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Share-based payments - Summary of the Restricted Common Stock Award/Unit Activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock      
Shares      
Nonvested balance at beginning of period (shares) 143,964    
Granted (shares) 110,600 133,000 110,000
Adjustment for performance-based shares (shares) 94,851    
Vested (shares) (100,479)    
Forfeited (shares) (18,060)    
Nonvested balance at end of period (shares) 120,276 143,964  
Weighted-Average Grant-Date Fair Value per Share ($)      
Nonvested balance at beginning of period (USD per share) $ 39.89    
Granted (USD per share) 43.39 $ 42.59 $ 37.34
Adjustment for performance-based shares (USD per share) 39.41    
Vested (USD per share) 38.27    
Forfeited (USD per share) 39.04    
Nonvested balance at end of period (USD per share) $ 40.99 $ 39.89  
Restricted Stock Unit      
Shares      
Nonvested balance at beginning of period (shares) 19,664    
Granted (shares) 110,564    
Vested (shares) (2,975)    
Forfeited (shares) (3,412)    
Nonvested balance at end of period (shares) 123,841 19,664  
Weighted-Average Grant-Date Fair Value per Share ($)      
Nonvested balance at beginning of period (USD per share) $ 38.14    
Granted (USD per share) 43.39    
Vested (USD per share) 38.64    
Forfeited (USD per share) 43.59    
Nonvested balance at end of period (USD per share) $ 42.67 $ 38.14