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Share-Based Arrangements (Employee Stock Purchase Plan) (Details) - ESPP Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award      
Market price after discount, purchase date 85.00%    
Number of shares authorized (in shares) 1,500,000    
Employee stock issued (in shares) 63,874 74,273 79,294
Number of shares available for purchase (in shares) 171,191    
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award      
Employee stock issued (in dollars per share) $ 46.79 $ 32.95 $ 31.64
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award      
Employee stock issued (in dollars per share) $ 57.27 $ 42.40 $ 36.46