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Capital Stock (Option Grant Assumptions) (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capital Stock [Abstract]      
Weighted-average fair value per share $ 22.43 $ 17.73 $ 22.55
Risk-free interest rate 2.00% 1.30% 1.80%
Expected dividend yield 1.50% 1.60% 1.60%
Expected stock price volatility 19.40% 20.50% 25.50%
Expected option life (years) 6 years 6 years 1 month 7 years 3 months