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ASSET RETIREMENT OBLIGATIONS (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of movements in the Company's ARO        
Obligations at beginning of year $ 520 $ 989    
Liabilities disposed 0 (59)    
Liabilities incurred 0 15    
Liabilities settled (30) (12)    
Change in estimate and other 0 (417)    
Foreign currency translation adjustment (6) (18)    
Accretion expense 12 22    
Obligations at end of year 496 520    
Accounts payable and other     $ 56 $ 7
Other long-term liabilities     440 513
Asset retirement obligations $ 496 $ 520 $ 496 $ 520
Minimum        
Asset Retirement Obligations [Line Items]        
Discount rate (as a percent) 1.80%      
Maximum        
Asset Retirement Obligations [Line Items]        
Discount rate (as a percent) 9.00%