Pensions and other benefits - Changes in Projected Benefit Obligation (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in projected benefit obligation:
Projected benefit obligation at January 1	$ 10,347	$ 13,387
Current service cost	81	159	$ 184
Interest cost	508	399	367
Employee contributions	48	42
Benefits paid	(693)	(702)
Foreign currency changes	2	16
Addition of KCS plans	31	0
Plan amendments and other	17	27
Net actuarial loss (gain)	428	(2,981)
Projected benefit obligation at December 31	10,769	10,347	13,387
Pensions
Change in projected benefit obligation:
Projected benefit obligation at January 1	9,936	12,884
Current service cost	71	148	171
Interest cost	486	383	351
Employee contributions	48	42
Benefits paid	(656)	(680)
Foreign currency changes	(4)	16
Addition of KCS plans	0	0
Plan amendments and other	18	27
Net actuarial loss (gain)	407	(2,884)
Projected benefit obligation at December 31	10,306	9,936	12,884
Other benefits
Change in projected benefit obligation:
Projected benefit obligation at January 1	411	503
Current service cost	10	11	13
Interest cost	22	16	16
Employee contributions	0	0
Benefits paid	(37)	(22)
Foreign currency changes	6	0
Addition of KCS plans	31	0
Plan amendments and other	(1)	0
Net actuarial loss (gain)	21	(97)
Projected benefit obligation at December 31	$ 463	$ 411	$ 503