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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Other components of net periodic benefit (recovery) cost:
Total other components of net periodic benefit (recovery) cost	$ (83)	$ (101)	$ (169)	$ (202)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	17	37	35	74
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	122	96	243	192
Expected return on plan assets	(221)	(239)	(441)	(479)
Recognized net actuarial loss	8	38	16	76
Amortization of prior service costs	1	0	1	0
Total other components of net periodic benefit (recovery) cost	(90)	(105)	(181)	(211)
Net periodic benefit (recovery) cost	(73)	(68)	(146)	(137)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	3	3	5	5
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	6	4	11	8
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss	0	0	0	1
Amortization of prior service costs	1	0	1	0
Total other components of net periodic benefit (recovery) cost	7	4	12	9
Net periodic benefit (recovery) cost	$ 10	$ 7	$ 17	$ 14