Stockholders' equity - Stock option activity (Details) - Employee Stock Option	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Number of options
Outstanding, beginning balance (in shares) | shares	573,925
Issued (in shares) | shares	104,458
Expired (in shares) | shares	(14,835)
Outstanding, ending balance (in shares) | shares	663,548
Exercisable (in shares) | shares	565,350
Weighted average exercise price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 4.94
Issued (in dollars per share) | $ / shares	1.99
Expired (in dollars per share) | $ / shares	11.62
Outstanding, ending balance (in dollars per share) | $ / shares	4.33
Exercisable (in dollars per share) | $ / shares	$ 4.73
Stock options, exercisable term (in years)	10 years
Minimum
Weighted average exercise price
Stock options, vesting period (in years)	1 year
Maximum
Weighted average exercise price
Stock options, vesting period (in years)	3 years