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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted Stock      
Non-vested at beginning of period 345,365 382,186 295,340
Granted 253,943 218,169 239,062
Performance Shares Quantity Adjustment (7) 15,061 234,076
Vested (234,421) (270,051) (372,271)
Forfeited (6,368)   (14,021)
Non-vested at end of period 358,512 345,365 382,186
Weighted-Average Grant Date Fair Value      
Non-vested at beginning of period $ 41.68 $ 36.41 $ 30.75
Granted 42.49 55.55 45.81
Performance Shares Quantity Adjustment 48.79 55.72 33.09
Vested 42.64 44.73 35.83
Forfeited 44.26   37.35
Non-vested at end of period $ 41.23 $ 41.68 $ 36.41