Net loss per share - Schedule of Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income (loss) - basic	$ (8,545)	$ (7,384)	$ (5,858)	$ 889	$ (10,008)	$ (7,030)	$ (21,787)	$ (16,149)
Less: noncash income from change in fair value of warrants				7,090				906
Net loss - diluted	$ (8,545)			$ (6,201)			$ (21,787)	$ (17,055)
Denominator:
Basic weighted-average common shares outstanding	79,583,254			31,793,292			56,916,137	31,775,590
Effect of dilutive securities:
Options to purchase common stock				260,882				236,395
Warrants				389,473				223,543
Diluted weighted-average common shares outstanding	79,583,254			32,443,647			56,916,137	32,235,528
Net income (loss) per common share:
Basic	$ (0.11)			$ 0.03			$ (0.38)	$ (0.51)
Diluted	$ (0.11)			$ (0.19)			$ (0.38)	$ (0.53)