Common Stock - Additional Information (Detail) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Jun. 01, 2018 $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares
Class of Stock [Line Items]
Common stock, shares authorized | shares		150,000,000	150,000,000	51,000,000
Common stock, par value | $ / shares		$ 0.0001	$ 0.0001	$ 0.0001
Undesignated preferred stock | shares		10,000,000
Undesignated preferred stock outstanding | shares		0
Cash dividends | $			$ 0
Reverse stock split		1-for-4.312
Stock Split, conversion ratio		4.312
Convertible preferred stock converted into common stock | shares				8,418,149
IPO [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold | shares	5,247,958
Issuance price per shares | $ / shares	$ 19.00
Net proceeds from issuance of common stock | $	$ 90,103
Net proceeds after deducting underwriting discounts and commissions and offering costs | $	6,980
Underwriting discounts and commissions and offering costs | $	$ 2,628
Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock, shares issued and sold | shares	787,193
Net proceeds from issuance of common stock | $	$ 13,910
Net proceeds after deducting underwriting discounts and commissions and offering costs | $	1,047
Underwriting discounts and commissions and offering costs | $	$ 104,013
Convertible preferred stock converted into common stock | shares	15,320,213