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Share-based compensation - PSU activity (Details)
3 Months Ended
Mar. 31, 2024
Milestone
$ / shares
shares
Dec. 31, 2023
Milestone
Other disclosure    
Number of Milestones Achieved | Milestone 2  
Number of Milestones Achievement Considered Probable | Milestone 2 2
Minimum    
Other disclosure    
Service period 1 year  
Maximum    
Other disclosure    
Service period 3 years  
Performance Share Awards | 2014 Plan    
Number of shares    
Non-vested at beginning of period (in shares) | shares 222,550  
Forfeited (in shares) | shares (17,030)  
Non-vested at end of period (in shares) | shares 205,520  
Weighted average grant-date fair value    
Non-vested at beginning of period (in dollars per share) | $ / shares $ 28.09  
Forfeited (in dollars per share) | $ / shares 27.12  
Non-vested at end of period (in dollars per share) | $ / shares $ 27.99