Business acquisition - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions										3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2023 CAD ($)	Apr. 14, 2023 CAD ($)	Apr. 13, 2023 CAD ($)	Dec. 14, 2021 CAD ($) shares	Dec. 14, 2021 USD ($) $ / shares shares	Sep. 15, 2021 CAD ($)	Sep. 15, 2021 USD ($)	May 21, 2021 CAD ($)	May 21, 2021 USD ($)	Apr. 13, 2023 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2021 CAD ($)	Aug. 10, 2021
Business Acquisition [Line Items]
Merger termination fee (Note 11)												$ 0	$ 0	$ (845,000,000)
Amount remitted												0	0	12,299,000,000
Shares issued for Kansas City Southern acquisition (Note 21)													(2,000,000)	23,456,000,000
Rail investments accounted for on an equity basis	$ 0		$ 44,402,000,000							$ 44,402,000,000	$ 0	0	45,091,000,000
Remeasurement loss of Kansas City Southern (Note 11)												7,175,000,000	0	0
Deferred income tax (recovery) expense (Note 6)												7,885,000,000	(136,000,000)	(242,000,000)
Goodwill (Note 11, 14)	17,729,000,000										17,729,000,000	17,729,000,000	344,000,000	328,000,000
Goodwill, expected tax deductible amount	0										0	0
Revenue of acquiree since acquisition date, actual												1,351,000,000	4,390,000,000
KCS historical, net income attributable to controlling shareholders												280,000,000	1,287,000,000
Equity loss of Kansas City Southern										230,000,000		230,000,000	1,074,000,000	(141,000,000)
Acquisition-related costs (Note 11)												190,000,000
Acquisition-related financing fees (Note 11)												17,000,000	0	51,000,000
Equity loss of Kansas City Southern
Business Acquisition [Line Items]
Equity loss of Kansas City Southern										230,000,000			1,074,000,000
Share capital
Business Acquisition [Line Items]
Shares issued for Kansas City Southern acquisition (Note 21)														23,461,000,000
Other expense (income)
Business Acquisition [Line Items]
Acquisition-related costs (Note 11)												6,000,000
Kansas City Southern
Business Acquisition [Line Items]
Amount remitted						$ 1,773,000,000	$ 1,400
Kansas City Southern
Business Acquisition [Line Items]
Merger termination fee (Note 11)								$ 845,000,000	$ (700)
Percentage ownership acquired				100.00%	100.00%
Stock issued (in shares) | shares				262.6	262.6
Exchange ratio															2.884
Shares issued for Kansas City Southern acquisition (Note 21)				$ 23,461,000,000	$ 18,282
Cash consideration				10,526,000,000	8,203
Total consideration				$ 35,760,000,000	$ 27,885
Fair value of previously held equity method investment	37,227,000,000	$ 37,227,000,000
Remeasurement loss of Kansas City Southern (Note 11)			7,175,000,000
Deferred income tax (recovery) expense (Note 6)			7,832,000,000									69,000,000
Cash and cash equivalents	298,000,000	298,000,000									298,000,000	298,000,000
Accounts receivable, net		704,000,000
Accounts payable acquired		970,000,000
Intangible assets	3,022,000,000	3,022,000,000									3,022,000,000	3,022,000,000
Goodwill (Note 11, 14)	17,565,000,000	17,491,000,000	$ 17,565,000,000							$ 17,565,000,000	17,565,000,000	17,565,000,000
Revenue of acquiree since acquisition date, actual											3,467,000,000
KCS historical, net income attributable to controlling shareholders											682,000,000	(297,000,000)
Acquisition-related costs (Note 11)														599,000,000
Interest rate hedge, unwinding													212,000,000
Purchase accounting net income, after deferred tax recovery												(228,000,000)
Net working capital	$ (59,000,000)	51,000,000									$ (59,000,000)	(59,000,000)
Kansas City Southern | Concession land rights
Business Acquisition [Line Items]
Property, plant and equipment, useful life			50 years							50 years
Kansas City Southern | Environmental Liabilities
Business Acquisition [Line Items]
Liabilities arising from contingencies		132,000,000
Net working capital		15,000,000
Kansas City Southern | Litigation and Personal Injury Claims
Business Acquisition [Line Items]
Liabilities arising from contingencies		40,000,000
Net working capital		28,000,000
Kansas City Southern | Concession Rights
Business Acquisition [Line Items]
Intangible assets		$ 9,176,000,000
Acquired finite-lived intangible assets, weighted average useful life		74 years
Kansas City Southern | Contracts and Customer Relationships
Business Acquisition [Line Items]
Intangible assets		$ 3,022,000,000
Kansas City Southern | Contracts and Customer Relationships | Minimum
Business Acquisition [Line Items]
Acquired finite-lived intangible assets, weighted average useful life		9 years
Kansas City Southern | Contracts and Customer Relationships | Maximum
Business Acquisition [Line Items]
Acquired finite-lived intangible assets, weighted average useful life		22 years
Kansas City Southern | Share capital
Business Acquisition [Line Items]
Consideration transferred, per share (in CAD or USD per share) | $ / shares					$ 90
Kansas City Southern | Preferred stock
Business Acquisition [Line Items]
Consideration transferred, per share (in CAD or USD per share) | $ / shares					$ 37.50
Kansas City Southern | Purchased services and other
Business Acquisition [Line Items]
Acquisition-related costs (Note 11)													74,000,000	183,000,000
Kansas City Southern | Other expense (income)
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												(2,000,000)
Acquisition-related costs (Note 11)														247,000,000
Kansas City Southern | Equity loss of Kansas City Southern
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												(48,000,000)	163,000,000
Acquisition-related costs (Note 11)										$ 11,000,000			$ 49,000,000	$ 169,000,000
Kansas City Southern | Compensation and benefits
Business Acquisition [Line Items]
Acquisition-related costs (Note 11)												71,000,000
Kansas City Southern | Purchased services and other
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												(1,000,000)
Acquisition-related costs (Note 11)												111,000,000
Kansas City Southern | Cost, direct material
Business Acquisition [Line Items]
Acquisition-related costs (Note 11)												2,000,000
Kansas City Southern | Depreciation, Depletion and Amortization
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												(234,000,000)
Kansas City Southern | Interest Revenue (Expense), Net
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												(17,000,000)
Kansas City Southern | Net Income (Loss) Attributable to Noncontrolling Interest
Business Acquisition [Line Items]
KCS historical, net income attributable to controlling shareholders												$ 5,000,000