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 ($)	Apr. 13, 2023 CAD ($)	Apr. 13, 2023 USD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 USD ($)	Dec. 14, 2021 CAD ($)	Dec. 14, 2021 USD ($)
Income Tax Examination [Line Items]
Deferred tax recovery, Kansas City Southern outside basis difference				$ 23,000,000	$ 17				$ 19,000,000	$ 15	$ 33,000,000	$ 26
Derecognition of deferred tax liability								$ 7,832,000,000
Deferred revaluation recovery amount due to change in tax rate								13,000,000	12,000,000
Deferred tax liability, outside basis difference	$ 0					$ 7,526,000,000		0	7,526,000,000		7,079,000,000		$ 7,178,000,000	$ 5,607
Income tax operating losses carried forward	52,000,000					22,000,000		52,000,000	22,000,000
Capital losses carried forward	2,000,000					2,000,000		2,000,000	2,000,000
Unrecognized tax benefits from capital losses	4,000,000							4,000,000
Unrecognized tax benefits that would impact effective tax rate	17,000,000							17,000,000
Tax benefits related to prior years							$ 24,000,000	10,000,000	0		0
Deferred tax expense (recovery), reversal of uncertain tax position						24,000,000
Interest and penalties accrual (recovery)								(3,000,000)	5,000,000		4,000,000
Accrued interest and penalties associated with unrecognized tax benefits	15,000,000					$ 18,000,000		15,000,000	18,000,000		13,000,000
Settlement of Mexican tax audits (Note 6)								135,000,000	$ 0		$ 0
Mexican Tax Authority
Income Tax Examination [Line Items]
Settlement of Mexican tax audits (Note 6)			$ 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				0
Mexican Tax Authority | Tax Years 2021 to 2022
Income Tax Examination [Line Items]
Tax adjustments, settlements, and unusual provisions	$ 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