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Employee Future Benefits - Summary of Benefit Obligations and Fair Value (Narrative) (Details) - CAD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Defined Benefit Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Projected benefit obligation $ 1,940 $ 978
Plan with projected benefit obligation, fair value of plan assets 1,681 790
Accumulated benefit obligation 268 833
Plan with accumulated benefit obligation, fair value of plan assets 130 790
OPEB Plans    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation 320 310
Plan with accumulated benefit obligation, fair value of plan assets $ 36 $ 31