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Stockholders' equity - Stock option activity (Details)
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Number of options  
Outstanding, beginning balance (in shares) | shares 227,203
Issued (in shares) | shares 113,561
Expired (in shares) | shares 800
Outstanding, ending balance (in shares) | shares 339,964
Exercisable (in shares) | shares 238,943
Weighted average exercise price  
Outstanding, beginning balance (in dollars per share) | $ / shares $ 9.39
Issued (in dollars per share) | $ / shares 2.50
Expired ( in dollars per share) | $ / shares 9.90
Outstanding, ending balance (in dollars per share) | $ / shares 7.09
Exercisable (in dollars per share) | $ / shares $ 7.76