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Collaborations - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 31, 2020
Dec. 31, 2018
Feb. 28, 2018
Dec. 31, 2019
Dec. 08, 2016
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Deferred revenue       $ 15,000,000  
Pfizer Inc | Collaboration Agreement          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Milestone payment     $ 1,000,000 $ 0  
Pfizer Inc | Collaboration Agreement | Maximum          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Contingent cash payments upon achievement of clinical, regulatory and commercial milestones         $ 17,500,000
ISU Abxis | Amended and Restated License Agreement          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Regulatory and development milestone payment   $ 2,500,000      
ISU Abxis | Amended and Restated License Agreement | Maximum          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Milestone payment   19,500,000      
Commercial milestone payments   $ 17,000,000      
Biogen | Exclusive License          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Research services       30 months  
Research services option to extend       12 months  
Deferred revenue       $ 15,000,000  
Biogen | Exclusive License | Subsequent Event          
Revenue Recognition Multiple Deliverable Arrangements [Line Items]          
Up-front payments received $ 15,000,000        
Development milestones payment receivable 340,000,000        
Sales milestones receivable. $ 340,000,000