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Stock-Based Compensation - Assumptions (Details) - Stock Options
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility   26.00%  
Expected term (Years) 5 years    
Dividend Yield (as a percent) 0.00% 0.00% 0.00%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 24.00%   25.00%
Risk-free rate 1.70% 1.20% 0.70%
Expected term (Years)   5 years 2 years
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 25.00%   34.00%
Risk-free rate 1.80%   1.90%
Expected term (Years)     7 years