Pensions and other benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Details) - CAD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	$ (1,843)	$ (1,676)
Deferred investment (losses) gains	(191)	(301)
Prior service cost	(48)	(32)
Deferred income tax	619	599
Total (Note 8)	(1,463)	(1,410)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	(1,871)	(1,711)
Deferred investment (losses) gains	(191)	(301)
Prior service cost	(47)	(31)
Deferred income tax	626	608
Total (Note 8)	(1,483)	(1,435)
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment (losses) gains	28	35
Deferred investment (losses) gains	0	0
Prior service cost	(1)	(1)
Deferred income tax	(7)	(9)
Total (Note 8)	$ 20	$ 25