Income taxes (Details) - CAD ($)			1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Apr. 13, 2023	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Business Acquisition [Line Items]
Effective tax rate				24.88%	18.01%	166.83%	18.97%
Effective tax rate, excluding discrete items				24.96%	24.25%	24.91%	24.25%
Acquisition-related costs				$ 0		$ 12,000,000
Deferred tax expense (recovery)				3,000,000	$ 38,000,000	(7,925,000,000)	$ 151,000,000
Remeasurement loss of Kansas City Southern				0	0	7,175,000,000	0
Re-evaluation of deferred income tax						51,000,000
Equity earnings of Kansas City Southern				0	221,000,000	230,000,000	627,000,000
Outside basis deferred tax recovery					9,000,000	23,000,000	8,000,000
Settlement of Mexican tax audits (Note 4)				75,000,000	0	75,000,000	0
Value added tax receivable	$ 10,000,000	$ 55,000,000	$ 10,000,000	10,000,000		10,000,000
Mexican Tax Authority
Business Acquisition [Line Items]
Reserve for potential future audits				15,000,000		15,000,000
Settlement of Mexican tax audits (Note 4)			75,000,000
Increase (decrease) in value added tax receivable	(9,000,000)		(16,000,000)			(25,000,000)
Mexican Tax Authority | Tax Years 2016 to 2020
Business Acquisition [Line Items]
Tax adjustments, settlements			$ 9,000,000
Mexican Tax Authority | Tax Years 2021 to 2022
Business Acquisition [Line Items]
Tax adjustments, settlements	$ 6,000,000
Iowa & Arkansas Tax Authorities
Business Acquisition [Line Items]
Deferred tax expense (recovery)				14,000,000
Iowa Tax Authority
Business Acquisition [Line Items]
Deferred tax expense (recovery)					12,000,000		12,000,000
KCS
Business Acquisition [Line Items]
Acquisition-related costs				24,000,000	$ 18,000,000	158,000,000	$ 57,000,000
Amortization of fair value adjustments				$ 87,000,000		$ 162,000,000
Deferred tax expense (recovery)		(7,832,000,000)
Remeasurement loss of Kansas City Southern		$ 7,175,000,000