Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended
	Aug. 31, 2018	Jul. 31, 2018	Feb. 28, 2018	Jan. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options exercised					0		0
Proceeds from exercise of stock options					$ 0		$ 0
Issuance of common stock for services (Shares)					0	0	75,183	46,858
Issuance of common stock for services, amount					$ 300	$ 200	$ 300	$ 200
Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense					14,400		$ 14,400
Unrecognized stock based compensation expense, weighted-average remaining recognition period							2 years 9 months 18 days
Number of options exercised						4,833		33,159
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense					$ 1,200		$ 1,200
Unrecognized stock based compensation expense, weighted-average remaining recognition period							1 year 8 months 12 days
Number of shares issued upon vesting of restricted stock units					0	0	0	15,188
Performance-Based Restricted Stock Units (PSRUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, Granted		903,374		161,865
Number of shares, Outstanding		903,374		125,895
Number of vested shares					0	0	0	0
Recognized stock-based compensation related expense in connection with vesting of award granted					$ 100		$ 500
Unrecognized stock based compensation expense					100		$ 100
Unrecognized stock based compensation expense, weighted-average remaining recognition period							1 month 6 days
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock based compensation expense					$ 100		$ 100
Unrecognized stock based compensation expense, weighted-average remaining recognition period							4 months 24 days
Number of shares issued under ESPP	52,370		68,589
Purchase price	$ 3.66		$ 4.38
Proceeds	$ 200		$ 300