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Stock options - Plans narrative (Details) - 2017 Plan - shares
1 Months Ended 9 Months Ended
Jan. 31, 2020
Jun. 30, 2017
Sep. 30, 2020
Stock options      
Number of shares authorized (in shares)   2,255,000  
Cumulative annual increase in number of shares issuable (as a percent)   4.00%  
Additional shares authorized (in shares) 1,815,520    
Number of shares available for future issuance (in shares)     1,320,916
Number of options granted in period (in shares)     2,165,306