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Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Restricted stock units (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Weighted-average grant date fair value per share        
Share-based compensation expense   $ 4,884 $ 2,168 $ 928
Restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)   422,027    
Number of restricted stock units        
Granted (in shares)   422,027    
Vested (in shares)   (99,990)    
Cancelled or Forfeited (in shares)   (375)    
Unvested restricted common stock at the end of the period (in shares)   321,662    
Weighted-average grant date fair value per share        
Granted (in dollars per share)   $ 15.65    
Vested (in dollars per share)   15.02    
Cancelled or Forfeited (in dollars per share)   18.40    
Unvested restricted common stock at the end of the period (in dollars per share)   $ 15.84    
Share-based compensation expense   $ 1,700    
Total unrecognized compensation cost related to unvested stock awards   $ 4,900    
Period of recognition of unrecognized compensation cost   7 months 6 days    
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)   84,637    
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)   84,637    
2017 Plan | Restricted stock units | Subject to vesting with 50% vesting on December 31, 2018 and December 31, 2019        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)   18,200    
Vesting percentage 1 (as a percent)   50.00%    
Vesting percentage 2 (as a percent)   50.00%    
Number of restricted stock units        
Granted (in shares)   18,200    
2017 Plan | Restricted stock units | Subject to a four year vesting schedule with 25% vesting on each anniversary        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)   3,300    
Vesting period 1   4 years    
Vesting percentage 1 (as a percent)   25.00%    
Number of restricted stock units        
Granted (in shares)   3,300    
2017 Plan | Restricted stock units | Vesting on December 31, 2018        
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 on August 31, 2018        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares)   50,000    
Number of restricted stock units        
Granted (in shares)   50,000    
2017 Plan | Restricted stock units | Vests evenly over 40 months on the last day of each month starting September 30,2018        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock issued (in shares) 250,000 250,000    
Vesting period 3   40 months    
Number of restricted stock units        
Granted (in shares) 250,000 250,000