Debt (Debt Textuals) (Details) (USD $)	6 Months Ended
	Jun. 30, 2012 MW	Dec. 31, 2011 MW
Debt Instrument [Line Items]
Term loan interest rate spread option Federal Funds effective rate	0.50%
Term loan interest rate spread option Prime Rate	2.25%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	3.25%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	1.25%
Percentage of principal amount of Term Loan to be paid quarterly	0.25%
Percentage of Term Loan amounts held by lenders	50.00%
Power generation capacity	10,500	11,391
Letters of credit issued	$ 561,000,000	$ 581,000,000
Upsize Facility Total	300,000,000
Percentage added to Federal Funds Effective Rate to arrive at base rate	0.50%
Repayment time for drawings under letters of credit	2
Excess amount of asset sales requiring mandatory prepayments	3,000,000,000
Minimum [Member]
Debt Instrument [Line Items]
Applicable margin range percentage above base rate	2.00%
Applicable Margin Range Percentage Above British Bankers' Association Interest Settlement Rates	3.00%
Unused commitment fee range percentage	0.50%
Maximum [Member]
Debt Instrument [Line Items]
Applicable margin range percentage above base rate	2.25%
Applicable Margin Range Percentage Above British Bankers' Association Interest Settlement Rates	3.25%
Unused commitment fee range percentage	0.75%
Russell City Project [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	845,000,000
Power generation capacity	619
Construction Loan Facility	700,000,000
Project Letter of Credit Facility	77,000,000
Term Loan Period	10
Debt Service Letter of Credit Facility	68,000,000
Amount Drawn Under Construction Loan	414,000,000
Letters of credit issued	61,000,000
Noncontrolling Interest, Ownership Percentage by Parent	75.00%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	25.00%
Applicable Margin Range Percentage Above British Bankers' Association Interest Settlement Rates	2.25%
Los Esteros Project [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	373,000,000
Power generation capacity	188
Power generation capacity after upgrade	309
Construction Loan Facility	305,000,000
Project Letter of Credit Facility	38,000,000
Term Loan Period	10
Debt Service Letter of Credit Facility	30,000,000
Amount Drawn Under Construction Loan	139,000,000
Letters of credit issued	$ 30,000,000
Applicable Margin Range Percentage Above British Bankers' Association Interest Settlement Rates	2.25%
One Month [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	1
Two Months [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	2
Three Months [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	3
Six Months [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	6
Nine Months [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	9
Twelve Months [Member]
Debt Instrument [Line Items]
Interest periods for LIBOR rate borrowings	12