Significant Unobservable Inputs for Each Year Included In Valuation (Detail) (Discounted Cash Flows Valuation Technique, USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2012 MWh	Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Generation output (MWh)		224,836
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Generation output (MWh)	11,916
Average realized price ($/MWh)	49.00	84.50
Operating costs	$ 86	$ 12,687
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Generation output (MWh)	15,456
Average realized price ($/MWh)	71.50	111
Operating costs	595	20,430
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Generation output (MWh)	15,097	224,836
Average realized price ($/MWh)	60.36	92.31
Operating costs	$ 400	$ 16,163