Warrants - Summary of Assumptions to Use Option Pricing Model (Detail)	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Jan. 31, 2017	Sep. 30, 2016
Common Stock Warrant | 2018 Direct Registered Offering | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)		7 years
Common Stock Warrant | 2018 Direct Registered Offering | Expected Volatility
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs		0.625
Common Stock Warrant | 2018 Direct Registered Offering | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs		0.028
Common Stock Warrant | 2018 Direct Registered Offering | Expected Dividend Yield
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs		0.00
2017 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)	5 years 1 month 6 days		6 years 1 month 6 days	7 years
2017 Placement Warrants | Expected Volatility
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.608		0.623	0.629
2017 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.025		0.023	0.022
2017 Placement Warrants | Expected Dividend Yield
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.00		0.00	0.00
2016 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)	4 years 8 months 12 days		5 years 8 months 12 days		7 years
2016 Placement Warrants | Expected Volatility
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.609		0.621		0.616
2016 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.025		0.022		0.014
2016 Placement Warrants | Expected Dividend Yield
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.00		0.00		0.00