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Fair Value of Financial Instruments - Additional Information (Detail)
 6 Months Ended
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Fair Value Of Financial Instruments [Line Items]    
Cash and cash equivalents $ 7,968,000 $ 71,157,000
Restricted cash 5,316,000 $ 527,000
Short-term investments $ 24,909,000  
Fair value of note payable to related party, description The fair value measurement of the note payable is based on discounted cash flow model and it is sensitive to the change in yield (Level 3 in the fair value hierarchy). If the yield increases by approximately 2% from 23% to 25%, the fair value of the note payable with the conversion feature would change from $59.5 million to $57.9 million, or a decrease of $1.6 million and 2.7%; if the yield decreases 2% from 23% to 21%, the fair value of the note payable with conversion feature would change from $59.5 million to $61.2 million, or an increase of $1.7 million or 2.9%. If the yield increases by approximately 4% from 23% to 27%, the fair value of the note payable with the conversion feature would change from $59.5 million to $56.3 million, or a decrease of $3.2 million and 5.4%; if the yield decreases 4% from 23% to 19%, the fair value of the note payable with conversion feature would change from $59.5 million to $62.9 million, or an increase of $3.4 million and 5.7%.  
Fair value of the liability, percentage decrease 6.00%  
Embedded derivatives $ 0  
Minimum    
Fair Value Of Financial Instruments [Line Items]    
Probabilities of meeting the milestones, amount $ 50,000,000  
Probabilities of meeting the milestones, percentage decrease 5.00%  
Fair value of the liability, percentage decrease 13.00%  
Maximum    
Fair Value Of Financial Instruments [Line Items]    
Probabilities of meeting the milestones, amount $ 200,000,000  
Probabilities of meeting the milestones, percentage decrease 10.00%  
Fair value of the liability, percentage decrease 24.00%  
Note Payable Yield Changes 1    
Fair Value Of Financial Instruments [Line Items]    
Note payable, percentage of interest rate increases (decreases) 2.00%  
Note payable, percentage of yield 0.25 0.23
Note payable to related party, fair value $ 57,900,000 $ 59,500,000
Change in fair value of note payable value due to change in interest rate percentage $ 1,600,000  
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage 2.70%  
Note Payable Yield Changes 2    
Fair Value Of Financial Instruments [Line Items]    
Note payable, percentage of interest rate increases (decreases) 2.00%  
Note payable, percentage of yield 0.21 0.23
Note payable to related party, fair value $ 61,200,000 $ 59,500,000
Change in fair value of note payable value due to change in interest rate percentage $ 1,700,000  
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage 2.90%  
Note Payable Yield Changes 3    
Fair Value Of Financial Instruments [Line Items]    
Note payable, percentage of interest rate increases (decreases) 4.00%  
Note payable, percentage of yield 0.27 0.23
Note payable to related party, fair value $ 56,300,000 $ 59,500,000
Change in fair value of note payable value due to change in interest rate percentage $ 3,200,000  
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage 5.40%  
Note Payable Yield Changes 4    
Fair Value Of Financial Instruments [Line Items]    
Note payable, percentage of interest rate increases (decreases) 4.00%  
Note payable, percentage of yield 0.19 0.23
Note payable to related party, fair value $ 62,900,000 $ 59,500,000
Change in fair value of note payable value due to change in interest rate percentage $ 3,400,000  
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage 5.70%  
Money Market Funds    
Fair Value Of Financial Instruments [Line Items]    
Cash and cash equivalents $ 8,000,000 71,200,000
Restricted cash 5,300,000 $ 500,000
U.S. Treasury bills or notes.    
Fair Value Of Financial Instruments [Line Items]    
Short-term investments $ 24,900,000