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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, 2022
Dec. 31, 2021
Dec. 31, 2020
Pension Benefits      
Disclosure of net defined benefit liability (asset) [line items]      
Current Service Costs $ 16 $ 16 $ 13
Past Service Costs - Curtailment and Plan Amendments 0 (1) 0
Interest Costs (2) (3) (3) (3)
Re-measurements:      
(Gains) Losses From Experience Adjustments 1 4 1
(Gains) Losses From Changes in Demographic Assumptions 0 (1) 0
(Gains) Losses From Changes in Financial Assumptions (64) (18) 15
Return on Plan Assets (Excluding Interest Income) (26) 9 5
Pension and OPEB (Liability) (3) (25) (61)  
Pension Benefits | Defined Benefit Obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined Benefit Obligation, Beginning of Year 220 188  
Plan Acquisition Upon the Arrangement (1) 0 41  
Current Service Costs 16 16  
Interest Costs (2) (7) (6)  
Benefits Paid (12) (17)  
Plan Participant Contributions 2 2  
Re-measurements:      
(Gains) Losses From Experience Adjustments 1 4  
(Gains) Losses From Changes in Demographic Assumptions 0 (1)  
(Gains) Losses From Changes in Financial Assumptions (64) (18)  
Exchange Rate Movements and Other 2 0  
Defined Benefit Obligation, End of Year 172 220 188
Pension Benefits | Plan Assets      
Disclosure of net defined benefit liability (asset) [line items]      
Fair Value of Plan Assets, Beginning of Year 159 117  
Plan Acquisition Upon the Arrangement (1) 0 32  
Interest Costs (2) 4 3  
Employer Contributions 16 9  
Benefits Paid (10) (13)  
Plan Participant Contributions 2 2  
Re-measurements:      
Return on Plan Assets (Excluding Interest Income) (26) 9  
Exchange Rate Movements and Other 2 0  
Fair Value of Plan Assets, End of Year 147 159 117
OPEB      
Disclosure of net defined benefit liability (asset) [line items]      
Current Service Costs 8 9 1
Past Service Costs - Curtailment and Plan Amendments 0 (3) 0
Interest Costs (2) (7) (6) 0
Re-measurements:      
(Gains) Losses From Experience Adjustments (2) 10 (2)
(Gains) Losses From Changes in Demographic Assumptions 0 (3) 0
(Gains) Losses From Changes in Financial Assumptions (57) (30) 1
Return on Plan Assets (Excluding Interest Income) 0 0 0
Pension and OPEB (Liability) (3) (174) (225)  
OPEB | Defined Benefit Obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined Benefit Obligation, Beginning of Year 225 20  
Plan Acquisition Upon the Arrangement (1) 0 224  
Current Service Costs 8 9  
Interest Costs (2) (7) (6)  
Benefits Paid (8) (8)  
Plan Participant Contributions 0 0  
Re-measurements:      
(Gains) Losses From Experience Adjustments (2) 10  
(Gains) Losses From Changes in Demographic Assumptions 0 (3)  
(Gains) Losses From Changes in Financial Assumptions (57) (30)  
Exchange Rate Movements and Other 1 0  
Defined Benefit Obligation, End of Year 174 225 20
OPEB | Plan Assets      
Disclosure of net defined benefit liability (asset) [line items]      
Fair Value of Plan Assets, Beginning of Year 0 0  
Plan Acquisition Upon the Arrangement (1) 0 0  
Interest Costs (2) 0 0  
Employer Contributions 8 3  
Benefits Paid (8) (3)  
Plan Participant Contributions 0 0  
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 $ 0