Business acquisition - Schedule of Allocation of Net Assets and Consideration Paid (Details) - CAD ($) $ in Millions				3 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Apr. 14, 2023	Sep. 30, 2023	Apr. 13, 2023	Dec. 31, 2022
Business Acquisition [Line Items]
Goodwill	$ 18,147			$ 18,147		$ 344
KCS
Business Acquisition [Line Items]
Cash and cash equivalents	298	$ 298		298	$ 298
Net working capital	(64)	51		(64)
Properties	28,749	28,748		28,749
Intangible assets	3,022	3,022		3,022
Other long-term assets	495	496		495
Long-term debt	(4,545)	(4,545)		(4,545)
Deferred income taxes	(6,972)	(6,984)		(6,972)
Other long-term liabilities	(403)	(406)		(403)
Total identifiable net assets	20,580	20,680		20,580
Goodwill	17,591	17,491		17,591	$ 17,591
Total identifiable net assets and goodwill	38,171	38,171		38,171
Fair value of previously held equity method investment	37,227	37,227	$ 37,227
Intercompany net balances acquired	12	12		12
Fair value of non-controlling interest	932	932		932
Total	$ 38,171	$ 38,171
Measurement Period Adjustments [Abstract]
Cash and cash equivalents				0
Net working capital				(115)
Properties				1
Intangible assets				0
Other long-term assets				(1)
Long-term debt				0
Deferred income taxes				12
Other long-term liabilities				3
Total identifiable net assets				(100)
Goodwill				100
Total net identifiable net assets and goodwill				0
Fair value of previously held equity method investment				0
Intercompany net balances acquired				0
Fair value of non-controlling interest				0
Total				$ 0