Fair Value Measurement - Assumptions Used for Determining Fair Values of Common Stock Warrants (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares
May 2016 Public Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 2.70
Exercise price	$ 3.90
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	1.23%
Expected life (in years)	5 years
Expected volatility	90.00%
Public Offering October 19,2016 [Member] | Over-allotment Options [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 0.93
Exercise price	$ 1.0331
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	0.25%
Expected life (in years)	29 days
Expected volatility	12.90%
Public Offering October 19,2016 [Member] | Warrants [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 0.93
Exercise price	$ 1.10
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	1.24%
Expected life (in years)	5 years
Expected volatility	80.00%
March 31, 2017 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 2.13
Exercise price	$ 2.50
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	1.93%
Expected life (in years)	5 years
Expected volatility	80.00%
August 9, 2017 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 1.39
Exercise price	$ 1.50
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	1.81%
Expected life (in years)	5 years
Expected volatility	100.00%
December 8, 2017 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 0.7399
Exercise price	$ 0.85
Expected dividend yield	0.00%
Discount rate-bond equivalent yield	2.09%
Expected life (in years)	4 years 6 months
Expected volatility	100.00%