Income taxes - Narrative (Details) $ in Millions			1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2023 CAD ($)	Sep. 30, 2023 CAD ($)	Sep. 30, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2024 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 USD ($)	Apr. 13, 2023 CAD ($)
Income Tax Examination [Line Items]
Deferred revaluation recovery amount due to change in tax rate					$ 81,000,000	$ 13,000,000
Deferred tax recovery, Kansas City Southern outside basis difference						23,000,000	$ 17	$ 19,000,000	$ 15
Derecognition of deferred tax liability						7,832,000,000
Income tax operating losses carried forward	$ 52,000,000				33,000,000	52,000,000
Capital losses carried forward	2,000,000				18,000,000	2,000,000
Unrecognized tax benefits from capital losses	4,000,000				6,000,000	4,000,000
Unrecognized tax benefits that would impact effective tax rate					24,000,000
Deferred tax expense (recovery), reversal of uncertain tax position				$ (24,000,000)
Interest and penalties accrual (recovery)					(4,000,000)	(3,000,000)		5,000,000
Accrued interest and penalties associated with unrecognized tax benefits	15,000,000			$ 18,000,000	11,000,000	15,000,000		18,000,000
Settlement of Mexican taxes					12,000,000	135,000,000		$ 0
Income taxes payable	255,000,000				379,000,000	$ 255,000,000
Mexican Tax Authority
Income Tax Examination [Line Items]
Settlement of Mexican taxes			$ 135,000,000
Increase (decrease) In value added tax receivable	$ (9,000,000)		(16,000,000)		(25,000,000)
Income tax expense, reserve for future audit settlements					3,000,000
Refundable value added tax					0					$ 55,000,000
Mexican Tax Authority | Tax Years 2021 to 2022
Income Tax Examination [Line Items]
Income taxes payable					3,000,000
Mexican Tax Authority | Tax Years 2016 to 2020
Income Tax Examination [Line Items]
Tax adjustments, settlements, and unusual provisions			$ 13,000,000
Mexican Tax Authority | Tax Years Through 2020 Excluding 2014
Income Tax Examination [Line Items]
Goodwill, period increase (decrease)					$ 96,000,000
Mexican Tax Authority | Tax Years 2009, 2010, 2013, 2015 to 2022 for Corporate Income Tax and VAT
Income Tax Examination [Line Items]
Goodwill, period increase (decrease)		$ 90,000,000