Warrants - Summary of Assumptions to Use Option Pricing Model (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
2017 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)	5 years 3 months 18 days	6 years 1 month 6 days
2017 Placement Warrants | Expected Volatility
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	59.60%	62.30%
2017 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	3.00%	2.30%
2017 Placement Warrants | Expected Dividend Yield
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.00%	0.00%
2016 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)	4 years 10 months 24 days	5 years 8 months 12 days
2016 Placement Warrants | Expected Volatility
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	59.70%	62.10%
2016 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	2.90%	2.20%
2016 Placement Warrants | Expected Dividend Yield
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	0.00%	0.00%