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Stock-Based Compensation (Fair Value Assumptions) (Details) - Performance-based shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 2.00% 1.40% 1.20%
Expected term (years) 3 years 3 years 3 years
Expected stock price volatility 19.90% 21.10% 19.80%