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Investments and Fair Value Instruments - Summary of Warrant Liabilities Measured and Recorded on Recurring Basis using Black-Scholes Model (Detail) - Common Stock Warrant Liability [Member] - Level 3 [Member]
Dec. 31, 2020
Dec. 31, 2019
Expected Term [Member] | September 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Expected term (in years) 4 years 2 months 12 days 5 years 2 months 12 days
Expected Term [Member] | June 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Expected term (in years) 3 years 11 months 15 days 4 years 11 months 15 days
Expected Term [Member] | March 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Expected term (in years) 2 years 2 months 19 days 3 years 2 months 19 days
Expected Term [Member] | June 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Expected term (in years) 0 years 3 years 11 months 15 days
Expected Volatility [Member] | September 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 135.0 120.0
Expected Volatility [Member] | June 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 135.0 120.0
Expected Volatility [Member] | March 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 130.0 125.0
Expected Volatility [Member] | June 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input   123.0
Risk-Free Interest Rate [Member] | September 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0.3 1.7
Risk-Free Interest Rate [Member] | June 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0.3 1.7
Risk-Free Interest Rate [Member] | March 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0.1 1.6
Risk-Free Interest Rate [Member] | June 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input   1.7
Expected Dividend Yield [Member] | September 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0 0
Expected Dividend Yield [Member] | June 2019 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0 0
Expected Dividend Yield [Member] | March 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input 0 0
Expected Dividend Yield [Member] | June 2018 Offering [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Warrants and rights outstanding, measurement input   0