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Fair Value of Financial Instruments - Summary of Changes in Fair Value of Level 3 Financial Liabilities (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Fair value, beginning of period $ 22,420 $ 2,723
Fair value of common stock warrants reclassified from liability to additional paid-in capital upon exercise (342) 0
Change in fair value of Level 3 financial liabilities (116) 6,739
Fair value, end of period 21,962 12,835
2017 Placement Warrants    
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Issuance of Placement Warrants $ 0 $ 3,373