Pensions and other benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Details) - CAD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	$ (1,449)	$ (1,843)
Deferred investment gains (losses)	405	(191)
Prior service cost	(59)	(48)
Deferred income tax	365	619
Total (Note 8)	(738)	(1,463)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	(1,501)	(1,871)
Deferred investment gains (losses)	405	(191)
Prior service cost	(58)	(47)
Deferred income tax	377	626
Total (Note 8)	(777)	(1,483)
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	52	28
Deferred investment gains (losses)	0	0
Prior service cost	(1)	(1)
Deferred income tax	(12)	(7)
Total (Note 8)	$ 39	$ 20