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Share-Based Payments (Weighted Average Assumptions for Employee Stock Purchase Plan and Other Options) (Details) - Employee Stock Purchase Plan [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Risk-free interest rate 1.07% 0.47% 0.10%
Expected term 6 months 6 months 6 months
Dividend yield 1.30% 1.70% 2.00%
Expected volatility 24.00% 31.00% 20.00%