Warrants - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended						12 Months Ended
	Aug. 22, 2016	Dec. 31, 2017	Jan. 31, 2017	Sep. 30, 2016	Aug. 31, 2016	Aug. 31, 2015	Jul. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Common Stock Warrants [Line Items]
(Gain) loss related to change in fair value								$ 16,598	$ (3)	$ (45)
Convertible preferred stock warrant liability							$ 93			93
Fair value of warrants		$ 22,420						22,420	2,723
Adjustments to additional paid In capital warrant exercised		274
Private Placement
Common Stock Warrants [Line Items]
Proceed from private placement and equity issuances, gross			$ 26,100	$ 13,800				26,100	13,800
Fair value of warrants upon issuance under Private Placement	$ 2,700
2015 Placement Common Stock Warrants
Common Stock Warrants [Line Items]
Estimated fair value of the warrants issued to placement agents										$ 316
2015 Placement Common Stock Warrants | Insiders
Common Stock Warrants [Line Items]
Warrants issued						198,760	198,760
Exercise price						$ 5.00	$ 5.00
Common stock warrants, expiry term						5 years	5 years
2016 Placement Warrant
Common Stock Warrants [Line Items]
Warrants issued				1,380,745	1,380,745
Exercise price				$ 2.95	$ 2.95
(Gain) loss related to change in fair value								$ 7,400	$ (3)
Common stock warrants, expiry term				7 years	7 years
Expected term (in years)								5 years 8 months 12 days	6 years 8 months 12 days
Expected volatility								62.10%	63.60%
Risk-free interest rate								2.20%	2.30%
Expected dividend yield								0.00%	0.00%
Fair value of warrants		$ 9,933						$ 9,933	$ 2,723
Class of warrant exercised		25,104						25,104
Placement warrants outstanding		1,355,641						1,355,641
2016 Placement Warrants
Common Stock Warrants [Line Items]
Expected term (in years)	7 years
Expected volatility	61.60%
Risk-free interest rate	1.40%
Expected dividend yield	0.00%
2017 Placement Warrants
Common Stock Warrants [Line Items]
Warrants issued			1,720,512
Exercise price			$ 3.17
(Gain) loss related to change in fair value								$ 9,200
Fair value of warrants upon issuance under Private Placement			$ 3,400
Expected term (in years)			7 years					6 years 1 month 6 days
Expected volatility			62.90%					62.30%
Risk-free interest rate			2.20%					2.30%
Expected dividend yield			0.00%					0.00%
Fair value of warrants		$ 12,487						$ 12,487
Warrant exercisable date			Jul. 31, 2017
Warrant expiration date			Jan. 31, 2024
Class of warrant exercised		9,389						9,389
Placement warrants outstanding		1,711,123						1,711,123
Term Loan | Series C Convertible Preferred Stock
Common Stock Warrants [Line Items]
Warrants issued							128,231				128,231
Exercise price											$ 5.84
Common stock warrants, expiration date											Dec. 16, 2023