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Pensions and Other Post-Employment Benefits - Summary of Defined Benefit and OPEB Plan Obligation and Funded Status (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
DB Pension Plan    
Disclosure of net defined benefit liability (asset) [line items]    
Current Service Costs $ 14 $ 10
Past Service Costs - Curtailment and Plan Amendments 0 0
Interest Income 1 1
Re-measurements:    
(Gains) Losses From Experience Adjustments 0 4
(Gains) Losses From Changes in Financial Assumptions (3) 13
Return on Plan Assets Excluding Interest Income 11 10
Defined Benefit Pension and OPEB Asset (Liability) (2) (13) (24)
DB Pension Plan | Defined Benefit Obligation    
Disclosure of net defined benefit liability (asset) [line items]    
Defined Benefit Obligation, Beginning of Year 202 172
Current Service Costs 14 10
Interest Income 9 9
Benefits Paid (12) (8)
Plan Participant Contributions 3 3
Re-measurements:    
(Gains) Losses From Experience Adjustments 0 4
(Gains) Losses From Changes in Financial Assumptions (3) 13
Exchange Rate Movements and Other 1 (1)
Defined Benefit Obligation, End of Year 214 202
DB Pension Plan | Plan Assets    
Disclosure of net defined benefit liability (asset) [line items]    
Fair Value of Plan Assets, Beginning of Year 178 147
Interest Income 8 8
Benefits Paid (12) (7)
Plan Participant Contributions 3 3
Employer Contributions 11 18
Re-measurements:    
Return on Plan Assets Excluding Interest Income 11 10
Exchange Rate Movements and Other 2 (1)
Fair Value of Plan Assets, End of Year 201 178
OPEB Plans    
Disclosure of net defined benefit liability (asset) [line items]    
Current Service Costs 2 14
Past Service Costs - Curtailment and Plan Amendments 0 10
Interest Income 12 10
Re-measurements:    
(Gains) Losses From Experience Adjustments 1 1
(Gains) Losses From Changes in Financial Assumptions (6) 50
Return on Plan Assets Excluding Interest Income 0 0
Defined Benefit Pension and OPEB Asset (Liability) (2) (252) (249)
OPEB Plans | Defined Benefit Obligation    
Disclosure of net defined benefit liability (asset) [line items]    
Defined Benefit Obligation, Beginning of Year 249 174
Current Service Costs 2 14
Interest Income 12 10
Benefits Paid (9) (9)
Plan Participant Contributions 0 0
Re-measurements:    
(Gains) Losses From Experience Adjustments 1 1
(Gains) Losses From Changes in Financial Assumptions (6) 50
Exchange Rate Movements and Other 3 (1)
Defined Benefit Obligation, End of Year 252 249
OPEB Plans | Plan Assets    
Disclosure of net defined benefit liability (asset) [line items]    
Fair Value of Plan Assets, Beginning of Year 0 0
Interest Income 0 0
Benefits Paid (9) (9)
Plan Participant Contributions 0 0
Employer Contributions 9 9
Re-measurements:    
Return on Plan Assets Excluding Interest Income 0 0
Exchange Rate Movements and Other 0 0
Fair Value of Plan Assets, End of Year $ 0 $ 0