v3.25.4
Financial and derivative instruments - Summary of estimated fair value of financial instruments (Detail) - CAD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Derivative Asset [Abstract]    
Derivative assets $ 59 $ 59
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other assets, including intangibles - net, Accounts receivable - net, Materials, supplies and prepaid expenses Other assets, including intangibles - net, Accounts receivable - net, Materials, supplies and prepaid expenses
Derivative Liability [Abstract]    
Derivative liabilities $ 32 $ 82
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Accounts Payable and Accrued Liabilities, Current, Other long-term obligations Accounts Payable and Accrued Liabilities, Current, Other long-term obligations
Effect of counterparty netting    
Derivative Asset [Abstract]    
Derivative assets $ (18) $ (38)
Derivative Liability [Abstract]    
Derivative liabilities (18) (38)
Effect of collateral netting    
Derivative Asset [Abstract]    
Derivative assets (2) 0
Derivative Liability [Abstract]    
Derivative liabilities 0 (14)
Net carrying value    
Derivative Asset [Abstract]    
Derivative assets 39 21
Derivative Liability [Abstract]    
Derivative liabilities 14 30
Level 1    
Derivative Asset [Abstract]    
Derivative assets 20 38
Derivative Liability [Abstract]    
Derivative liabilities 18 52
Level 2    
Derivative Asset [Abstract]    
Derivative assets 39 21
Derivative Liability [Abstract]    
Derivative liabilities 14 30
Level 3    
Derivative Asset [Abstract]    
Derivative assets 0 0
Derivative Liability [Abstract]    
Derivative liabilities $ 0 $ 0