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Fair Value of Financial Instruments - Summary of Changes in Fair Value of Level 3 Financial Liabilities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Fair value, beginning of period $ 2,723 $ 0 $ 138
Change in fair value of Level 3 financial liabilities 16,598 (3) (45)
Conversion of convertible preferred stock warrants to common stock warrants 0 0 (93)
Fair value, end of period 22,420 2,723 0
2017 Placement Warrants Liability      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Issuance of Placement Warrants 3,373    
Fair value of Placement Warrants at exercise (74)    
2016 Placement Warrants Liability      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Issuance of Placement Warrants 0 $ 2,726 $ 0
Fair value of Placement Warrants at exercise $ (200)