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Financial Instruments - Summary of Fair Value Hierarchy for Risk Management Assets and Liabilities Carried at Fair Value (Detail) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Levels Of Fair Value Hierarchy [Line Items]    
Fair Value of Risk Management Assets and Liabilities $ 53  
Level 2 of Fair Value Hierarchy [Member]    
Disclosure Of Levels Of Fair Value Hierarchy [Line Items]    
Fair Value of Risk Management Assets and Liabilities $ (53) $ 3