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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
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Restricted Stock    
Non-vested at beginning of period 382,186 295,340
Granted 217,550 239,062
Performance Shares Quantity Adjustment 22,973 234,076
Vested (246,508) (372,271)
Forfeited   (14,021)
Non-vested at end of period 376,201 382,186
Weighted-Average Grant Date Fair Value    
Non-vested at beginning of period $ 36.41 $ 30.75
Granted 55.56 45.81
Performance Shares Quantity Adjustment 55.51 33.09
Vested 45.01 35.83
Forfeited   37.35
Non-vested at end of period $ 43.02 $ 36.41