Warrants - Summary of Assumptions to Use Option Pricing Model (Detail)		1 Months Ended		12 Months Ended
	Jul. 23, 2015	Jan. 31, 2017	Aug. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
2016 Placement Warrant
Fair Value Inputs Liabilities Quantitative Information [Line Items]
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%
2017 Placement Warrants
Fair Value Inputs Liabilities Quantitative Information [Line Items]
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%
Series C Convertible Preferred Stock Warrant
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Expected term (in years)	5 years
Expected volatility	31.80%
Risk-free interest rate	1.70%
Expected dividend yield	0.00%
Common Stock Warrants
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Expected term (in years)			5 years
Expected volatility			31.80%
Risk-free interest rate			1.60%
Expected dividend yield			0.00%