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REVENUE - Narrative (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue from Contract with Customer [Abstract]      
Mark-to-market gains (losses) $ 265 $ 346 $ (1,100)
Revenue recognized 174    
Increase (decrease) in contract with customers, liability 591    
Remaining performance obligation 59,500    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01      
Revenue from Contract with Customer [Abstract]      
Remaining performance obligation $ 6,800    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]      
Remaining performance obligation, period 1 year