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Fair Value of Financial Instruments - Summarizes Changes in Fair Value of Level 3 Financial Liabilities (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Fair value, beginning of period $ 22,420 $ 2,723 $ 0
Change in fair value of Level 3 financial liabilities (9,379) 16,598 (3)
Fair value, end of period 11,844 22,420 2,723
2016 Placement Warrants      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Issuance of Placement Warrants 0 0 2,726
Fair value of Placement Warrants at exercise (1,187) (200) 0
2017 Placement Warrants      
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]      
Issuance of Placement Warrants 0 3,373 0
Fair value of Placement Warrants at exercise $ (10) $ (74) $ 0