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Stock-Based Compensation - Assumptions Used to Calculate Fair Value (Details) - Performance RSUs
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, minimum 2.48% 1.76% 0.85%
Risk-free interest rate, maximum 2.63% 1.89% 1.20%
Expected term (years) 3 years 3 years 3 years
Expected share price volatility, minimum (percent) 14.76% 11.01% 17.72%
Expected share price volatility, maximum (percent) 17.71% 14.70% 18.22%