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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Income Taxes [Line Items]    
Net operating loss carryforwards $ 43,623 $ 52,034
Deferred tax assets related to U.S. federal research and development credits $ 13,400  
Net operating loss carryforwards beginning expiration year 2031  
U.S. research and development credit beginning expiration year 2024  
Orphan drug credit beginning expiration year 2030  
Deferred tax assets relating to U.S. state NOL carryforwards $ 8,400  
Unrecognized tax benefits that would impact effective tax rate 11,200  
U.S. Federal    
Income Taxes [Line Items]    
Net operating loss carryforwards 35,200  
Orphan Drug    
Income Taxes [Line Items]    
Net deferred tax assets $ 24,400  
District of Columbia    
Income Taxes [Line Items]    
Net operating loss carryforwards beginning expiration year 2032  
Other States    
Income Taxes [Line Items]    
Net operating loss carryforwards beginning expiration year 2029