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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Re-measurements:      
Defined Benefit Plan Cost (Recovery) $ 33 $ 29 $ 30
Pension Benefits [Member]      
Disclosure Of Defined Benefit Plans [Line Items]      
Current Service Costs 13 11 13
Past Service Costs – Curtailments     (2)
Net Interest Costs 3 3 3
Re-measurements:      
Return on Plan Assets (Excluding Interest Income) (5) (15) 7
(Gains) Losses From Experience Adjustments 1 (4)  
(Gains) Losses From Changes in Financial Assumptions 15 12  
Defined Benefit Plan Cost (Recovery) 27 7 21
Defined Contribution Plan Cost 22 21 22
Total Plan Cost 49 28 43
OPEB [Member]      
Disclosure Of Defined Benefit Plans [Line Items]      
Current Service Costs 1 1 1
Net Interest Costs   1 1
Re-measurements:      
(Gains) Losses From Experience Adjustments (2)    
(Gains) Losses From Changes in Financial Assumptions $ 1 1 (1)
Defined Benefit Plan Cost (Recovery)   3 1
Total Plan Cost   $ 3 $ 1