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Investments and Fair Value of Financial Instruments - Summary of Changes in Fair Value of Liabilities (Detail) - Level 3 [Member]
$ in Thousands
3 Months Ended
Mar. 31, 2019
USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]  
Balance as of December 31, 2018 $ 5,083
Change in fair value 8,495
Balance as of March 31, 2019 13,578
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, 2018 2,354
Change in fair value 3,970
Balance as of March 31, 2019 6,324
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, 2018 2,729
Change in fair value 4,525
Balance as of March 31, 2019 $ 7,254