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Stock options - Activity under stock option plan (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Number of Shares    
Outstanding at beginning of period (in shares) 4,720,772  
Granted (in shares) 1,412,653  
Exercised (in shares) (43,055)  
Cancelled (in shares) (41,082)  
Outstanding at end of period (in shares) 6,049,288  
Options exercisable, at end of period (in shares) 2,846,100  
Weighted- Average Exercise Price    
Outstanding at beginning of period (in dollars per share) $ 5.24  
Granted (in dollars per share) 6.10  
Exercised (in dollars per share) 2.77  
Cancelled (in dollars per share) 8.50  
Outstanding at end of period (in dollars per share) 5.44  
Options exercisable, at end of period (in dollars per share) 4.68  
Additional Disclosures    
Weighted-average grant date fair value of options granted (in dollars per share) $ 3.79 $ 2.39
Cash received from the exercise of stock options (in dollars) $ 119 $ 42