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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 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Weighted-average grant date fair value per share        
Share-based compensation expense $ 1,828 $ 1,867 $ 4,713 $ 3,393
Vested immediately upon grant        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting percentage 1 (as a percent)     10.4167%  
Restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     227,653  
Number of restricted stock units        
Unvested restricted common stock at the beginning of the period (in shares)     321,662  
Granted (in shares)     227,653  
Vested (in shares)     (83,996)  
Cancelled (in shares)     (42,983)  
Unvested restricted common stock at the end of the period (in shares) 422,336   422,336  
Weighted-average grant date fair value per share        
Unvested restricted common stock at the beginning of the period (in dollars per share)     $ 15.84  
Granted (in dollars per share)     24.21  
Vested (in dollars per share)     16.65  
Cancelled (in dollars per share)     18.17  
Unvested restricted common stock at the end of the period (in dollars per share) $ 19.90   $ 19.90  
Share-based compensation expense $ 800 $ 1,100 $ 2,000 $ 1,200
Total unrecognized compensation cost related to unvested stock awards $ 7,600   $ 7,600  
Period of recognition of unrecognized compensation cost     2 years 11 months 9 days  
Restricted stock units | Vested in equal amounts annually over three years        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     4,540  
Vesting period 1     3 years  
Number of restricted stock units        
Granted (in shares)     4,540  
Restricted stock units | Vested immediately upon grant        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     1,983  
Number of restricted stock units        
Granted (in shares)     1,983  
Restricted stock units | Vesting ratably over the remaining 45 months        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     31,732  
Vesting period 3     45 months  
Number of restricted stock units        
Granted (in shares)     31,732  
Restricted stock units | Vest upon the one year        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     3,000  
Vesting period 1     1 year  
Number of restricted stock units        
Granted (in shares)     3,000  
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]        
Restricted common stock issued (in shares)     135,058  
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)     135,058  
2017 Plan | Restricted stock units | Vesting on December 31, 2019        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     15,890  
Number of restricted stock units        
Granted (in shares)     15,890  
2017 Plan | Restricted stock units | Vested in equal amounts annually over four years        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     24,233  
Vesting period 2     43 months  
Number of restricted stock units        
Granted (in shares)     24,233  
2017 Plan | Restricted stock units | Vesting equally over four years        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     10,194  
Number of restricted stock units        
Granted (in shares)     10,194  
2017 Plan | Restricted stock units | Vested immediately upon grant        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)     3,006  
Number of restricted stock units        
Granted (in shares)     3,006