Key Assumptions Used to Value the Warrants (Detail)	Mar. 31, 2019 $ / shares	Mar. 29, 2019 $ / shares	Dec. 31, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2017 $ / shares	May 31, 2016 $ / shares
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Stock price		$ 0.82
Settlement Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Stock price				$ 2.90	$ 2.90	$ 2.79
Measurement Input, Price Volatility | Settlement Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility				62.74	96.01	101.52
Measurement Input, Expected Term | Settlement Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected term (in years)				1 year	1 year	1 year
Measurement Input, Risk Free Interest Rate | Settlement Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility				2.37	2.09	1.76
Measurement Input, Expected Dividend Rate | Settlement Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility				0	0	0
PIPE Warrants
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Stock price	$ 0.82		$ 1.60				$ 0.89
PIPE Warrants | Measurement Input, Price Volatility
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility	111.99		82.64				76.25
PIPE Warrants | Measurement Input, Expected Term
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected term (in years)	2 years 3 months		2 years 6 months				5 years
PIPE Warrants | Measurement Input, Risk Free Interest Rate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility	2.27		2.47				1.22
PIPE Warrants | Measurement Input, Expected Dividend Rate
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected price volatility	0		0				0