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Summary of Significant Accounting Policies - Significant Customers (Details) - Customer Concentration Risk
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue | Merck      
Concentration Risk [Line Items]      
Concentration risk, percentage   61.00% 49.00%
Revenue | BARDA      
Concentration Risk [Line Items]      
Concentration risk, percentage 65.00% 13.00%  
Revenue | Vertex      
Concentration Risk [Line Items]      
Concentration risk, percentage   10.00%  
Revenue | AstraZeneca      
Concentration Risk [Line Items]      
Concentration risk, percentage     34.00%
Revenue | U.S. Government (excluding BARDA)      
Concentration Risk [Line Items]      
Concentration risk, percentage 24.00%    
Accounts Receivable | Merck      
Concentration Risk [Line Items]      
Concentration risk, percentage   13.00%  
Accounts Receivable | BARDA      
Concentration Risk [Line Items]      
Concentration risk, percentage 22.00% 54.00%  
Accounts Receivable | Vertex      
Concentration Risk [Line Items]      
Concentration risk, percentage   17.00%  
Accounts Receivable | European Commission      
Concentration Risk [Line Items]      
Concentration risk, percentage 28.00%    
Accounts Receivable | United Kingdom Government      
Concentration Risk [Line Items]      
Concentration risk, percentage 11.00%    
Accounts Receivable | South Korea Government      
Concentration Risk [Line Items]      
Concentration risk, percentage 24.00%