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Stock Plans and Stock Based Compensation - Activity Related to Stock Options and SARs Under All Plans(Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares/Rights      
Granted (in shares) 59,333 73,716 109,750
Outstanding, December 31, 2017 (in shares) 269,506    
Exercisable, December 31, 2017 (in shares) 130,610    
Weighted Average Exercise Price      
Outstanding, December 31, 2017 (in dollars per share) $ 47.34    
Exercisable, December 31, 2017 (in dollars per share) $ 41.88    
Weighted Average Remaining Contractual Term      
Outstanding (in years) 5 years 3 months 26 days    
Stock Options And Stock Appreciation Rights      
Number of Shares/Rights      
Outstanding, January 1, 2017 (in shares) 338,516    
Granted (in shares) 0    
Exercised (in shares) (66,878)    
Forfeited (in shares) (2,132)    
Outstanding, December 31, 2017 (in shares) 269,506 338,516  
Exercisable, December 31, 2017 (in shares) 130,610    
Weighted Average Exercise Price      
Outstanding, January 1, 2017 (in dollars per share) $ 45.56    
Granted (in dollars per share) 0.00    
Exercised (in dollars per share) 37.96    
Forfeited (in dollars per share) 58.58    
Outstanding, December 31, 2017 (in dollars per share) 47.34 $ 45.56  
Exercisable, December 31, 2017 (in dollars per share) $ 41.88    
Weighted Average Remaining Contractual Term      
Outstanding (in years) 5 years 3 months 26 days    
Exercisable (in years) 3 years 9 months 4 days    
Aggregate Intrinsic Value      
Outstanding $ 9,167,831    
Exercisable $ 5,155,065