Shareholders' equity - Share-based compensation - Outstanding and exercisable options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Outstanding and exercisable options, weighted average exercise price
Granted	$ 15.19	$ 6.65
Exercised	1.4400	$ 1.3200
Outstanding at end of period	7.89
Exercisable at end of period	$ 1.92
Weighted average remaining contractual life (in years)	8 years 2 months 12 days	8 years 6 months
Weighted average remaining contractual life for vested options (in years)	7 years	6 years 7 months 6 days
Options forfeited	7,728
Share-based compensation expense	$ 981	$ 211
Unrecognized compensation cost	$ 17,442	$ 3,047
Expected period for recognition	2 years 10 months 24 days	3 years
Option Plans 2011 and 2019
Outstanding and exercisable options, number of shares
Outstanding at end of period	3,288,376
Exercisable at end of period	1,333,297
2011 Plan
Outstanding and exercisable options, number of shares
Outstanding at beginning of period	2,364,526	2,295,045
Forfeited	(4,978)
Granted		116,742
Exercised	(53,722)	(18,153)
Outstanding at end of period	2,305,826	2,393,634
Exercisable at end of period	1,327,637	670,168
Outstanding and exercisable options, weighted average exercise price
Outstanding at beginning of period	$ 2.15	$ 1.77
Forfeited	2.12	1.32
Granted		9.42
Exercised	1.44
Outstanding at end of period	2.17	1.99
Exercisable at end of period	$ 1.85	1.35
Weighted average fair values of options granted		$ 6.65
2019 Plan
Outstanding and exercisable options, number of shares
Outstanding at beginning of period	220,140
Forfeited	(2,750)
Granted	765,160
Outstanding at end of period	982,550
Exercisable at end of period	5,660
Outstanding and exercisable options, weighted average exercise price
Outstanding at beginning of period	$ 20.78
Forfeited	21.48
Granted	21.49
Outstanding at end of period	21.33
Exercisable at end of period	18.43
Weighted average fair values of options granted	$ 15.19