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Financial Instruments - Summary of Urealized Risk Management Positions (Detail) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk Management Net $ 53  
Level 2 of Fair Value Hierarchy [Member]    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk Management Net (53) $ 3
Level 2 of Fair Value Hierarchy [Member] | Commodity Price Risk [Member] | Crude Oil, Natural Gas and Condensate [Member]    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk Management Asset 5 5
Risk Management Liability 58 2
Risk Management Net $ (53) $ 3