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Financial Instruments - Summary of Unrealized Risk Management Positions (Detail) - CAD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management net asset (liability) $ 64 $ 7
Power Swap Contracts    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management net asset (liability) 3  
Level 2 of fair value hierarchy    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management asset 73 20
Risk management liability 9 13
Risk management net asset (liability) 64 7
Level 2 of fair value hierarchy | Commodity price risk | Crude Oil, Natural Gas, Condensate and Refined Products    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management asset 9 9
Risk management liability 1 10
Risk management net asset (liability) 8 (1)
Level 2 of fair value hierarchy | Commodity price risk | Power Swap Contracts    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management asset 3 6
Risk management liability 0 0
Risk management net asset (liability) 3 6
Level 2 of fair value hierarchy | Commodity price risk | Renewable Power Contracts    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management asset 61 5
Risk management liability 4 0
Risk management net asset (liability) 57 5
Level 2 of fair value hierarchy | Currency risk | Foreign Exchange Rate Contracts    
Disclosure Of Recurring Fair Value Measurement Of Assets And Liabilities [Line Items]    
Risk management asset 0 0
Risk management liability 4 3
Risk management net asset (liability) $ (4) $ (3)