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Common Stock - Narrative (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Sep. 30, 2021
Dec. 31, 2019
Class of Stock [Line Items]      
Common stock and temporary equity, shares authorized (in shares) 35,192,637 35,192,637  
Stock Options      
Class of Stock [Line Items]      
Contractual term 10 years    
Requisite service period 4 years    
Stock Options | 2013 Stock Option and Grant Plan      
Class of Stock [Line Items]      
Contractual term 10 years    
Requisite service period 4 years    
Stock Options | 2019 Equity Incentive Plan      
Class of Stock [Line Items]      
Contractual term 10 years    
Requisite service period 4 years    
Common Stock      
Class of Stock [Line Items]      
Common stock, shares authorized (in shares) 128,734,881 143,234,881 107,168,070
Common stock par value (in dollars per share) $ 0.000001 $ 0.000001 $ 0.000001
Common Stock | Stock Options | 2013 Stock Option and Grant Plan      
Class of Stock [Line Items]      
Common stock, shares authorized (in shares) 0    
Common Stock | Stock Options | 2019 Equity Incentive Plan      
Class of Stock [Line Items]      
Common stock, shares authorized (in shares) 845,650   845,650
Redeemable Convertible Preferred Stock      
Class of Stock [Line Items]      
Mezzanine equity, shares authorized (in shares) 159,303,110 173,803,110 138,679,570