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Organization and Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Dec. 31, 2020
USD ($)
shares
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
shares
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2020
USD ($)
segment
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
shares
Organization and Summary of Significant Accounting Policies [Line Items]                      
Revenues $ 25,800,000 $ 57,763,000 $ 21,553,000 $ 13,076,000 $ 54,851,000 $ 21,958,000 $ 17,548,000 $ 8,071,000 $ 118,192,000 $ 102,428,000 $ 84,452,000
Net loss (40,675,000) $ (1,508,000) $ (35,965,000) $ (42,974,000) $ 4,501,000 $ (27,361,000) $ (30,356,000) $ (42,203,000) (121,122,000) $ (95,419,000) $ (68,889,000)
Goodwill and intangible asset impairment                 0    
Impairment of long-lived assets held-for-use                 0    
Letter of credit $ 1,500,000               $ 1,500,000    
Stock options and RSUs outstanding (in shares) | shares 14,237,871       10,750,550       14,237,871 10,750,550 9,048,793
Number of operating segments | segment                 1    
Pfizer SB-525 | Change in collaboration agreement scope | Collaborative arrangement                      
Organization and Summary of Significant Accounting Policies [Line Items]                      
Revenues                 $ 8,900,000 $ 5,700,000  
Net loss                 $ 8,900,000 $ 5,700,000  
Earnings per share, basic (in dollars per share) | $ / shares                 $ 0.06 $ 0.05  
Minimum                      
Organization and Summary of Significant Accounting Policies [Line Items]                      
Estimated useful lives of related assets                 3 years    
Maximum                      
Organization and Summary of Significant Accounting Policies [Line Items]                      
Estimated useful lives of related assets                 5 years