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Stock-Based Compensation (Weighted-Average Assumptions-PSUs) (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility, minimum 31.00% 31.00% 37.00%
Volatility, maximum 36.00% 35.00% 45.00%
Dividend yield 0.25% 0.25% 0.25%
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term 3 years 3 years 3 years
Volatility, minimum 13.00% 13.00% 13.00%
Volatility, maximum 32.00% 31.00% 32.00%
Risk-free rate of return 2.41% 1.49% 0.88%
Dividend yield 0.00% 0.00% 0.00%
Weighted-average fair value per share (in dollars per share) $ 284.78 $ 147.25 $ 88.51