The company - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended				3 Months Ended
	Jun. 30, 2020	Feb. 29, 2020	May 31, 2019	Oct. 31, 2018	Dec. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated deficit						$ (458,455)	$ (318,524)
IPO [Member]
Proceeds of underwritten public offering common stock			$ 84,300	$ 69,600
At The Market Offering [Member]
Proceeds of underwritten public offering common stock					$ 48,000
Subsequent Public Offering [Member]
Proceeds of underwritten public offering common stock	$ 107,400	$ 140,200