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Other comprehensive income (loss) information - Amounts Reclassified Out of Accumulated Other Comprehensive Income (Loss) (Detail) - CAD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Accumulated Defined Benefit Plans Adjustment Attributable to Parent        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Amortization of postretirement benefits liability adjustment included in net benefit cost $ (7) $ (16) $ (20) $ (50)