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Financial Instruments - Summary of Earnings Impact of (Gain) Loss from Risk Management Positions (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]      
(Gain) Loss on Risk Management $ 995 $ 308 [1] $ 156 [1]
Realized (Gain) Loss      
Disclosure of detailed information about financial instruments [line items]      
(Gain) Loss on Risk Management 993 252 7
Unrealized (Gain) Loss      
Disclosure of detailed information about financial instruments [line items]      
(Gain) Loss on Risk Management $ 2 $ 56 $ 149
[1] See Note 3(w) for revisions to comparative results.