Pensions and other benefits - Net Periodic Benefit Cost (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Current service cost	$ 81	$ 159	$ 184
Interest cost on benefit obligation	508	399	367
Expected return on plan assets	(882)	(959)	(959)
Recognized net actuarial loss (gain)	45	148	205
Amortization of prior service costs	2	1	0
Total other components of net periodic benefit (recovery) cost	(327)	(411)	(387)
Net periodic benefit (recovery) cost	(246)	(252)	(203)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Current service cost	71	148	171
Interest cost on benefit obligation	486	383	351
Expected return on plan assets	(882)	(959)	(959)
Recognized net actuarial loss (gain)	32	153	206
Amortization of prior service costs	2	1	0
Total other components of net periodic benefit (recovery) cost	(362)	(422)	(402)
Net periodic benefit (recovery) cost	(291)	(274)	(231)
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Current service cost	10	11	13
Interest cost on benefit obligation	22	16	16
Expected return on plan assets	0	0	0
Recognized net actuarial loss (gain)	13	(5)	(1)
Amortization of prior service costs	0	0	0
Total other components of net periodic benefit (recovery) cost	35	11	15
Net periodic benefit (recovery) cost	$ 45	$ 22	$ 28