Warrants - Summary of Assumptions to Use Option Pricing Model (Detail)		1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 22, 2016	Jan. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
2016 Placement Warrants
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Expected term (in years)	7 years		5 years 4 months 24 days	5 years 8 months 12 days
Expected volatility	61.60%		61.20%	62.10%
Risk-free interest rate	1.40%		2.60%	2.20%
Expected dividend yield	0.00%		0.00%	0.00%
2017 Placement Warrants
Fair Value Inputs Liabilities Quantitative Information [Line Items]
Expected term (in years)		7 years	5 years 9 months 18 days	6 years 1 month 6 days
Expected volatility		62.90%	61.30%	62.30%
Risk-free interest rate		2.20%	2.60%	2.30%
Expected dividend yield		0.00%	0.00%	0.00%