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Employee Future Benefits - Summary of Cash Payments (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans $ 151 $ 155
Retiree welfare plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 11 11
Defined benefit plans [Member] | Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 61 71
Defined benefit plans [Member] | Retiree welfare plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans 11 11
Defined contribution plans [Member] | Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Cash payment for future benefit plans $ 90 $ 84