Business acquisition - Narrative (Details) mi in Thousands, $ in Millions				3 Months Ended			6 Months Ended
	Apr. 14, 2023 CAD ($) mi	Apr. 13, 2023 CAD ($)	Dec. 14, 2021 mi	Jun. 30, 2023 CAD ($)	Jun. 30, 2023 CAD ($)	Jun. 30, 2022 CAD ($)	Jun. 30, 2023 CAD ($)	Jun. 30, 2022 CAD ($)	Dec. 31, 2022 CAD ($)
Business Acquisition [Line Items]
Miles of transportation network | mi	20
Investment in Kansas City Southern (Note 9)		$ 44,402		$ 0	$ 0		$ 0		$ 45,091
Remeasurement loss of Kansas City Southern (Note 8)				7,175		$ 0	7,175	$ 0
Deferred income tax expense (recovery)				7,953		(114)	7,928	(113)
Gross amount due under contracts		664
Goodwill				17,674	17,674		17,674		$ 344
Total revenues				3,174		2,202	5,440	4,040
Net income				1,324		765	2,124	1,355
Equity earnings of Kansas City Southern				26		208	230	406
Acquisition-related costs				9			12
Income (loss) from equity method investments
Business Acquisition [Line Items]
Equity earnings of Kansas City Southern				26		208	230	406
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	37,227
Remeasurement loss of Kansas City Southern (Note 8)		7,175
Deferred income tax expense (recovery)		7,832					(7,832)
Cash and cash equivalents		298
Accounts receivables, net		644
Gross intercompany receivables		24
Expected amount under contracts uncollectible		20
Intangible assets		3,022
Goodwill		17,491
Total revenues					998
Net income					$ 138
Acquisition-related costs				119		19	134	39
KCS | Litigation And Personal Injury Claims
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired		41
KCS | Environmental Liabilities
Business Acquisition [Line Items]
Liabilities arising from contingencies acquired		$ 132
KCS | Concession Rights
Business Acquisition [Line Items]
Renewal term of concession rights and related assets		50 years
Intangible assets		$ 9,176
Amortization periods of intangible assets acquired		74 years
KCS | Contracts and Customer Relationships
Business Acquisition [Line Items]
Intangible assets		$ 3,022
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			6
Compensation and benefits | KCS
Business Acquisition [Line Items]
Acquisition-related costs				63			63
Purchased services and other | KCS
Business Acquisition [Line Items]
Acquisition-related costs				53		19	65	39
Income (loss) from equity method investments | KCS
Business Acquisition [Line Items]
Acquisition-related costs				$ 1		$ 14	$ 11	$ 27