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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
Sep. 30, 2021
Dec. 31, 2020
Disclosure Of Levels Of Fair Value Hierarchy [Line Items]    
Level 2 – Prices Sourced From Observable Data or Market Corroboration $ (292) $ (53)
Level 2 of fair value hierarchy    
Disclosure Of Levels Of Fair Value Hierarchy [Line Items]    
Level 2 – Prices Sourced From Observable Data or Market Corroboration $ (292) $ (53)