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ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Dec. 31, 2018
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 57,763 $ 21,958 $ 92,392 $ 47,577    
Decrease in net loss 1,508 27,361 80,447 99,920    
Letter of credit established as a deposit 1,500   1,500      
Cash and cash equivalents 259,393 114,291 259,393 114,291 $ 80,428 $ 140,418
Current restricted cash 0 2,000 0 2,000 0 0
Non-current restricted cash 1,500 1,500 1,500 1,500 1,500 3,500
Cash, cash equivalents and restricted cash as reported within the accompanying Condensed Consolidated Statements of Cash Flows 260,893 117,791 260,893 117,791 $ 81,928 $ 143,918
Biogen MA, Inc.            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 9,621 $ 0 $ 17,799 $ 0    
Biogen MA, Inc. | Customer Concentration Risk | Revenue Benchmark            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Concentration risk, percentage 16.00% 0.00% 19.00% 0.00%    
Kite Pharma Inc            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 7,294 $ 8,861 $ 21,935 $ 26,233    
Kite Pharma Inc | Customer Concentration Risk | Revenue from Contract with Customer            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Concentration risk, percentage 13.00% 40.00% 24.00% 55.00%    
Pfizer            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 9,200 $ 468 $ 12,985 $ 1,538    
Pfizer | Customer Concentration Risk | Revenue from Contract with Customer            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Concentration risk, percentage 68.00% 16.00% 51.00% 11.00%    
Sanofi            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 1,498 $ 2,754 $ 3,789 $ 7,763    
Sanofi | Customer Concentration Risk | Revenue from Contract with Customer            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Concentration risk, percentage 3.00% 40.00% 4.00% 29.00%    
Change in Collaboration Agreement Scope | Collaboration Agreement | Pfizer            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues $ 5,800          
Decrease in net loss $ 5,800          
Earnings per share, basic (in dollars per share) $ 0.04          
Change in Collaboration Agreement Scope | Collaboration Agreement | Pfizer and Sanofi            
Organization And Summary Of Significant Accounting Policies [Line Items]            
Increase in revenues     $ 8,900      
Decrease in net loss     $ 8,900      
Earnings per share, basic (in dollars per share)     $ 0.06