Schedule of DOE Loan Facility Draw-Downs (Detail) (USD $)	3 Months Ended											9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Jan. 20, 2012
Debt [Line Items]
Loan Facility Available for Future Draw-downs, Beginning balance														$ 465,000,000
Draw-downs received during the period	(33,256,000)	(71,274,000)	(84,267,000)	(51,252,000)	(90,822,000)	(31,693,000)	(30,656,000)	(15,271,000)	(11,138,000)	(15,499,000)	(29,920,000)	(188,796,000)	(153,172,000)
Loan Facility Available for Future Draw-downs, Beginning balance				$ 188,797,000				$ 393,220,000
Minimum [Member]
Debt [Line Items]
Interest rate	1.00%	1.00%	0.90%	1.00%	1.00%	1.80%	2.10%	1.70%	1.70%	2.50%	2.90%
Maximum [Member]
Debt [Line Items]
Interest rate	1.20%	1.30%	1.60%	1.50%	1.40%	2.70%	3.00%	2.80%	2.60%	3.40%	3.40%