Warrants - Summary of Assumptions to Use Option Pricing Model (Detail)		1 Months Ended	6 Months Ended	12 Months Ended
	Aug. 22, 2016	Jan. 31, 2017	Jun. 30, 2018	Dec. 31, 2017
2016 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)	7 years		5 years 2 months 12 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	61.60%		59.80%	62.10%
2016 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs	1.40%		2.70%	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%	0.00%
2017 Placement Warrants | Expected Term
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Expected term (in years)		7 years	5 years 7 months 6 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		62.90%	60.00%	62.30%
2017 Placement Warrants | Risk-free Interest Rate
Fair Value Inputs1 Liabilities Quantitative Information [Line Items]
Warrants, fair value measurement inputs		2.20%	2.80%	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%	0.00%