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SHAREHOLDERS' EQUITY - Normal Course Issuer Bid (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Apr. 30, 2019
Apr. 30, 2018
Disclosure of classes of share capital [line items]        
Shares repurchased $ 655,000,000      
Class B Non-Voting Shares        
Disclosure of classes of share capital [line items]        
Shares repurchased $ 655,000,000      
Number of shares purchased (in shares) 9,900,000      
2019 NCIB | Class B Non-Voting Shares        
Disclosure of classes of share capital [line items]        
Authorized amount (in shares)     35,700,000  
Authorized amount     $ 500,000,000  
Shares repurchased $ 500,000,000      
Number of shares purchased (in shares) 7,700,000      
2018 NCIB | Class B Non-Voting Shares        
Disclosure of classes of share capital [line items]        
Authorized amount (in shares)       35,800,000
Authorized amount       $ 500,000,000
Shares repurchased $ 155,000,000 $ 0    
Number of shares purchased (in shares) 2,200,000