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STOCK OPTION AND STOCK UNIT PLANS - WEIGHTED AVERAGE ASSUMPTIONS (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
CAD ($)
$ / shares
Dec. 31, 2018
CAD ($)
$ / shares
Dec. 31, 2017
CAD ($)
$ / shares
Dec. 31, 2019
$ / shares
Dec. 31, 2018
$ / shares
Dec. 31, 2017
$ / shares
Weighted average assumptions used to determine the fair value of options            
Compensation expense | $ $ 117 $ 106 $ 165      
INCENTIVE STOCK OPTIONS            
Weighted average assumptions used to determine the fair value of options            
Fair value per option (in dollars per share) | $ / shares $ 4.37 $ 3.86 $ 6.00      
Expected option term (in years) 5 years 5 years 5 years      
Expected volatility (as a percent) 19.90% 21.90% 20.40%      
Expected dividend yield (as a percent) 6.10% 6.40% 4.40%      
Risk-free interest rate (as a percent) 2.00% 2.20% 1.20%      
Expected option term - historical exercise practice 6 years          
Compensation expense | $ $ 32 $ 28 $ 40      
Unrecognized compensation cost related to non-vested share-based compensation arrangements granted | $ $ 18          
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 $ 4.04 $ 3.75 $ 5.66      
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       $ 4.09 $ 3.30 $ 5.72
INCENTIVE STOCK OPTIONS | Retirement-eligible employees            
Weighted average assumptions used to determine the fair value of options            
Expected option term (in years) 3 years