Stockholders' Equity - Additional Information (Detail)							12 Months Ended
	Nov. 15, 2018 USD ($) shares	Oct. 12, 2018 USD ($)	Nov. 06, 2017 USD ($)	Nov. 01, 2017 $ / shares shares	May 11, 2016 USD ($) $ / shares shares	Apr. 25, 2016	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)
Class Of Stock [Line Items]
Number of common stock issued upon conversion of value							$ 28,548,000	$ 141,000,000	$ 167,139,000
Common stock par value per share | $ / shares							$ 0.0001	$ 0.0001
Proceeds from common stock offering							$ 28,971,000	$ 141,000,000	$ 170,507,000
Accretion amount recognized							$ 0
Sales Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares | shares	1,659,300
Proceeds from common stock offering	$ 29,000,000
Percentage of gross proceeds from common stock as sales agent cash commission		3.00%
Sales Agreement [Member] | General and Administrative [Member]
Class Of Stock [Line Items]
Legal accounting and other fees	$ 400,000
Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering		$ 100,000,000
IPO [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares | shares					6,900,000
Common stock price per share | $ / shares					$ 18.00
Proceeds from issuance initial public offering					$ 115,500,000
Reverse stock split ratio of common stock						0.588
Reverse stock split description							one-for-1.7 reverse stock split
Effective date of reverse stock split							Apr. 25, 2016
Convertible preferred stock conversion ratio							one-for-0.6465903
Conversion of convertible preferred stock into common stock | shares					23,481,956
Private Placement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares | shares					3,055,554
Number of common stock issued upon conversion of value					$ 55,000,000
Public Offering [Member] | Underwriting Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares | shares				6,250,000
Common stock par value per share | $ / shares				$ 0.0001
Proceeds from common stock offering			$ 141,000,000