Warrants - Assumptions for Warrants Issued (Detail)	Sep. 30, 2019 shares	Dec. 31, 2018 shares	Aug. 31, 2018	Aug. 13, 2018 shares
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of shares	23,508,191	22,283,191
2018 public offerings | Confidentially Marketed Public Offering
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equivalent term (years)	3 years 10 months 13 days	4 years 7 months 13 days	5 years	5 years
Number of shares	14,725,000	15,600,000		20,000,000
2018 public offerings | Confidentially Marketed Public Offering | Measurement Input, Exercise Price [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	0.40	0.40		0.40
2018 public offerings | Confidentially Marketed Public Offering | Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	128.7	129.8		121.8
2018 public offerings | Confidentially Marketed Public Offering | Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	1.56	2.50		2.75