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Stock-based compensation - Summary of incentive plan for members of management (Details) - Restricted Stock Units (RSUs) - Executive Officers - Performance Based Shares - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Restricted Stock    
Non-vested at beginning of period 345,365 382,186
Granted 278,326 218,169
Performance Shares Quantity Adjustment (7) 15,061
Vested (245,002) (270,051)
Forfeited (3,555)  
Non-vested at end of period 375,127 345,365
Weighted-Average Grant Date Fair Value    
Non-vested at beginning of period $ 41.68 $ 36.41
Granted 43.21 55.55
Performance Shares Quantity Adjustment 48.79 55.72
Vested 43.10 44.73
Forfeited 43.72  
Non-vested at end of period $ 41.52 $ 41.68