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Stockholders' equity - Warrants (Details) - Warrant
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2019
USD ($)
$ / shares
shares
Number of options  
Outstanding, beginning balance (in shares) | shares 1,745,868
Exercised (in shares) | shares (76,797)
Outstanding, ending balance (in shares) | shares 1,669,071
Exercisable (in shares) | shares 1,669,071
Weighted average exercise price  
Outstanding, beginning balance (in dollars per share) | $ / shares $ 3.25
Exercised (in dollars per share) | $ / shares 3.25
Outstanding, ending balance (in dollars per share) | $ / shares 3.25
Exercisable (in dollars per share) | $ / shares $ 3.25
Proceeds from Warrant Exercises | $ $ 250