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STOCK OPTION AND STOCK UNIT PLANS - WEIGHTED AVERAGE ASSUMPTIONS (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
CAD ($)
$ / shares
Dec. 31, 2017
CAD ($)
$ / shares
Dec. 31, 2016
CAD ($)
$ / shares
Dec. 31, 2018
$ / shares
Dec. 31, 2017
$ / shares
Dec. 31, 2016
$ / shares
Weighted average assumptions used to determine the fair value of options            
Compensation expense | $ $ 106 $ 165 $ 130      
INCENTIVE STOCK OPTIONS            
Weighted average assumptions used to determine the fair value of options            
Fair value per option (in dollars per share) | $ / shares $ 3.86 $ 6.00 $ 7.37      
Expected option term (in years) 5 years 5 years 5 years      
Expected volatility (as a percent) 21.90% 20.40% 25.10%      
Expected dividend yield (as a percent) 6.40% 4.40% 4.40%      
Risk-free interest rate (as a percent) 2.20% 1.20% 0.80%      
Expected option term based on historical practice 6 years          
Expected option term based on historical practice for retirement eligible employees 3 years          
Compensation expense | $ $ 28 $ 40 $ 43      
Unrecognized compensation cost related to non-vested share-based compensation arrangements granted | $ $ 23          
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) | $ / shares $ 3.75 $ 5.66 $ 7.01      
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       $ 3.30 $ 5.72 $ 6.60