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Common stock reserved for future issuance (Details) - shares
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Payment Arrangement [Abstract]      
Stock option grants outstanding (see Note 6) 1,994,806 2,392,786 2,374,124
Reserved for future stock option grants 393,351 308,871  
Total common stock reserved for future issuance 2,388,157 2,701,657