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Stockholders' equity - Summary of options outstanding and exercisable (Details)
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of options outstanding (in shares) 347,464
Number of options exercisable (in shares) 326,382
Weighted average exercise price of outstanding and exercisable options (in dollars per share) | $ / shares $ 6.77
$2.50 - $5.70  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of options outstanding (in shares) 211,534
Weighted average remaining estimated life (in years) 5 years 6 months
Number of options exercisable (in shares) 204,568
$6.90 - $9.60  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of options outstanding (in shares) 53,175
Weighted average remaining estimated life (in years) 3 years 6 months
Number of options exercisable (in shares) 53,175
$10.50 - $10.80  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of options outstanding (in shares) 4,000
Weighted average remaining estimated life (in years) 4 years 10 months 24 days
Number of options exercisable (in shares) 4,000
$14.20 - $23.40  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of options outstanding (in shares) 78,755
Weighted average remaining estimated life (in years) 7 years
Number of options exercisable (in shares) 64,639
Minimum | $2.50 - $5.70  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares $ 2.50
Minimum | $6.90 - $9.60  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 6.90
Minimum | $10.50 - $10.80  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 10.50
Minimum | $14.20 - $23.40  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 14.20
Maximum | $2.50 - $5.70  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 5.70
Maximum | $6.90 - $9.60  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 9.60
Maximum | $10.50 - $10.80  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares 10.80
Maximum | $14.20 - $23.40  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Weighted average price of outstanding options (in dollars per share) | $ / shares $ 23.40