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Employee Future Benefits - Summary of Re-measurement Effects Recognized in Other Comprehensive Income (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Experience $ (3) $ (2)
Demographic assumption changes (7) 67
Economic assumption changes 194 (333)
Return on plan assets (excluding interest income) 70 310
Change in effect of asset limit (37) 5
Total re-measurement effects 217 47
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Experience   14
Demographic assumption changes (1) 12
Economic assumption changes 29 (49)
Return on plan assets (excluding interest income) (1) 33
Total re-measurement effects $ 27 $ 10