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Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Details)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Expected dividend yield 3.40% 3.60%
Risk-free interest rate 2.80% 2.00%
Expected volatility 19.10% 17.90%
Expected life (years) 6 years 1 month 6 days 6 years 1 month 6 days