Pensions and other benefits - Changes in Projected Benefit Obligation (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in projected benefit obligation:
Projected benefit obligation as at January 1	$ 10,769	$ 10,347
Current service cost	97	81	$ 159
Interest cost	491	508	399
Employee contributions	50	48
Benefits paid	(695)	(693)
Foreign currency changes	14	2
Addition of KCS plans	0	31
Plan amendments and other	18	17
Net actuarial (gain) loss	(139)	428
Projected benefit obligation as at December 31	10,605	10,769	10,347
Pensions
Change in projected benefit obligation:
Projected benefit obligation as at January 1	10,306	9,936
Current service cost	84	71	148
Interest cost	468	486	383
Employee contributions	50	48
Benefits paid	(659)	(656)
Foreign currency changes	15	(4)
Addition of KCS plans	0	0
Plan amendments and other	18	18
Net actuarial (gain) loss	(116)	407
Projected benefit obligation as at December 31	10,166	10,306	9,936
Other benefits
Change in projected benefit obligation:
Projected benefit obligation as at January 1	463	411
Current service cost	13	10	11
Interest cost	23	22	16
Employee contributions	0	0
Benefits paid	(36)	(37)
Foreign currency changes	(1)	6
Addition of KCS plans	0	31
Plan amendments and other	0	(1)
Net actuarial (gain) loss	(23)	21
Projected benefit obligation as at December 31	$ 439	$ 463	$ 411