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Share-based compensation - PSU activity (Details) - $ / shares
1 Months Ended 6 Months Ended
Dec. 31, 2021
Sep. 30, 2021
Jun. 30, 2022
Other disclosure      
Service period 3 years 1 year  
2014 Plan      
Other disclosure      
Vesting period (in years)     4 years
Performance Share Awards | 2014 Plan      
Number of shares      
Non-vested at beginning of period (in shares)     632,930
Granted (in shares)     34,700
Vested (in shares)     (77,330)
Forfeited (in shares)     (24,400)
Non-vested at end of period (in shares) 632,930   565,900
Weighted average grant-date fair value      
Non-vested at beginning of period (in dollars per share)     $ 33.54
Granted (in dollars per share)     15.11
Vested (in dollars per share)     57.56
Forfeited (in dollars per share)     29.35
Non-vested at end of period (in dollars per share) $ 33.54   $ 29.31