Pension and other benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	$ 25	$ 24	$ 74	$ 72
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	122	122	366	368
Expected return on plan assets	(232)	(223)	(695)	(668)
Recognized net actuarial loss	1	10	4	30
Amortization of prior service costs	2	2	4	5
Total other components of net periodic benefit (recovery) cost	(107)	(89)	(321)	(265)
Net periodic benefit (recovery) cost	(82)	(65)	(247)	(193)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	22	21	64	63
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	117	116	350	350
Expected return on plan assets	(232)	(223)	(695)	(668)
Recognized net actuarial loss	1	10	5	30
Amortization of prior service costs	2	2	4	5
Total other components of net periodic benefit (recovery) cost	(112)	(95)	(336)	(283)
Net periodic benefit (recovery) cost	(90)	(74)	(272)	(220)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	3	3	10	9
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	5	6	16	18
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss	0	0	(1)	0
Amortization of prior service costs	0	0	0	0
Total other components of net periodic benefit (recovery) cost	5	6	15	18
Net periodic benefit (recovery) cost	$ 8	$ 9	$ 25	$ 27