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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Reconciliation Of Changes In Derivative Financial Assets (Liabilities), Net [Roll Forward]      
Derivative financial asset (liability), net $ 356 $ 101  
Net change in fair value of outstanding derivatives financial instruments recognized in: Risk management activities (13) 35 $ (37)
Net change in fair value of outstanding derivatives financial instruments recognized in: Foreign exchange (231) 260  
Net change in fair value of outstanding derivatives financial instruments recognized in: Other comprehensive income (loss) 66 (40)  
Derivative financial asset (liability), net 178 356 $ 101
Asset (liability), included in current portion of other long-term (liabilities) assets (104) 75  
Asset (liability), included in other long-term assets $ 282 $ 281