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Share-Based Compensation - MSUs Assumptions (Details) - MSUs - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Assumptions:      
Weighted average grant date fair value (in dollars per share) $ 34.22 $ 35.79 $ 19.59
Expected volatility 32.62% 32.10% 34.02%
Risk-free rate 2.37% 1.39% 0.86%
Expected term (in years) 2 years 11 months 12 days 2 years 11 months 14 days 2 years 11 months 12 days
Expected dividends $ 0 $ 0 $ 0