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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Effect of Fair Value and Cash Flow Hedging Relationships (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Interest Expense | Interest rate contracts      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Hedged items $ (19) $ (71) $ (74)
Derivatives designated as hedging instruments 1 (4) 1
Reclassification of gains/(losses) on derivative instruments from AOCI to net income (12) (22) (17)
Revenue | Commodity contracts      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Reclassification of gains/(losses) on derivative instruments from AOCI to net income $ (7) $ (5) $ 20