Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance	2,069,167
Options granted, Number of Shares	2,054,540
Options exercised, Number of Shares	(64,938)
Options expired, Number of Shares	(520)
Options forfeited, Number of Shares	(700,985)
Options Outstanding, Number of Shares, ending balance	3,357,264	2,069,167
Exercisable, Number of Shares, ending balance	1,248,644
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 12.18
Options granted, Weighted Average Exercise Price	9.10
Options exercised, Weighted Average Exercise Price	9.28
Options expired, Weighted Average Exercise Price	10.56
Options forfeited, Weighted Average Exercise Price	14.86
Options Outstanding, Weighted Average Exercise Price, ending balance	9.79	$ 12.18
Exercisable, Weighted Average Exercise Price, ending balance	$ 11.03
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	8 years 3 months 7 days	7 years 9 months 3 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 4 months 20 days
Options Outstanding, Aggregate Intrinsic Value		$ 3,187
Exercisable, Aggregate Intrinsic Value	$ 0