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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, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension Benefits      
Disclosure of net defined benefit liability (asset) [line items]      
Current Service Costs $ 16 $ 13 $ 11
Past Service Costs - Curtailment and Plan Amendments (1) 0 0
Net Interest Costs 3 3 3
Re-measurements:      
(Gains) Losses From Experience Adjustments 4 1 (4)
(Gains) Losses From Changes in Demographic Assumptions (1) 0 0
(Gains) Losses From Changes in Financial Assumptions (18) 15 12
Return on Plan Assets (Excluding Interest Income) 9 5 $ 15
Pension and OPEB (Liability) $ (61) $ (71)  
Actuarial assumption of discount rates 2.95% 2.50% 3.00%
Pension Benefits | Defined Benefit Obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined Benefit Obligation, Beginning of Year $ 188 $ 158  
Plan Acquisition Upon the Arrangement (1) 41 0  
Current Service Costs 16 13  
Net Interest Costs 6 5  
Benefits Paid (17) (6)  
Plan Participant Contributions 2 2  
Re-measurements:      
(Gains) Losses From Experience Adjustments 4 1  
(Gains) Losses From Changes in Demographic Assumptions (1) 0  
(Gains) Losses From Changes in Financial Assumptions (18) 15  
Defined Benefit Obligation, End of Year 220 188 $ 158
Pension Benefits | Plan Assets      
Disclosure of net defined benefit liability (asset) [line items]      
Fair Value of Plan Assets, Beginning of Year 117 107  
Plan Acquisition Upon the Arrangement (1) 32 0  
Net Interest Costs (3) (2)  
Employer Contributions 9 6  
Benefits Paid (13) (5)  
Plan Participant Contributions 2 2  
Re-measurements:      
Return on Plan Assets (Excluding Interest Income) 9 5  
Fair Value of Plan Assets, End of Year 159 117 107
OPEB      
Disclosure of net defined benefit liability (asset) [line items]      
Current Service Costs 9 1 1
Past Service Costs - Curtailment and Plan Amendments (3) 0 0
Net Interest Costs 6 0 1
Re-measurements:      
(Gains) Losses From Experience Adjustments 10 (2) 0
(Gains) Losses From Changes in Demographic Assumptions (3) 0 0
(Gains) Losses From Changes in Financial Assumptions (30) 1 1
Return on Plan Assets (Excluding Interest Income) 0 0 $ 0
Pension and OPEB (Liability) $ (225) $ (20)  
Actuarial assumption of discount rates 2.98% 2.50% 3.00%
OPEB | Defined Benefit Obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Defined Benefit Obligation, Beginning of Year $ 20 $ 22  
Plan Acquisition Upon the Arrangement (1) 224 0  
Current Service Costs 9 1  
Net Interest Costs 6 0  
Benefits Paid (8) (2)  
Plan Participant Contributions 0 0  
Re-measurements:      
(Gains) Losses From Experience Adjustments 10 (2)  
(Gains) Losses From Changes in Demographic Assumptions (3) 0  
(Gains) Losses From Changes in Financial Assumptions (30) 1  
Defined Benefit Obligation, End of Year 225 20 $ 22
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  
Net Interest Costs 0 0  
Employer Contributions 3 0  
Benefits Paid (3) 0  
Plan Participant Contributions 0 0  
Re-measurements:      
Return on Plan Assets (Excluding Interest Income) 0 0  
Fair Value of Plan Assets, End of Year $ 0 $ 0 $ 0