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Stock-based compensation - Summary of incentive plan for members of management (Details) - Performance Based Shares - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Restricted Stock    
Non-vested at beginning of period 358,512 345,365
Granted 191,479 253,943
Performance Shares Quantity Adjustment 54,306 (7)
Vested (272,990) (234,421)
Forfeited (2,429) (6,368)
Non-vested at end of period 328,878 358,512
Weighted-Average Grant Date Fair Value    
Non-vested at beginning of period $ 41.23 $ 41.68
Granted 69.38 42.49
Performance Shares Quantity Adjustment 54.21 48.79
Vested 55.15 42.64
Forfeited 42.52 44.26
Non-vested at end of period $ 47.90 $ 41.23