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INCOME TAXES - Additional Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Net operating loss carryforwards          
Tax loss and credit carryforwards $ 1,046,000,000 $ 1,238,000,000      
Valuation allowance 673,000,000 1,159,000,000      
Valuation allowance release 259,000,000 0      
Deferred income tax liabilities on the unremitted earnings of foreign investments 648,000,000 619,000,000      
Income tax payments, net of refunds 713,000,000 338,000,000 $ 247,000,000    
Interest expense (recover) reflected within net tax expense 4,000,000 (1,000,000) 0    
Accrued interest expense 7,000,000 3,000,000      
Income tax penalties expense 0 0 $ 0    
Income tax penalties accrued 0 0      
Canada federal and provincial          
Net operating loss carryforwards          
Unused net operating loss carryforwards 1,929,000,000 1,867,000,000      
Capital loss carryforwards unrecognized 598,000,000 821,000,000      
Canada federal and provincial | Alternative minimum tax          
Net operating loss carryforwards          
Minimum tax credits $ 102,000,000 $ 91,000,000      
U.S. federal          
Net operating loss carryforwards          
Unused net operating loss carryforwards       $ 1,098 $ 889
Mexican Tax Authority          
Net operating loss carryforwards          
Tax loss and credit carryforwards       $ 4 $ 3