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Development Derivative Liability - Additional Information (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
Jan. 29, 2020
USD ($)
Sep. 23, 2019
USD ($)
Jun. 27, 2019
USD ($)
Feb. 28, 2019
USD ($)
Installment
Feb. 28, 2019
USD ($)
Installment
Jan. 31, 2020
USD ($)
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Jun. 07, 2019
USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Minimum payment of additional funding amount               $ 20,000,000 $ 60,000,000    
Loss from remeasurement of development derivative liability               68,406,000 736,000    
Development derivative liability $ 223,245,000             $ 223,245,000 60,736,000 $ 134,839,000  
Level 3 [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Implied cost of borrowing discount rates               13.47%      
SFJ Agreement [Member]                      
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]                      
Proceeds from SFJ agreement       $ 20,000,000.0   $ 60,000,000.0 $ 20,000,000.0   60,000,000.0    
Additional funding amount upon achievement of development milestones   $ 20,000,000.0 $ 20,000,000.0 40,000,000.0 $ 60,000,000.0 $ 60,000,000.0          
Number of milestone payments | Installment         3 3          
Development funding for minimum period of operating expense           10 months          
Additional funding for development costs 50,000,000.0                    
Increase in additional funding for development costs                     $ 20,000,000.0
Proceeds SFJ agreement upon achievement of milestone       $ 20,000,000.0              
Number of additional annual payments | Installment           6          
Minimum payment of additional funding amount           $ 120,000,000.0          
Increase in additional funding amount under amendment                     $ 20,000,000.0
Loss from remeasurement of development derivative liability               $ 68,400,000 $ 700,000    
Proceeds from collaborators of additional milestones                   60,000,000.0  
Development derivative liability $ 223,200,000             $ 223,200,000   $ 134,800,000  
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 5,000,000.0          
Aggregate amount of additional annual payments         226,000,000.0 226,000,000.0          
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 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 $ 452,000,000.0