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Capital Stock and Shares Compensation Plans - Weighted-average assumptions used in estimate of compensation costs of share options (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Restricted Stock Units [Abstract]      
Weighted-average fair value of stock options granted $ 15.18 $ 19.20 $ 18.94
Assumptions used to determine fair value:      
Dividend yield 1.28% 0.99% 0.95%
Risk-free rate of return 1.49% 1.53% 0.84%
Expected life of options (years) 5 years 5 years 5 years
Expected volatility 24.70% 27.20% 36.30%