Variable Interest Entities - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Oct. 29, 2010 Variable Interest Entity, Primary Beneficiary Crump Geothermal Company LLC	Dec. 31, 2012 Variable Interest Entity, Primary Beneficiary Crump Geothermal Company LLC
Variable Interest Entity [Line Items]
Payment for ownership interest	$ 200,000	$ 100,000	$ 100,000
Additional payment for ownership interest over three-year period to the other joint owner			2,400,000
Payment period for additional ownership interest			3 years
Voting interest			50.00%
Obligation to fund during development stage			15,000,000
Aggregate amount of financing directly to the joint owner			15,000,000
Development costs				$ 11,100,000