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Common stock reserved for future issuance (Details) - shares
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Payment Arrangement [Abstract]      
Stock option grants outstanding (see Note 4) 2,392,786 2,374,124 2,247,026
Reserved for future stock option grants (see Note 4) 308,871 116,703  
Total common stock reserved for future issuance 2,701,657 2,490,827