Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	5,317,326
Granted (in shares)	899,085
Exercised (in shares)	(3,035)
Cancelled or forfeited (in shares)	(148,221)
Outstanding, ending balance (in shares)	6,065,155
Exercisable (in shares)	2,723,252
Vested and expected to vest (in shares)	6,065,155
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 15.98
Granted (in dollars per share)	12.88
Exercised (in dollars per share)	11.30
Cancelled or forfeited (in dollars per share)	15.66
Outstanding (in dollars per share)	15.53
Exercisable (in dollars per share)	14.77
Vested and expected to vest (in dollars per share)	$ 15.53
Remaining Contractual Life
Outstanding	7 years 10 months 24 days
Exercisable	6 years 7 months 6 days
Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value
Outstanding	$ 391
Exercisable	350
Vested and expected to vest	391
Options disclosures
Stock-based compensation	4,040	$ 3,518
Outstanding stock options
Options disclosures
Stock-based compensation	$ 3,208	$ 2,776
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.25	$ 6.23
Stock-based compensation	$ 3,200	$ 2,800
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0	0
Options disclosures
Stock-based compensation	$ 100	$ 100