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Share-based payments - Summary of the Restricted Common Stock Award Activity (Details) - Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shares      
Nonvested balance at beginning of period (shares) 252,190    
Granted (shares) 109,903 119,000 125,000
Adjustment for performance-based shares (shares) (36,997)    
Vested (shares) (83,286)    
Forfeited (shares) (35,483)    
Nonvested balance at end of period (shares) 206,327 252,190  
Weighted-Average Grant-Date Fair Value per Share      
Nonvested balance at beginning of period (USD per share) $ 34.88    
Granted (USD per share) 37.34 $ 37.16 $ 32.24
Adjustment for performance-based shares (USD per share) 32.91    
Vested (USD per share) 33.23    
Forfeited (USD per share) 35.03    
Nonvested balance at end of period (USD per share) $ 37.18 $ 34.88