Stockholders' equity - Warrants (Details)	Dec. 31, 2019 $ / shares shares
Warrants Expiring November 1, 2020
Class of Stock [Line Items]
Number of warrants outstanding (in shares) | shares	1,866,831
Common stock warrant exercise price (in dollars per share) | $ / shares	$ 8.16
Warrants Expiring September 12, 2022
Class of Stock [Line Items]
Number of warrants outstanding (in shares) | shares	810,357
Common stock warrant exercise price (in dollars per share) | $ / shares	$ 6.72