Business acquisition - Narrative (Details)					3 Months Ended				6 Months Ended
	Apr. 13, 2024 CAD ($) renewal_period	Apr. 14, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Jun. 30, 2024 CAD ($)	Mar. 31, 2024 CAD ($)	Jun. 30, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Jun. 30, 2024 CAD ($)	Jun. 30, 2023 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 14, 2021
Business Acquisition [Line Items]
Investment in Kansas City Southern			$ 44,402,000,000	$ 44,402,000,000				$ 44,402,000,000
Remeasurement loss of Kansas City Southern					$ 0		$ 7,175,000,000		$ 0	$ 7,175,000,000
Deferred income tax recovery					(18,000,000)		7,953,000,000		(35,000,000)	7,928,000,000
Goodwill					18,411,000,000				18,411,000,000		$ 17,729,000,000
Equity earnings of Kansas City Southern			$ 26,000,000		0		26,000,000	$ (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					28,000,000		119,000,000		54,000,000	134,000,000
KCS historical, net income (loss) attributable to controlling shareholders					(86,000,000)		(81,000,000)		(170,000,000)	(123,000,000)
KCS earnings (loss), after deferred tax recovery					(62,000,000)		(61,000,000)		(123,000,000)	(103,000,000)
KCS deferred tax recovery					24,000,000		20,000,000		47,000,000	20,000,000
KCS | Environmental Liabilities
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired	160,000,000
Net working capital	15,000,000
KCS | Litigation and Personal Injury Claims
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired	40,000,000
Net working capital	44,000,000
KCS | Income (loss) from equity method investments
Business Acquisition [Line Items]
Equity earnings of Kansas City Southern							26,000,000			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							1,000,000			11,000,000
KCS historical, net income (loss) attributable to controlling shareholders							6,000,000			48,000,000
Compensation and benefits | KCS
Business Acquisition [Line Items]
Other tax expense						$ 10,000,000
Acquisition-related costs					2,000,000		63,000,000		6,000,000	63,000,000
Materials | KCS
Business Acquisition [Line Items]
Acquisition-related costs					2,000,000				4,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs					24,000,000		53,000,000		44,000,000	65,000,000
Other expense (income) | KCS
Business Acquisition [Line Items]
Acquisition-related costs							3,000,000			6,000,000
KCS historical, net income (loss) attributable to controlling shareholders					0		(1,000,000)		(1,000,000)	(1,000,000)
Depreciation and amortization | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders					(82,000,000)		(68,000,000)		(161,000,000)	(68,000,000)
Purchased services and other | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders					(1,000,000)				(2,000,000)
Net interest expense | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders					(5,000,000)		$ (6,000,000)		(10,000,000)	$ (6,000,000)
Net loss attributable to non-controlling interest | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders					$ 2,000,000				$ 4,000,000