																2,017	2,024
Put Option Exercise Period															2,025