Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 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	696,100
Options forfeited, Number of Shares	(59,063)
Options Outstanding, Number of Shares, ending balance	2,015,389	1,378,352
Exercisable, Number of Shares, ending balance	713,412
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.38
Options granted, Weighted Average Exercise Price	17.35
Options forfeited, Weighted Average Exercise Price	12.37
Options Outstanding, Weighted Average Exercise Price, ending balance	12.04	$ 9.38
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.26
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	8 years 4 months 2 days	7 years 9 months 22 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 5 months 9 days
Options Outstanding, Aggregate Intrinsic Value	$ 9,806	$ 9,829
Exercisable, Aggregate Intrinsic Value	$ 5,299