Pensions and other benefits - Changes in Projected Benefit Obligation (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pensions
Change in projected benefit obligation:
Projected benefit obligation at January 1	$ 12,884	$ 13,799
Current service cost	148	171	$ 140
Interest cost	383	351	406
Employee contributions	42	42
Benefits paid	(680)	(667)
Foreign currency changes	16	0
Plan amendments and other	27	0
Actuarial gain	(2,884)	(812)
Projected benefit obligation at December 31	9,936	12,884	13,799
Other benefits
Change in projected benefit obligation:
Projected benefit obligation at January 1	503	553
Current service cost	11	13	12
Interest cost	16	16	17
Employee contributions	0	0
Benefits paid	(22)	(31)
Foreign currency changes	0	0
Plan amendments and other	0	0
Actuarial gain	(97)	(48)
Projected benefit obligation at December 31	$ 411	$ 503	$ 553