Pensions and other benefits - Changes in Fund Assets (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Change in plan assets:
Fair value of plan assets at January 1	$ 12,867	$ 14,943
Actual return on plan assets	1,208	(1,464)
Employer contributions	52	36
Employee contributions	48	42
Benefits paid	(693)	(702)
Foreign currency changes	(4)	12
Fair value of plan assets at December 31	13,478	12,867
Funded status – plan surplus (deficit)	2,709	2,520
Pensions
Change in plan assets:
Fair value of plan assets at January 1	12,862	14,938
Actual return on plan assets	1,207	(1,464)
Employer contributions	15	14
Employee contributions	48	42
Benefits paid	(656)	(680)
Foreign currency changes	(4)	12
Fair value of plan assets at December 31	13,472	12,862
Funded status – plan surplus (deficit)	3,166	2,926
Other benefits
Change in plan assets:
Fair value of plan assets at January 1	5	5
Actual return on plan assets	1	0
Employer contributions	37	22
Employee contributions	0	0
Benefits paid	(37)	(22)
Foreign currency changes	0	0
Fair value of plan assets at December 31	6	5
Funded status – plan surplus (deficit)	$ (457)	$ (406)