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Warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Restricted stock units (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Weighted-average grant date fair value per share        
Share-based compensation expense $ 2,501 $ 1,601 $ 4,610 $ 2,885
Restricted stock units        
Number of restricted stock units        
Unvested restricted common stock at the beginning of the period (in shares)     370,123  
Granted (in shares)     224,792  
Vested (in shares)     (93,816)  
Cancelled (in shares)     (21,103)  
Unvested restricted common stock at the end of the period (in shares) 479,996   479,996  
Weighted-average grant date fair value per share        
Unvested restricted common stock at the beginning of the period (in dollars per share)     $ 20.48  
Granted (in dollars per share)     27.33  
Vested (in dollars per share)     27.26  
Cancelled (in dollars per share)     19.94  
Unvested restricted common stock at the end of the period (in dollars per share) $ 23.41   $ 23.41  
Share-based compensation expense $ 1,100 $ 700 $ 2,000 $ 1,200
Total unrecognized compensation cost related to unvested stock awards $ 10,400   $ 10,400  
Period of recognition of unrecognized compensation cost     2 years 10 months 24 days  
Restricted stock        
Number of restricted stock units        
Granted (in shares)     0  
Unvested restricted common stock at the end of the period (in shares) 39,806   39,806  
Weighted-average grant date fair value per share        
Unvested restricted common stock at the end of the period (in dollars per share) $ 3.12   $ 3.12  
2017 Plan | Restricted stock units | Subject to a four year vesting schedule with 25% vesting on the first anniversary and the remaining vesting ratably on a monthly basis over the remaining three years        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period 1     4 years  
Vesting period 2     3 years  
Vesting percentage 1 (as a percent)     25.00%  
Vesting percentage 2 (as a percent)     75.00%  
Number of restricted stock units        
Granted (in shares)     202,336  
2017 Plan | Restricted stock units | Vesting on December 31, 2020        
Number of restricted stock units        
Granted (in shares)     15,890  
2017 Plan | Restricted stock units | Vested immediately upon grant        
Number of restricted stock units        
Granted (in shares)     5,106  
2017 Plan | Restricted stock units | Vested on May 31, 2020        
Number of restricted stock units        
Granted (in shares)     1,460