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Common stock reserved for future issuance (Details) - shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Stock option grants outstanding (see Note 5) 2,247,026 2,161,085 2,022,353
Reserved for future stock option grants (see Note 5) 85,337 95,981  
Reserved for note conversion (see Note 2) 972,884  
Total common stock reserved for future issuance 2,332,363 3,229,950