Business acquisition - Narrative (Details) mi in Thousands						3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2023 CAD ($)	Jun. 30, 2023 CAD ($)	Apr. 14, 2023 CAD ($) mi	Apr. 13, 2023 CAD ($)	Dec. 14, 2021 mi	Sep. 30, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Sep. 30, 2022 CAD ($)	Sep. 30, 2023 CAD ($)	Sep. 30, 2023 CAD ($)	Sep. 30, 2022 CAD ($)	Dec. 31, 2022 CAD ($)
Business Acquisition [Line Items]
Miles of transportation network | mi			20
Investment in Kansas City Southern	$ 0			$ 44,402,000,000		$ 0	$ 44,402,000,000		$ 0	$ 0		$ 45,091,000,000
Remeasurement loss of Kansas City Southern						0		$ 0		7,175,000,000	$ 0
Deferred income tax expense (recovery)						(3,000,000)		(38,000,000)		7,925,000,000	(151,000,000)
Goodwill	18,147,000,000					18,147,000,000			18,147,000,000	18,147,000,000		$ 344,000,000
Total revenues						3,339,000,000		2,312,000,000		8,779,000,000	6,352,000,000
Net income						780,000,000		891,000,000		2,904,000,000	2,246,000,000
Equity earnings of Kansas City Southern						0		221,000,000		230,000,000	627,000,000
Acquisition-related costs						0				12,000,000
Income (loss) from equity method investments
Business Acquisition [Line Items]
Equity earnings of Kansas City Southern								221,000,000		230,000,000	627,000,000
KCS
Business Acquisition [Line Items]
Percentage of voting interest acquired					100.00%
Miles of transportation network | mi					7
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
Deferred income tax expense (recovery)				7,832,000,000
Cash and cash equivalents	298,000,000	298,000,000		298,000,000		298,000,000	298,000,000		298,000,000	298,000,000
Accounts receivables, net				698,000,000			698,000,000
Accounts payable and accrued liabilities				943,000,000			943,000,000
Intangible assets	3,022,000,000	3,022,000,000				3,022,000,000			3,022,000,000	3,022,000,000
Goodwill	$ 17,591,000,000	$ 17,491,000,000		17,591,000,000		17,591,000,000	17,591,000,000		17,591,000,000	17,591,000,000
Total revenues						1,208,000,000				2,206,000,000
Net income							224,000,000		$ 362,000,000
Acquisition-related costs						24,000,000		18,000,000		158,000,000	57,000,000
Goodwill, purchase accounting adjustments						100,000,000
KCS | Litigation and Personal Injury Claims
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired				41,000,000			41,000,000
KCS | Environmental Liabilities
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired				$ 132,000,000			132,000,000
KCS | Concession Rights
Business Acquisition [Line Items]
Renewal term of concession rights and related assets				50 years
Intangible assets				$ 9,176,000,000			9,176,000,000
Amortization periods of intangible assets acquired				74 years
KCS | Contracts and Customer Relationships
Business Acquisition [Line Items]
Intangible assets				$ 3,022,000,000			$ 3,022,000,000
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						0				6,000,000
Compensation and benefits | KCS
Business Acquisition [Line Items]
Acquisition-related costs						1,000,000				64,000,000
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs						22,000,000		18,000,000		87,000,000	57,000,000
Income (loss) from equity method investments | KCS
Business Acquisition [Line Items]
Acquisition-related costs						0		$ 12,000,000		11,000,000	$ 39,000,000
Cost, Direct Material | KCS
Business Acquisition [Line Items]
Acquisition-related costs						$ 1,000,000				$ 1,000,000