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Stock-based compensation - Summary of incentive plan for members of management (Details) - Performance Based Shares - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Restricted Stock      
Non-vested at beginning of period 358,512 345,365 382,186
Granted 191,479 253,943 218,169
Performance Shares Quantity Adjustment 54,306 (7) 15,061
Vested (273,974) (234,421) (270,051)
Forfeited (8,440) (6,368)  
Non-vested at end of period 321,883 358,512 345,365
Weighted-Average Grant Date Fair Value      
Non-vested at beginning of period $ 41.23 $ 41.68 $ 36.41
Granted 69.38 42.49 55.55
Performance Shares Quantity Adjustment 54.21 48.79 55.72
Vested 55.11 42.64 44.73
Forfeited 43.48 44.26  
Non-vested at end of period $ 47.98 $ 41.23 $ 41.68