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Financial Instruments - Summary of Earnings Impact of (Gain) Loss from Risk Management Positions (Detail) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Disclosure of detailed information about financial instruments [line items]        
(Gain) Loss on Risk Management $ 157 $ 3 $ 951 $ 233
Realized (Gain) Loss        
Disclosure of detailed information about financial instruments [line items]        
(Gain) Loss on Risk Management 184 138 725 226
Unrealized (Gain) Loss        
Disclosure of detailed information about financial instruments [line items]        
(Gain) Loss on Risk Management $ (27) $ (135) $ 226 $ 7