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Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Details)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Expected dividend yield 3.40% 3.70%
Risk-free interest rate 2.70% 2.00%
Expected volatility 19.50% 19.70%
Expected life (years) 6 years 6 years 2 months 12 days