v3.21.1
Shareholders' equity - Share-based compensation - Non-vested share options (Details) - $ / shares
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Weighted average fair value    
Granted   $ 15.19
Option Plans 2011 and 2019    
Number of options    
Non-vested share options at end of period 3,427,110 1,955,079
Weighted average fair value    
Non-vested at end of period $ 6.08 $ 8.12
2011 Plan    
Number of options    
Non-vested at beginning of period 543,192 1,152,300
Vested, outstanding (93,087) (169,133)
Forfeited   (4,978)
Non-vested share options at end of period 450,105 978,189
Weighted average fair value    
Non-vested at beginning of period $ 1.81 $ 1.88
Vested, outstanding 1.61 5.81
Forfeited/expired   1.52
Non-vested at end of period $ 1.85 $ 1.21
2019 Plan    
Number of options    
Non-vested at beginning of period 1,438,026 218,975
Granted 1,863,950 765,160
Vested, outstanding (324,971) (4,495)
Forfeited   (2,750)
Non-vested share options at end of period 2,977,005 976,890
Weighted average fair value    
Non-vested at beginning of period $ 10.28 $ 14.00
Granted 4.79 15.19
Vested, outstanding 11.38 12.85
Forfeited/expired   15.25
Non-vested at end of period $ 6.72 $ 11.80