Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance		824,604
Options granted, Number of Shares	0	287,750
Options exercised, Number of Shares		(191,878)
Options forfeited, Number of Shares		(49,349)
Options expired, Number of Shares		(12,170)
Options Outstanding, Number of Shares, ending balance	858,957	858,957	824,604
Exercisable, Number of Shares, ending balance	152,802	152,802
Options Outstanding, Weighted Average Exercise Price, beginning balance		$ 6.79
Options granted, Weighted Average Exercise Price		11.46
Options exercised, Weighted Average Exercise Price		5.99
Options forfeited, Weighted Average Exercise Price		9.58
Options expired, Weighted Average Exercise Price		9.26
Options Outstanding, Weighted Average Exercise Price, ending balance	$ 9.72	9.72	$ 6.79
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.29	$ 9.29
Options Outstanding, Weighted Average Remaining Contractual Life (Years), beginning balance		7 years 9 months 18 days	7 years 1 month 28 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance		6 years 4 months 13 days
Options Outstanding, Aggregate Intrinsic Value, beginning balance		$ 789
Options Outstanding, Aggregate Intrinsic Value, ending balance	$ 3,736	3,736	$ 789
Exercisable, Aggregate Intrinsic Value, ending balance	$ 2,231	$ 2,231