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Capital Stock (Weighted-Average Assumptions Used to Estimate the Fair Value of Options Granted) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Capital [Abstract]      
Document Fiscal Year Focus 2017    
Risk-free interest rate 2.31% 1.76% 1.96%
Volatility factor 32.00% 32.00% 31.00%
Dividend yield (annual) 1.60% 2.00% 1.70%
Weighted-average expected life (years) 4 years 4 years 1 month 6 days 4 years