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Share-Based Compensation and Benefit Plans (Employee Share Purchase Plan Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation [Abstract]      
Stock Issued During Period, Shares, Employee Stock Purchase Plans 18,986 20,797 14,655
Weighted Average Fair Value Of Shares Purchased Under Employee Share Purchase Plan $ 89.89 $ 82.33 $ 74.66
Employee Stock Ownership Plan (ESOP), Compensation Expense $ 0.3 $ 0.4 $ 0.2