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]	Jun. 30, 2021	Dec. 31, 2020
Remaining Term [Member] | January 2021 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	5 years 29 days	0 years
Remaining Term [Member] | September 2019 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	3 years 8 months 12 days	4 years 2 months 12 days
Remaining Term [Member] | June 2019 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	3 years 5 months 15 days	3 years 11 months 15 days
Remaining Term [Member] | March 2018 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	1 year 8 months 19 days	2 years 2 months 19 days
Expected Volatility [Member] | January 2021 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	125.8
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	137.7	135.0
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	140.4	135.0
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	91.1	130.0
Risk-Free Interest Rate [Member] | January 2021 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.9
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	0.6	0.3
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	0.6	0.3
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	0.2	0.1
Expected Dividend Yield [Member] | January 2021 Offering [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
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