Income taxes - Reconciliation of Uncertain Tax Positions Related to Unrecognized Tax Benefits (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation of Uncertain Tax Positions to Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits at January 1		$ 20	$ 49	$ 55
Tax benefits related to the current year		2	1	0
Tax benefits related to prior years	$ 24	10	0	0
Tax benefits acquired with KCS		2	0	0
Gross uncertain tax benefits related to prior years		(6)	(30)	(6)
Settlements with taxing authorities		(6)	0	0
Unrecognized tax benefits at December 31		$ 22	$ 20	$ 49