Common stock - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 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	294,250
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	865,457	824,604
Exercisable, Number of Shares, ending balance	418,064
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 6.79
Options granted, Weighted Average Exercise Price	11.51
Options exercised, Weighted Average Exercise Price	5.99
Options forfeited, Weighted Average Exercise Price	9.58
Options expired, Weighted Average Exercise Price	7.95
Options Outstanding, Weighted Average Exercise Price, ending balance	9.75	$ 6.79
Exercisable, Weighted Average Exercise Price, ending balance	$ 8.33
Options Outstanding, Weighted Average Remaining Contractual Life (Years), beginning balance	7 years 6 months 22 days	7 years 1 month 28 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 3 months 22 days
Options Outstanding, Aggregate Intrinsic Value, beginning balance	$ 2,393	$ 789
Exercisable, Aggregate Intrinsic Value, ending balance	$ 1,748