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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	$ 20	$ 40	$ 60	$ 119
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	127	99	381	299
Expected return on plan assets	(220)	(240)	(661)	(719)
Recognized net actuarial loss	8	39	24	116
Amortization of prior service costs			2	0
Total other components of net periodic benefit (recovery) cost	(85)	(102)	(254)	(304)
Net periodic benefit (recovery) cost	(65)	(62)	(194)	(185)
Pensions
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	18	37	53	111
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	122	95	365	287
Expected return on plan assets	(220)	(240)	(661)	(719)
Recognized net actuarial loss	8	39	24	115
Amortization of prior service costs			1	0
Total other components of net periodic benefit (recovery) cost	(90)	(106)	(271)	(317)
Net periodic benefit (recovery) cost	(72)	(69)	(218)	(206)
Other benefits
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	2	3	7	8
Other components of net periodic benefit (recovery) cost:
Interest cost on benefit obligation	5	4	16	12
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss	0	0	0	1
Amortization of prior service costs			1	0
Total other components of net periodic benefit (recovery) cost	5	4	17	13
Net periodic benefit (recovery) cost	$ 7	$ 7	$ 24	$ 21