Income taxes - Deferred Income Tax Assets and Liabilities (Details) $ in Millions, $ in Millions	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2021 CAD ($)	Dec. 14, 2021 CAD ($)	Dec. 14, 2021 USD ($)
Deferred income tax assets
Tax losses and other attributes carried forward	$ 173	$ 70
Liabilities carrying value in excess of tax basis	276	108
Unrealized foreign exchange losses	18	50
Environmental remediation costs	50	22
Other	7	5
Total deferred income tax assets	524	255
Valuation allowance	(36)	(4)
Total net deferred income tax assets	488	251
Deferred income tax liabilities
Investment in Kansas City Southern (Note 12)	0	7,526	$ 7,079	$ 7,178	$ 5,607
Properties carrying value in excess of tax basis	9,481	4,149
Pensions carrying value in excess of tax basis	751	691
Intangibles carrying value in excess of tax basis	789	0
Investments carrying value in excess of tax basis	473	38
Other	46	44
Total deferred income tax liabilities	11,540	12,448
Total net deferred income tax liabilities	$ 11,052	$ 12,197