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STOCK OPTION AND STOCK UNIT PLANS - WEIGHTED AVERAGE ASSUMPTIONS (Details)
CAD / shares in Units, CAD in Millions
12 Months Ended
Dec. 31, 2017
CAD
CAD / shares
Dec. 31, 2016
CAD
CAD / shares
Dec. 31, 2015
CAD
CAD / shares
Dec. 31, 2017
$ / shares
Dec. 31, 2016
$ / shares
Dec. 31, 2015
$ / shares
Weighted average assumptions used to determine the fair value of options            
Compensation expense CAD 165 CAD 130 CAD 97      
INCENTIVE STOCK OPTIONS            
Weighted average assumptions used to determine the fair value of options            
Fair value per option (in dollars per share) | CAD / shares CAD 6.00 CAD 7.37 CAD 6.48      
Expected option term (in years) 5 years 5 years 5 years      
Expected volatility (as a percent) 20.40% 25.10% 19.90%      
Expected dividend yield (as a percent) 4.20% 4.40% 3.20%      
Risk-free interest rate (as a percent) 1.20% 0.80% 0.90%      
Expected option term based on historical practice 6 years          
Expected option term based on historical practice for retirement eligible employees 3 years          
Compensation expense CAD 40 CAD 43 CAD 35      
Unrecognized compensation cost related to non-vested share-based compensation arrangements granted CAD 47          
Weighted average period over which compensation cost is expected to be recognized 2 years          
INCENTIVE STOCK OPTIONS | Canadian employees            
Weighted average assumptions used to determine the fair value of options            
Fair value per option (in dollars per share) | CAD / shares CAD 5.66 CAD 7.01 CAD 6.22      
INCENTIVE STOCK OPTIONS | United States employees            
Weighted average assumptions used to determine the fair value of options            
Fair value per option (in dollars per share) | $ / shares       $ 5.72 $ 6.60 $ 6.16