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, 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	4,016,500
Options exercised, Number of Shares	(326,094)	(64,938)
Options forfeited, Number of Shares	(747,162)
Options Outstanding, Number of Shares, ending balance	5,840,538	2,897,294
Exercisable, Number of Shares, ending balance	1,805,043
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.04
Options granted, Weighted Average Exercise Price	2.91
Options exercised, Weighted Average Exercise Price	4.77
Options forfeited, Weighted Average Exercise Price	8.71
Options Outstanding, Weighted Average Exercise Price, ending balance	5.11	$ 9.04
Exercisable, Weighted Average Exercise Price, ending balance	$ 9.85
Options Outstanding, Weighted Average Remaining Contractual Life (Years), ending balance	7 years 8 months 19 days	7 years 11 months 15 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	3 years 4 months 9 days
Options Outstanding, Aggregate Intrinsic Value	$ 45,037
Exercisable, Aggregate Intrinsic Value	$ 7,264