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Stock-based compensation - Summary of incentive plan for members of management (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Performance Based Shares      
Restricted Stock      
Non-vested at beginning of period 382,186 295,340 383,982
Granted 218,169 239,062 212,200
Performance Shares Quantity Adjustment 15,061 234,076 (232,989)
Vested (270,051) (372,271) (67,853)
Forfeited   (14,021)  
Non-vested at end of period 345,365 382,186 295,340
Weighted-Average Grant Date Fair Value      
Non-vested at beginning of period $ 36.41 $ 30.75 $ 26.35
Granted 55.55 45.81 42.57
Performance Shares Quantity Adjustment 55.72 33.09 29.10
Vested 44.73 35.83 48.54
Forfeited   37.35  
Non-vested at end of period $ 41.68 $ 36.41 $ 30.75
Restricted Stock Units (RSUs) | Executive Officers      
Restricted Stock      
Granted 152,773 166,648 138,516