Income taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Income Tax [Line Items]
Recognized tax benefits	$ (7,850,000)	$ 1,397,000	$ (3,783,000)	$ 3,177,000
Pretax income on tax benefits	5,600,000	7,500,000	22,900,000	19,200,000
Provisions for income taxes before discrete items	3,300,000	1,400,000	8,500,000	4,300,000
U.S. statutory rate			35.00%		35.00%
Unrecognized tax benefits	30,359,000		30,359,000		35,597,000
Unrecognized tax benefits decrease in next 12 months	4,500,000		4,500,000
Accrued interest and penalties related to unrecognized tax benefits	$ 1,200,000		$ 1,200,000		$ 1,700,000