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Stock Based Compensation (Weighted Average Assumptions Used to Value Option Grants) (Details) - Employee Stock Option
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate (%) 2.61% 2.09%
Expected option life (years) 5 years 3 months 25 days 5 years 3 months 6 days
Expected volatility (%) 18.00% 18.87%
Expected dividend yield (%) 0.59% 0.69%