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Notes Payable - Summary of Future Principal Payments under Hercules Loan Agreement (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Notes payable, net of debt discount $ 28,937 $ 8,385
Hercules Loan Agreement [Member] | Term Loan [Member]    
Debt Instrument [Line Items]    
2020 1,466  
2021 18,527  
2022 10,007  
Total 30,000  
Less: Unamortized discount (1,063)  
Notes payable, net of debt discount $ 28,937