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Share-Based Compensation - Stock Option Award Vesting and Weighted Average Assumptions (Details) - Stock Options - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Weighted average assumptions and weighted average fair value of options    
Number of options granted (in shares) 341,659 481,712
Weighted average exercise price (in dollars per share) $ 87.08 $ 54.47
Risk-free interest rate (as a percent) 2.55% 1.77%
Expected volatility (as a percent) 21.80% 21.72%
Expected term (years) 4 years 9 months 18 days 4 years 7 months 6 days
Dividend yield (as a percent) 0.60% 0.73%
Weighted average fair value (in dollars per share) $ 19.34 $ 10.73
Maximum    
Share-based Compensation    
Award vesting period 3 years