Business acquisition - Schedule of Allocation of Net Assets and Consideration Paid (Details) - CAD ($) $ in Millions			9 Months Ended
	Dec. 31, 2023	Apr. 14, 2023	Dec. 31, 2023	Apr. 13, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Goodwill	$ 17,729		$ 17,729		$ 344	$ 328
Kansas City Southern
Business Acquisition [Line Items]
Cash and cash equivalents	298	$ 298	298
Net working capital	(59)	51	(59)
Properties	28,749	28,748	28,749
Intangible assets	3,022	3,022	3,022
Other long-term assets	491	496	491
Debt including debt maturing within one year	(4,545)	(4,545)	(4,545)
Deferred income taxes	(6,942)	(6,984)	(6,942)
Other long-term liabilities	(408)	(406)	(408)
Total identifiable net assets	20,606	20,680	20,606
Goodwill	17,565	17,491	17,565	$ 17,565
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			(110)
Properties			1
Intangible assets			0
Other long-term assets			(5)
Debt including debt maturing within one year			0
Deferred income taxes			42
Other long-term liabilities			(2)
Total identifiable net assets			(74)
Goodwill			74
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