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Fair Value Measurements - Narrative (Details)
12 Months Ended
Oct. 05, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Derivative [Line Items]        
Marketable securities   $ 0 $ 13,580,000  
Accrued interest on marketable securities     100,000  
Payments of debt fees   $ 1,875,000 0 $ 0
Oxford Finance Loan | Medium-term Notes        
Derivative [Line Items]        
Payments of debt fees $ 1,900,000      
Success Fee Derivative        
Derivative [Line Items]        
Success Fee amount $ 1,900,000   $ 1,200,000  
Measurement Input, Probability Of Liquidity Event | Success Fee Derivative        
Derivative [Line Items]        
Derivative liability, measurement input     0.65