Pension and other benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	$ 24	$ 24
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	122	123
Expected return on plan assets	(232)	(223)
Recognized net actuarial loss	2	10
Amortization of prior service costs	1	2
Total other components of net periodic benefit (recovery) cost	(107)	(88)
Net periodic benefit (recovery) cost	(83)	(64)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	21	21
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	117	117
Expected return on plan assets	(232)	(223)
Recognized net actuarial loss	2	10
Amortization of prior service costs	1	2
Total other components of net periodic benefit (recovery) cost	(112)	(94)
Net periodic benefit (recovery) cost	(91)	(73)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	3	3
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	5	6
Expected return on plan assets	0	0
Recognized net actuarial loss	0	0
Amortization of prior service costs	0	0
Total other components of net periodic benefit (recovery) cost	5	6
Net periodic benefit (recovery) cost	$ 8	$ 9