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Share-based payments - Summary of the Restricted Common Stock Award Activity (Details) - Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Nonvested balance at beginning of period (shares) 206,327    
Granted (shares) 132,957 110,000 119,000
Adjustment for performance-based shares (shares) (59,348)    
Vested (shares) (59,260)    
Forfeited (shares) (76,712)    
Nonvested balance at end of period (shares) 143,964 206,327  
Weighted-Average Grant-Date Fair Value per Share ($)      
Nonvested balance at beginning of period (USD per share) $ 37.18    
Granted (USD per share) 42.59 $ 37.34 $ 37.16
Adjustment for performance-based shares (USD per share) 42.78    
Vested (USD per share) 36.98    
Forfeited (USD per share) 37.28    
Nonvested balance at end of period (USD per share) $ 39.89 $ 37.18