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Share-Based Compensation - Stock Option Award Vesting and Weighted Average Assumptions (Details) - Stock Options - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Weighted average assumptions and weighted average fair value of options    
Number of options granted (in shares) 223,520 341,659
Weighted average exercise price (in dollars per share) $ 91.93 $ 87.08
Risk-free interest rate (as a percent) 2.48% 2.55%
Expected volatility (as a percent) 22.79% 21.80%
Expected term (years) 4 years 9 months 18 days 4 years 9 months 18 days
Dividend yield (as a percent) 0.57% 0.60%
Weighted average fair value (in dollars per share) $ 21.85 $ 19.34
Maximum    
Share-based Compensation    
Award vesting period 3 years