Business acquisition - Schedule of Allocation of Net Assets and Consideration Paid (Details) - CAD ($) $ in Millions			12 Months Ended
	Mar. 31, 2024	Apr. 14, 2023	Mar. 31, 2024	Dec. 31, 2023
Business Acquisition [Line Items]
Goodwill	$ 18,228		$ 18,228	$ 17,729
KCS
Business Acquisition [Line Items]
Cash and cash equivalents	298	$ 298	298
Net working capital	(110)	51	(110)
Properties	28,749	28,748	28,749
Intangible assets	3,022	3,022	3,022
Other long-term assets	490	496	490
Debt including debt maturing within one year	(4,545)	(4,545)	(4,545)
Deferred income taxes	(6,922)	(6,984)	(6,922)
Other long-term liabilities	(443)	(406)	(443)
Total identifiable net assets	20,539	20,680	20,539
Goodwill	17,632	17,491	17,632
Total identifiable net assets and goodwill	38,171	38,171	38,171
Fair value of previously held equity method investment	37,227	37,227
Intercompany payable balance, net acquired	12	12	12
Fair value of non-controlling interest	932	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