Pensions and other benefits - Net Periodic Benefit Cost (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Total other components of net periodic benefit (recovery) cost	$ (387)	$ (342)	$ (381)
Pensions
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	171	140	107
Interest cost on benefit obligation	351	406	450
Expected return on fund assets	(959)	(945)	(947)
Recognized net actuarial loss	206	177	84
Amortization of prior service (recoveries) costs	0	(1)	(1)
Total other components of net periodic benefit (recovery) cost	(402)	(363)	(414)
Net periodic benefit (recovery) cost	(231)	(223)	(307)
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	13	12	11
Interest cost on benefit obligation	16	17	20
Expected return on fund assets	0	0	0
Recognized net actuarial loss	(1)	4	12
Amortization of prior service (recoveries) costs	0	0	1
Total other components of net periodic benefit (recovery) cost	15	21	33
Net periodic benefit (recovery) cost	$ 28	$ 33	$ 44