Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance		2,897,294
Options granted, Number of Shares		298,000
Options exercised, Number of Shares	(13,104)	(13,104)
Options forfeited, Number of Shares		(278,197)
Options Outstanding, Number of Shares, ending balance	2,903,993	2,903,993	2,897,294
Exercisable, Number of Shares, ending balance	2,288,581	2,288,581
Options Outstanding, Weighted Average Exercise Price, beginning balance		$ 9.04
Options granted, Weighted Average Exercise Price		3.54
Options exercised, Weighted Average Exercise Price		3.07
Options forfeited, Weighted Average Exercise Price		11.84
Options Outstanding, Weighted Average Exercise Price, ending balance	$ 8.23	8.23	$ 9.04
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.41	$ 9.41
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance		3 years 11 months 12 days	7 years 11 months 15 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance		3 years 7 months 6 days
Options Outstanding, Aggregate Intrinsic Value	$ 3	$ 3