Business acquisition - Schedule of Allocation of Net Assets and Consideration Paid (Details) - CAD ($) $ in Millions				9 Months Ended
	Dec. 31, 2023	Apr. 14, 2023	Apr. 13, 2023	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2022
Business Acquisition [Line Items]
Goodwill	$ 17,729			$ 17,729	$ 19,350	$ 344
Kansas City Southern
Business Acquisition [Line Items]
Cash and cash equivalents		$ 298			298
Net working capital		51			(110)
Properties		28,748			28,749
Intangible assets		3,022			3,022
Other long-term assets		496			490
Debt including debt maturing within one year		(4,545)			(4,545)
Deferred income taxes		(6,984)			(6,922)
Other long-term liabilities		(406)			(443)
Total identifiable net assets		20,680			20,539
Goodwill		17,491	$ 17,632		17,632
Total identifiable net assets and goodwill		38,171			38,171
Fair value of previously held equity method investment	37,227	37,227	$ 37,227
Intercompany payable balance, net acquired		12			12
Fair value of non-controlling interest		932			$ 932
Total	$ 38,171	$ 38,171
Measurement period adjustments
Cash and cash equivalents				0
Net working capital				(161)
Properties				1
Intangible assets				0
Other long-term assets				(6)
Debt including debt maturing within one year				0
Deferred income taxes				62
Other long-term liabilities				(37)
Total identifiable net assets				(141)
Goodwill				141
Total net identifiable net assets and goodwill				0
Fair value of previously held equity method investment				0
Intercompany payable balance, net acquired				0
Fair value of non-controlling interest				0
Total				$ 0