Shareholders' equity - Share-based compensation - Non-vested share options (Details) - $ / shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Option Plans 2011 and 2019
Number of options
Non-vested share options at end of period	1,955,079
Weighted average fair value
Non-vested at end of period	$ 8.12
2011 Plan
Number of options
Non-vested at beginning of period	1,152,300	1,706,303
Granted		116,742
Vested, outstanding	(169,133)	(99,579)
Forfeited	(4,978)
Non-vested share options at end of period	978,189	1,723,466
Weighted average fair value
Non-vested at beginning of period	$ 1.88	$ 1.35
Forfeited/expired	1.52
Granted		6.65
Vested, outstanding	5.81	1.18
Non-vested at end of period	$ 1.21	$ 1.72
2019 Plan
Number of options
Non-vested at beginning of period	218,975
Granted	765,160
Vested, outstanding	(4,495)
Forfeited	(2,750)
Non-vested share options at end of period	976,890
Weighted average fair value
Non-vested at beginning of period	$ 14.44
Forfeited/expired	15.25
Granted	15.19
Vested, outstanding	12.85
Non-vested at end of period	$ 11.80