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Stockholders' equity - Restricted stock activity (Details) - Restricted Stock
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Shares  
Beginning balance (in shares) | shares 8,867
Granted (in shares) | shares 176,561
Vested (in shares) | shares (40,623)
Ending balance (in shares) | shares 144,805
Weighted average grant date fair value  
Beginning balance (in dollars per share) | $ / shares $ 3.51
Granted (in dollars per share) | $ / shares 2.13
Vested (in dollars per share) | $ / shares 2.21
Ending balance (in dollars per share) | $ / shares $ 2.19