Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance	865,457
Options granted, Number of Shares	658,710
Options exercised, Number of Shares	(55,663)
Options expired, Number of Shares	(8,008)
Options forfeited, Number of Shares	(82,144)
Options Outstanding, Number of Shares, ending balance	1,378,352	865,457
Exercisable, Number of Shares, ending balance	575,422
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.75
Options granted, Weighted Average Exercise Price	8.77
Options exercised, Weighted Average Exercise Price	7.91
Options expired, Weighted Average Exercise Price	9.52
Options forfeited, Weighted Average Exercise Price	9.39
Options Outstanding, Weighted Average Exercise Price, ending balance	9.38	$ 9.75
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.30
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	7 years 9 months 22 days	7 years 6 months 22 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 2 months 12 days
Options Outstanding, Aggregate Intrinsic Value	$ 9,829	$ 2,393
Exercisable, Aggregate Intrinsic Value	$ 4,152