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Stockholder's equity (deficit) - Undesignated preferred stock and Restricted common stock (Details) - $ / shares
1 Months Ended
Oct. 31, 2017
Mar. 31, 2017
Mar. 31, 2020
Restricted stock      
Class of Stock      
Shares issued (in shares)   226,400  
Stock options vesting period 2 years 4 years  
Number of shares subject to be repurchased 75,467    
Undesignated Preferred Stock      
Class of Stock      
Preferred stock, par value (in dollars per share)     $ 0.0001
Preferred stock, shares issued     0
Preferred stock, shares outstanding     0
Undesignated Preferred Stock | Maximum      
Class of Stock      
Preferred stock, shares authorized     10,000,000