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Employee Future Benefits - Summary of Pension and Retiree Welfare Plans (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Pension defined benefit plans [member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance $ 87  
Actuarial losses (gains) due to:    
Experience 5 $ 3
Demographic assumption changes   7
Economic assumption changes (835) (194)
Return on plan assets (excluding interest income) 869 (70)
Ending balance 120 87
Pension defined benefit plans [member] | Present value of defined benefit obligation [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance 4,560 4,901
Current service cost 43 44
Past service cost - amendment (6)  
Interest cost 127 115
Plan participants' contributions 0  
Actuarial losses (gains) due to:    
Experience 5 3
Demographic assumption changes 0 7
Economic assumption changes (835) (194)
Benefits paid (299) (303)
Impact of changes in foreign exchange rates 199 (13)
Ending balance 3,794 4,560
Pension defined benefit plans [member] | Plan assets [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance 4,510 4,595
Interest cost 127 109
Plan participants' contributions 0  
Actuarial losses (gains) due to:    
Benefits paid (299) (303)
Return on plan assets (excluding interest income) (869) 70
Impact of changes in foreign exchange rates 205 (13)
Employer contributions 59 61
Administration costs (11) (9)
Ending balance 3,722 4,510
Retiree welfare plans [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance (3)  
Actuarial losses (gains) due to:    
Experience (13)  
Demographic assumption changes   1
Economic assumption changes (112) (29)
Return on plan assets (excluding interest income) 91 1
Ending balance (57) (3)
Retiree welfare plans [Member] | Present value of defined benefit obligation [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance 584 638
Current service cost 0  
Past service cost - amendment 0  
Interest cost 16 15
Plan participants' contributions 3 3
Actuarial losses (gains) due to:    
Experience (13)  
Demographic assumption changes 0 1
Economic assumption changes (112) (29)
Benefits paid (40) (42)
Impact of changes in foreign exchange rates 28 (2)
Ending balance 466 584
Retiree welfare plans [Member] | Plan assets [Member]    
Disclosure of net defined benefit liability (asset) [Line Items]    
Beginning balance 587 606
Interest cost 16 14
Plan participants' contributions 3 3
Actuarial losses (gains) due to:    
Benefits paid (40) (42)
Return on plan assets (excluding interest income) (91) (1)
Impact of changes in foreign exchange rates 39 (2)
Employer contributions 11 11
Administration costs (2) (2)
Ending balance $ 523 $ 587