Redeemable Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	9 Months Ended
	Jul. 05, 2017	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017
Temporary Equity [Line Items]
Preferred stock issuance costs, paid				$ 2,049
Redeemable convertible preferred stock, authorized (in shares)			0
Redeemable convertible preferred stock, issued (in shares)			0
Redeemable convertible preferred stock, outstanding (in shares)			0
IPO [Member]
Temporary Equity [Line Items]
Initial public offering closing date			Jul. 05, 2017
IPO [Member] | Common Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued, price per share	$ 15.00
Preferred stock issuance costs, paid	$ 2,304
Number of common shares issued on conversion of redeemable convertible preferred stock	10,509,774
Series F Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued		483,501
Preferred stock shares issued, price per share		$ 1.36
Proceeds from issuance of redeemable convertible preferred stock		$ 626
Preferred stock issuance costs, paid		32
Stock purchase agreement, aggregate purchase price		$ 11,516
Series E-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares converted		4,411,765
Series E-3 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock shares issued on conversion		4,411,765