Pension and other benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	$ 25	$ 24	$ 49	$ 48
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	122	123	244	246
Expected return on plan assets	(231)	(222)	(463)	(445)
Recognized net actuarial loss (gain)	1	10	3	20
Amortization of prior service costs	1	1	2	3
Total other components of net periodic benefit (recovery) cost	(107)	(88)	(214)	(176)
Net periodic benefit (recovery) cost	(82)	(64)	(165)	(128)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	21	21	42	42
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	116	117	233	234
Expected return on plan assets	(231)	(222)	(463)	(445)
Recognized net actuarial loss (gain)	2	10	4	20
Amortization of prior service costs	1	1	2	3
Total other components of net periodic benefit (recovery) cost	(112)	(94)	(224)	(188)
Net periodic benefit (recovery) cost	(91)	(73)	(182)	(146)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	4	3	7	6
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	6	6	11	12
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss (gain)	(1)	0	(1)	0
Amortization of prior service costs	0	0	0	0
Total other components of net periodic benefit (recovery) cost	5	6	10	12
Net periodic benefit (recovery) cost	$ 9	$ 9	$ 17	$ 18