Stock-based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	5,485,078
Granted (in shares)	1,031,704
Exercised (in shares)	(3,811)
Cancelled or forfeited (in shares)	(261,109)
Outstanding, ending balance (in shares)	6,251,862
Exercisable (in shares)	3,165,450
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 14.77
Granted (in dollars per share)	7.42
Exercised (in dollars per share)	7.27
Cancelled or forfeited (in dollars per share)	14.01
Outstanding (in dollars per share)	13.59
Exercisable (in dollars per share)	$ 15.18
Remaining Contractual Life
Outstanding	7 years 8 months 12 days
Exercisable	6 years 6 months
Options disclosures
Stock-based compensation.	$ 3,601	$ 4,040
Stock options
Options disclosures
Stock-based compensation.	$ 2,523	3,208
Share-based Payment Arrangement, Employee | Stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 4.81
Stock-based compensation.	$ 2,500	$ 3,200
Share-based Payment Arrangement, Nonemployee | Stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0	0