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Share-Based Compensation (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Grant Date Weighted Average Fair Value [Abstract]        
Total pre-tax share-based compensation recognized in the Consolidated Statements of Earnings $ 5,900 $ 7,700 $ 1,600  
Tax related benefits 500 2,800 500  
Cash received from the exercise of stock options $ 300 $ 700 $ 400  
Non-vested Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized (in shares) 1,800      
Shares available to be granted under existing stock plans (in shares) 1,800      
Age participant attains for recognition of expenses from date of grant 65 years      
Expense recognition period for years prior to 2013 5 years      
Percentage of grants to elected officers that will be performance stock unit awards 100.00% 100.00% 100.00%  
Percentage of stated performance metrics award of grant 150.00% 150.00% 150.00%  
Number of years to measure performance metrics 3 years 3 years 3 years  
Non-vested Stock [Roll Forward]        
Outstanding, beginning of period (in shares) 435 542 604  
Granted (in shares) 115 119 154  
Vested (in shares) (114) (172) (74)  
Cancelled (in shares) (24) (54) (142)  
Outstanding, end of period (in shares) 412 435 542  
Grant Date Weighted Average Fair Value [Abstract]        
Outstanding, beginning of period (in dollars per share) $ 56.44 $ 48.94 $ 43.84  
Granted (in dollars per share) 74.26 69.92 60.65  
Vested (in dollars per share) 39.75 43.42 35.51  
Cancelled (in dollars per share) 63.62 52.41 47.02  
Outstanding, end of period (in dollars per share) $ 65.64 $ 56.44 $ 48.94  
Outstanding, aggregate intrinsic value $ 30,113 $ 34,184 $ 34,063 $ 36,454
Total intrinsic values of shares vested 8,800 $ 12,800 $ 4,800  
Non-vested Stock and Performance Stock Units [Member]        
Grant Date Weighted Average Fair Value [Abstract]        
Compensation cost net yet recognized $ 13,500      
Compensation cost not yet recognized, period for recognition 1 year 3 months 29 days