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FINANCIAL INSTRUMENTS - Estimated fair values of derivative financial instruments included in risk management asset (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation Of Changes In Derivative Financial Assets (Liabilities), Net [Roll Forward]      
Derivative financial asset (liability), net $ 489 $ 854  
Net change in fair value of outstanding derivatives financial instruments recognized in: Risk management activities (37) (25) $ (374)
Net change in fair value of outstanding derivatives financial instruments recognized in: Foreign exchange (375) (304)  
Net change in fair value of outstanding derivatives financial instruments recognized in: Other comprehensive income (loss) 24 (36)  
Derivative financial asset (liability), net 101 489 $ 854
Asset (liability), included in current portion of other long-term (liabilities) assets (103) 222  
Asset (liability), included in other long-term assets $ 204 $ 267