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Development Derivative Liability - Additional Information (Detail)
 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 28, 2019
USD ($)
Installment
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Jun. 07, 2019
USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Gain/(loss) from remeasurement of development derivative liability   $ 2,697,000 $ 2,770,000 $ (68,406,000) $ (263,000) $ (9,104,000) $ (736,000) $ (62,939,000) $ (10,103,000)    
Development derivative liability   217,778,000 $ 220,475,000 $ 223,245,000 130,103,000 $ 109,840,000 $ 60,736,000 $ 217,778,000 130,103,000 $ 134,839,000  
Level 3 [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Implied cost of borrowing discount rates               16.18%      
SFJ Agreement [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Proceeds from SFJ agreement $ 60,000,000.0             $ 20,000,000.0 120,000,000.0 $ 120,000,000.0  
Additional funding amount upon achievement of development milestones $ 60,000,000.0                    
Number of milestone payments | Installment 3                    
Development funding for minimum period of operating expense 10 months                    
Increase in additional funding for development costs                     $ 20,000,000.0
Number of additional annual payments | Installment 6                    
Gain/(loss) from remeasurement of development derivative liability   2,700,000     (300,000)     (62,900,000) $ (10,100,000)    
Proceeds from collaborators of milestones         $ 20,000,000.0            
Development derivative liability   $ 217,800,000           $ 217,800,000      
SFJ Agreement [Member] | Level 3 [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Implied cost of borrowing discount rates               8.00%      
SFJ Agreement [Member] | Maximum [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Obligated to pay initial payment $ 5,000,000.0                    
Aggregate amount of additional annual payments 226,000,000.0                    
SFJ Agreement [Member] | Regulatory Approval Granted by FDA and EMA [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Obligated to pay initial payment 10,000,000.0                    
SFJ Agreement [Member] | Regulatory Approval Granted by FDA and EMA [Member] | Maximum [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Aggregate amount of additional annual payments $ 452,000,000.0