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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	$ 24	$ 20	$ 48	$ 40
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	123	128	246	254
Expected return on plan assets	(222)	(221)	(445)	(441)
Recognized net actuarial loss	10	8	20	16
Amortization of prior service costs	1	2	3	2
Total other components of net periodic benefit (recovery) cost	(88)	(83)	(176)	(169)
Net periodic benefit (recovery) cost	(64)	(63)	(128)	(129)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	21	17	42	35
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	117	122	234	243
Expected return on plan assets	(222)	(221)	(445)	(441)
Recognized net actuarial loss	10	8	20	16
Amortization of prior service costs	1	1	3	1
Total other components of net periodic benefit (recovery) cost	(94)	(90)	(188)	(181)
Net periodic benefit (recovery) cost	(73)	(73)	(146)	(146)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost	3	3	6	5
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	6	6	12	11
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss	0	0	0	0
Amortization of prior service costs	0	1	0	1
Total other components of net periodic benefit (recovery) cost	6	7	12	12
Net periodic benefit (recovery) cost	$ 9	$ 10	$ 18	$ 17