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Share-based Compensation (Details) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Jan. 01, 2008
Stock options        
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Dividend yield (as a percent) 2.50%   2.37%  
Expected life of option (in years) 4 years 4 months 24 days   3 years 8 months 15 days  
Risk-free interest rate (as a percent) 1.73%   1.57%  
Expected stock price volatility (as a percent) 28.32%   26.76%  
Options Outstanding        
Balance at beginning of period (in shares) 1,737,213 1,595,007 1,595,007  
Granted (in shares) 168,000   445,100  
Exercised (in shares) (137,943)   (192,875)  
Lapsed or cancelled (in shares) (61,834)   (110,019)  
Balance at end of period (in shares) 1,705,436   1,737,213  
Options exercisable at end of period (in shares) 946,285   748,617  
Weighted average fair value of options granted during the period $ 3,733,043   $ 8,363,036  
Weighted Average Exercise Price        
Balance at beginning of year (in dollars per share) $ 98.44 $ 95.52 $ 95.52  
Granted (in dollars per share) 105.76   105.86  
Exercised (in dollars per share) 84.06   76.90  
Lapsed or cancelled (in dollars per share) 122.71   123.86  
Balance at end of period (in dollars per share) 99.45   98.44  
Options exercisable at end of period (in dollars per share) $ 91.61   $ 87.72  
Restricted Stock Awards        
Options Outstanding        
Weighted average fair value of options granted during the period $ 30,813 $ 10,650,077    
Summary of restricted stock        
Balance at beginning of year (in shares) 3,202,031 3,137,881 3,137,881  
Granted (in shares) 300 98,800    
Cancelled (in shares) 0 (34,650)    
Balance at end of period (in shares) 3,202,331   3,202,031  
Vested during the period (in shares) 84,236 83,822    
Compensation expense recorded $ 4,854,350 $ 7,153,966    
Employee Stock Purchase Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares of common stock available for issuance (shares)       500,000
Summary of restricted stock        
Duration of each offering period starting the first day of each calendar quarter (in months) 3 months      
Share Based Compensation Arrangement by Share Based Payment Award, Purchase Price as Percentage of Market Value of Common Stock 85.00%