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, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance	1,378,352
Options granted, Number of Shares	837,540
Options exercised, Number of Shares	(49,280)
Options forfeited, Number of Shares	(97,445)
Options Outstanding, Number of Shares, ending balance	2,069,167	1,378,352
Exercisable, Number of Shares, ending balance	912,098
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.38
Options granted, Weighted Average Exercise Price	16.63
Options exercised, Weighted Average Exercise Price	9.10
Options forfeited, Weighted Average Exercise Price	12.29
Options Outstanding, Weighted Average Exercise Price, ending balance	12.18	$ 9.38
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.52
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	7 years 9 months 3 days	7 years 9 months 22 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 4 months 6 days
Options Outstanding, Aggregate Intrinsic Value	$ 3,187	$ 9,829
Exercisable, Aggregate Intrinsic Value	$ 2,248