Business acquisition - Narrative (Details)				3 Months Ended					9 Months Ended
	Apr. 13, 2024 CAD ($) renewal_period	Apr. 14, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Sep. 30, 2024 CAD ($)	Mar. 31, 2024 CAD ($)	Sep. 30, 2023 CAD ($)	Jun. 30, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Sep. 30, 2024 CAD ($)	Sep. 30, 2023 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 14, 2021
Business Acquisition [Line Items]
Investment in Kansas City Southern			$ 44,402,000,000					$ 44,402,000,000
Remeasurement loss of Kansas City Southern				$ 0		$ 0			$ 0	$ 7,175,000,000
Deferred income tax recovery				(5,000,000)		(3,000,000)			(40,000,000)	7,925,000,000
Goodwill				18,160,000,000					18,160,000,000		$ 17,729,000,000
Equity earnings of Kansas City Southern				0		0		$ 230,000,000	0	230,000,000
KCS
Business Acquisition [Line Items]
Percentage of voting interest acquired												100.00%
Fair value of previously held equity method investment	$ 37,227,000,000	$ 37,227,000,000	$ 37,227,000,000
Remeasurement loss of Kansas City Southern							$ 7,175,000,000			7,175,000,000
Deferred income tax recovery							$ 7,832,000,000			7,832,000,000
Accounts receivables, net	697,000,000
Accounts payable and accrued liabilities	1,014,000,000
Intangible assets	3,022,000,000	3,022,000,000
Net working capital	(110,000,000)	51,000,000
Goodwill	17,632,000,000	$ 17,491,000,000
Goodwill, expected tax deductible amount	0
Acquisition-related costs				36,000,000		24,000,000			90,000,000	158,000,000
KCS historical, net income (loss) attributable to controlling shareholders				(89,000,000)		87,000,000			(259,000,000)	210,000,000
KCS earnings (loss), after deferred tax recovery				(65,000,000)		63,000,000			(188,000,000)	166,000,000
KCS deferred tax recovery				24,000,000		24,000,000			71,000,000	44,000,000
KCS | Environmental Liabilities
Business Acquisition [Line Items]
Net working capital	15,000,000
Liabilities arising from contingencies acquired	160,000,000
KCS | Litigation and Personal Injury Claims
Business Acquisition [Line Items]
Net working capital	44,000,000
Liabilities arising from contingencies acquired	40,000,000
KCS | Income (loss) from equity method investments
Business Acquisition [Line Items]
Equity earnings of Kansas City Southern										230,000,000
KCS | Concession Rights
Business Acquisition [Line Items]
Renewal term of concession rights and related assets			50 years
Intangible assets	$ 9,176,000,000
Amortization periods of intangible assets acquired	74 years
Number of renewal periods | renewal_period	1
KCS | Contracts and Customer Relationships | Minimum
Business Acquisition [Line Items]
Amortization periods of intangible assets acquired	9 years
KCS | Contracts and Customer Relationships | Maximum
Business Acquisition [Line Items]
Amortization periods of intangible assets acquired	22 years
Income (loss) from equity method investments | KCS
Business Acquisition [Line Items]
Acquisition-related costs						0				11,000,000
KCS historical, net income (loss) attributable to controlling shareholders						0				48,000,000
Compensation and benefits | KCS
Business Acquisition [Line Items]
Other tax expense (recovery)				(7,000,000)	$ 10,000,000
Acquisition-related costs				11,000,000		1,000,000			17,000,000	64,000,000
Materials | KCS
Business Acquisition [Line Items]
Acquisition-related costs				1,000,000		1,000,000			5,000,000	1,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs				24,000,000					68,000,000
Other expense (income) | KCS
Business Acquisition [Line Items]
Acquisition-related costs						0				6,000,000
KCS historical, net income (loss) attributable to controlling shareholders				(1,000,000)		1,000,000			(2,000,000)	2,000,000
Depreciation and amortization | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				(85,000,000)		81,000,000			(246,000,000)	149,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs						22,000,000				87,000,000
KCS historical, net income (loss) attributable to controlling shareholders				0					(2,000,000)
Net interest expense | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				(4,000,000)		$ 5,000,000			(14,000,000)	$ 11,000,000
Net loss attributable to non-controlling interest | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				$ 1,000,000					$ 5,000,000