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Commitments and Contingencies - Additional Information (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Dec. 31, 2015
USD ($)
Mar. 31, 2021
USD ($)
ft²
OfficeSpace
Milestone
Mar. 31, 2020
USD ($)
Dec. 31, 2020
Milestone
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Other Commitments [Line Items]              
Rental expense     $ 63,000 $ 47,000      
Debt outstanding     $ 0        
Number of Milestones Achieved | Milestone     0        
Milestone payment $ 300,000            
Indemnification obligations amount     $ 0        
Contingent liabilities     $ 0        
License Agreement [Member]              
Other Commitments [Line Items]              
Sales based milestone payment   $ 36,000,000          
Royalty payment period   8 years          
Office of Chief Scientist of Israeli Ministry of Economy and Industry [Member]              
Other Commitments [Line Items]              
Grants received   $ 537,000          
ALS Therapy Development Foundation, Inc. License Agreement [Member]              
Other Commitments [Line Items]              
Fee due for milestones achieved           $ 1,000,000 $ 1,000,000
Common stock issued in lieu of making a cash payment           $ 1,000,000 $ 1,000,000
Number of Milestones Achieved | Milestone     0   0    
Remaining milestone payments for first licensed product     $ 6,000,000        
Annual License Maintenance Fee     100,000        
ALS Therapy Development Foundation, Inc. License Agreement [Member] | Achievement of 500 Million Aggregate Sales [Member]              
Other Commitments [Line Items]              
Reaching of aggregate net sales     500,000,000        
Amount of one-time milestone payment     15,000,000        
ALS Therapy Development Foundation, Inc. License Agreement [Member] | Achievement of 1 Billion Aggregate Sales [Member]              
Other Commitments [Line Items]              
Reaching of aggregate net sales     1,000,000,000        
Amount of one-time milestone payment     $ 30,000,000        
Minimum [Member]              
Other Commitments [Line Items]              
Remaining term of office lease     3 months        
Minimum [Member] | License Agreement [Member]              
Other Commitments [Line Items]              
Sales required to achieve milestone payment   1,000,000,000          
Maximum [Member]              
Other Commitments [Line Items]              
Remaining term of office lease     6 months        
Maximum [Member] | License Agreement [Member]              
Other Commitments [Line Items]              
Development and regulatory milestone payments   $ 42,100,000          
Maximum [Member] | ALS Therapy Development Foundation, Inc. License Agreement [Member]              
Other Commitments [Line Items]              
Development and regulatory milestone payments     $ 2,500,000        
Irvine, California [Member]              
Other Commitments [Line Items]              
Area of office space | ft²     5,197        
Lease expiration date     Sep. 30, 2021        
Burlington, Massachusetts [Member]              
Other Commitments [Line Items]              
Lease expiration date     Jun. 30, 2021        
Number of office spaces operating lease | OfficeSpace     4