Nature of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended						9 Months Ended
	Jun. 25, 2018	Jul. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Nature Of Organization And Operations [Line Items]
Net proceeds from issuance of common stock	$ 104,013								$ 129,465
Convertible preferred stock converted into common stock	15,320,213
Net loss			$ (17,145)	$ (16,055)	$ (17,103)	$ (11,591)	$ (10,507)	$ (8,242)	(50,303)	$ (30,340)
Accumulated deficit			$ (122,042)						$ (122,042)		$ (71,739)
IPO [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	5,247,958
Issuance price per shares	$ 19.00
Net proceeds from issuance of common stock	$ 90,103
Underwriting discounts and commissions and offering costs	6,980
Other offering expenses	$ 2,628
Over-Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold	787,193
Net proceeds from issuance of common stock	$ 13,910
Underwriting discounts and commissions and offering costs	$ 1,047
July 2019 Follow-on Offering [Member]
Nature Of Organization And Operations [Line Items]
Net proceeds from issuance of common stock		$ 129,500
Common Stock [Member] | Over-Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold		975,000
Common Stock [Member] | July 2019 Follow-on Offering [Member]
Nature Of Organization And Operations [Line Items]
Common stock, shares issued and sold		7,475,000
Issuance price per shares		$ 18.50