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Pensions and Other Post-Employment Benefits - Summary of Pension and OPEB Costs (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
DB Pension Plan    
Disclosure of defined benefit plans [line items]    
Current Service Costs $ 14 $ 10
Past Service Costs – Curtailments and Plan Amendments 0 0
Net Interest Costs 1 1
Re-measurements:    
Return on Plan Assets Excluding Interest Income (11) (10)
(Gains) Losses From Experience Adjustments 0 4
(Gains) Losses From Changes in Financial Assumptions (3) 13
Defined Benefit Plan Cost (Recovery) 1 18
Defined contribution plan cost 107 99
Total Plan Cost 108 117
OPEB Plans    
Disclosure of defined benefit plans [line items]    
Current Service Costs 2 14
Past Service Costs – Curtailments and Plan Amendments 0 10
Net Interest Costs 12 10
Re-measurements:    
Return on Plan Assets Excluding Interest Income 0 0
(Gains) Losses From Experience Adjustments 1 1
(Gains) Losses From Changes in Financial Assumptions (6) 50
Defined Benefit Plan Cost (Recovery) 9 85
Defined contribution plan cost 0 0
Total Plan Cost $ 9 $ 85