Pensions and other benefits - Net Periodic Benefit Cost (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Current service cost	$ 97	$ 81	$ 159
Interest cost on benefit obligation	491	508	399
Expected return on plan assets	(891)	(882)	(959)
Recognized net actuarial loss (gain)	41	45	148
Amortization of prior service costs	7	2	1
Total other components of net periodic benefit (recovery) cost	(352)	(327)	(411)
Net periodic benefit (recovery) cost	(255)	(246)	(252)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Current service cost	84	71	148
Interest cost on benefit obligation	468	486	383
Expected return on plan assets	(891)	(882)	(959)
Recognized net actuarial loss (gain)	40	32	153
Amortization of prior service costs	7	2	1
Total other components of net periodic benefit (recovery) cost	(376)	(362)	(422)
Net periodic benefit (recovery) cost	(292)	(291)	(274)
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Current service cost	13	10	11
Interest cost on benefit obligation	23	22	16
Expected return on plan assets	0	0	0
Recognized net actuarial loss (gain)	1	13	(5)
Amortization of prior service costs	0	0	0
Total other components of net periodic benefit (recovery) cost	24	35	11
Net periodic benefit (recovery) cost	$ 37	$ 45	$ 22