Income taxes - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Recognized tax provision	$ 2,009,000	$ 2,049,000	$ 253,000	$ 4,067,000
Pretax net income on tax provision	11,433,000	9,054,000	18,039,000	17,306,000
Provisions for income taxes before discrete items	2,400,000	2,600,000	4,300,000	5,200,000
U.S. statutory income tax rate			35.00%
Benefit related to retroactive renewal of the U.S. federal research and development tax credit and certain international tax provisions on January 2, 2013			3.2
Gross unrecognized tax benefits	33,807,000		33,807,000		29,755,000
Gross unrecognized tax benefits decrease in next 12 months	4,000,000		4,000,000
Accrued interest and penalties related to unrecognized tax benefits	$ 1,500,000		$ 1,500,000		$ 1,200,000