Employee retirement benefits - Summary of Change in Accumulated Other Comprehensive Income (Detail) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
(Charge) credit to other comprehensive income, before-tax	$ (218)	$ 880	$ 1,076
Deferred income tax (charge) credit (note 17)	53	(215)	(264)
(Charge) credit to other comprehensive income, after-tax	$ (165)	$ 665	$ 812

Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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