Business acquisition - Narrative (Details) - CAD ($)				3 Months Ended			12 Months Ended
	Mar. 31, 2024	Apr. 14, 2023	Apr. 13, 2023	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Mar. 31, 2024	Dec. 31, 2023	Dec. 14, 2021
Business Acquisition [Line Items]
Investment in Kansas City Southern			$ 44,402,000,000
Deferred income tax expense (recovery)				$ (17,000,000)		$ (24,000,000)
Goodwill	$ 18,228,000,000			18,228,000,000			$ 18,228,000,000	$ 17,729,000,000
Total revenues				3,520,000,000		2,266,000,000
Net income				775,000,000		800,000,000
Equity earnings of Kansas City Southern				0		204,000,000
Goodwill, expected tax deductible amount	0			0			0
KCS revenue from date of control				0		1,187,000,000
KCS historical, net income (loss) attributable to controlling shareholders				0		246,000,000
KCS
Business Acquisition [Line Items]
Dividends received						300,000,000
Amortization of basis difference						42,000,000
Income (loss) from equity method investments
Business Acquisition [Line Items]
Equity earnings of Kansas City Southern						204,000,000
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
Remeasurement loss of Kansas City Southern					$ 7,175,000,000
Deferred income tax expense (recovery)				(23,000,000)	$ 7,832,000,000
Cash and cash equivalents	298,000,000	298,000,000		298,000,000			298,000,000
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		3,022,000,000			3,022,000,000
Goodwill	17,632,000,000	17,491,000,000		17,632,000,000			17,632,000,000
Acquisition-related costs				26,000,000		15,000,000
Goodwill, purchase accounting adjustments							141,000,000
Net working capital	$ (110,000,000)	51,000,000		(110,000,000)			$ (110,000,000)
KCS actual revenue after deferred tax recovery				61,000,000
KCS historical, net income (loss) attributable to controlling shareholders				84,000,000
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 | Environmental Liabilities
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired		160,000,000
Net working capital		15,000,000
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
KCS | Contracts and Customer Relationships | Maximum
Business Acquisition [Line Items]
Amortization periods of intangible assets acquired		22 years
KCS | Contracts and Customer Relationships | Minimum
Business Acquisition [Line Items]
Amortization periods of intangible assets acquired		9 years
Other expense (income) | KCS
Business Acquisition [Line Items]
Acquisition-related costs						3,000,000
KCS historical, net income (loss) attributable to controlling shareholders				(1,000,000)
Compensation and benefits | KCS
Business Acquisition [Line Items]
Acquisition-related costs				4,000,000
Other tax expense				10,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs				20,000,000
Income (loss) from equity method investments | KCS
Business Acquisition [Line Items]
Acquisition-related costs						10,000,000
KCS historical, net income (loss) attributable to controlling shareholders						42,000,000
Materials | KCS
Business Acquisition [Line Items]
Acquisition-related costs				2,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs						$ 12,000,000
KCS historical, net income (loss) attributable to controlling shareholders				(1,000,000)
Depreciation and amortization | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				(79,000,000)
Net interest expense | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				(5,000,000)
Net loss attributable to non-controlling interest | KCS
Business Acquisition [Line Items]
KCS historical, net income (loss) attributable to controlling shareholders				$ 2,000,000