Fair Value Measurement - Assumptions Used for Determining Fair Values of Common Stock Warrants (Detail)	Mar. 31, 2019 $ / shares	Mar. 19, 2019 $ / shares	Jan. 18, 2019 $ / shares	Sep. 20, 2018 $ / shares
Warrant Fair Value Black Scholes Method [Line Items]
Stock price		$ 1.37	$ 2.25	$ 3.285
January 30, 2018 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 10.17
January 30, 2018 Offering [Member] | Exercise Price [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	15.00
January 30, 2018 Offering [Member] | Expected Dividend Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	0.00
January 30, 2018 Offering [Member] | Discount Rate-Bond Equivalent Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	2.48
January 30, 2018 Offering [Member] | Expected Life [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Expected life (in years)	5 years
January 30, 2018 Offering [Member] | Expected Volatility [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	99.0
February 12, 2019 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 1.05
February 12, 2019 Offering [Member] | Exercise Price [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	1.20
February 12, 2019 Offering [Member] | Expected Dividend Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	0.00
February 12, 2019 Offering [Member] | Discount Rate-Bond Equivalent Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	2.49
February 12, 2019 Offering [Member] | Expected Life [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Expected life (in years)	5 years
February 12, 2019 Offering [Member] | Expected Volatility [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	147.7
March 19, 2019 Offering [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Stock price	$ 1.12
March 19, 2019 Offering [Member] | Exercise Price [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	1.25
March 19, 2019 Offering [Member] | Expected Dividend Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	0.00
March 19, 2019 Offering [Member] | Discount Rate-Bond Equivalent Yield [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	2.44
March 19, 2019 Offering [Member] | Expected Life [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Expected life (in years)	5 years 6 months
March 19, 2019 Offering [Member] | Expected Volatility [Member]
Warrant Fair Value Black Scholes Method [Line Items]
Exercise price	140.0