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Stockholders' equity - Stock option activity (Details)
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Number of options  
Outstanding, beginning balance (in shares) | shares 339,964
Issued (in shares) | shares 7,500
Outstanding, ending balance (in shares) | shares 347,464
Exercisable (in shares) | shares 326,382
Weighted average exercise price  
Outstanding, beginning balance (in dollars per share) | $ / shares $ 7.09
Issued (in dollars per share) | $ / shares 2.11
Outstanding, ending balance (in dollars per share) | $ / shares 6.98
Exercisable (in dollars per share) | $ / shares $ 6.77