Net loss per common stock - Schedule of basic and diluted net loss per common stock (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Securities excluded from computation of earnings per share	16,468,693	3,357,264	16,468,693	3,357,264
Outstanding stock options [Member]
Securities excluded from computation of earnings per share	6,203,993	3,357,264	6,203,993	3,357,264
Restricted stock units [Member]
Securities excluded from computation of earnings per share	72,000		72,000
Common stock to be issued on conversion of convertible preferred stock [Member]
Securities excluded from computation of earnings per share	10,192,700		10,192,700