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Stock-based compensation - Summary of incentive plan for members of management (Details) - Restricted Stock Units (RSUs) - Executive Officers - $ / shares
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Restricted Stock    
Granted 87,706 84,590
Performance Based Shares    
Restricted Stock    
Non-vested at beginning of period 345,365 382,186
Granted 152,521 218,169
Performance Shares Quantity Adjustment (7) 15,061
Vested (160,333) (270,051)
Non-vested at end of period 337,546 345,365
Weighted-Average Grant Date Fair Value    
Non-vested at beginning of period $ 41.68 $ 36.41
Granted 47.68 55.55
Performance Shares Quantity Adjustment 48.79 55.72
Vested 44.17 44.73
Non-vested at end of period $ 42.83 $ 41.68