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Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Details)
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Expected dividend yield 3.60% 3.80%
Risk-free interest rate 2.00% 1.40%
Expected volatility 17.80% 19.60%
Expected life (years) 6 years 1 month 6 days 6 years 2 months 12 days