Restatement of consolidated financial statements - Additional Information (Detail) (USD $)	12 Months Ended												3 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Adjustment	Dec. 31, 2010 Adjustment	Dec. 31, 2009 Adjustment	Sep. 30, 2012 Adjustment	Jun. 30, 2012 Adjustment	Mar. 31, 2012 Adjustment	Sep. 30, 2011 Adjustment	Jun. 30, 2011 Adjustment	Mar. 31, 2011 Adjustment	Sep. 30, 2011 Adjustment Short term deferred tax asset	Jun. 30, 2011 Adjustment Short term deferred tax asset	Mar. 31, 2011 Adjustment Short term deferred tax asset	Dec. 31, 2010 Adjustment Short term deferred tax asset
Net income (LOSS)	$ (363,238,000)	$ (1,325,620,000)	$ (150,929,000)	$ (2,236,000)	$ (12,856,000)	$ (8,000,000)
Deferred tax asset	17,883,000	22,677,000	61,128,000	25,700,000	66,700,000	12,900,000
Deferred tax liability				0	47,400,000	6,400,000
Accumulated other comprehensive income	14,325,000	32,550,000		(2,612,000)	1,565,000	1,565,000	(2,612,000)	(2,612,000)	(2,612,000)	(3,840,000)	4,134,000	2,088,000
Deferred tax liability presented as decrease in deferred tax asset	94,034,000	95,352,000	121,964,000	4,224,000	(47,354,000)		2,517,000	2,458,000	2,902,000	(42,996,000)	(50,970,000)	(49,705,000)
Adjustment to short term deferred tax asset													$ 10,600,000	$ 31,500,000	$ 30,700,000	$ 29,300,000