Investments and Fair Value of Financial Instruments - Summary of Changes in Fair Value of Liabilities (Detail) - Level 3 [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	$ 16,936
Change in fair value	(5,082)
Exercise of warrants (Note 9)	(4,018)
Balance as of June 30, 2020	7,836
Common Stock Warrant Liability [Member] | Purchase Agreement [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	4,579
Change in fair value	(1,911)
Exercise of warrants (Note 9)	0
Balance as of June 30, 2020	2,668
Common Stock Warrant Liability [Member] | June 2018 Offering [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	5,444
Change in fair value	(1,426)
Exercise of warrants (Note 9)	(4,018)
Balance as of June 30, 2020	0
Common Stock Warrant Liability [Member] | June 2019 Offering [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	1,993
Change in fair value	(703)
Exercise of warrants (Note 9)	0
Balance as of June 30, 2020	1,290
Common Stock Warrant Liability [Member] | September 2019 Offering [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	2,920
Change in fair value	(1,042)
Exercise of warrants (Note 9)	0
Balance as of June 30, 2020	1,878
Contingent Liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance as of December 31, 2019	2,000
Change in fair value	0
Exercise of warrants (Note 9)	0
Balance as of June 30, 2020	$ 2,000