Income taxes (Details) - CAD ($) $ in Millions		3 Months Ended			6 Months Ended
	Apr. 13, 2023	Jun. 30, 2024	Jun. 30, 2023	Apr. 13, 2023	Jun. 30, 2024	Jun. 30, 2023
Business Acquisition [Line Items]
Deferred tax recovery		$ 3			$ 3
Effective tax rate		24.40%	120.88%		24.72%	139.47%
Effective tax rate, excluding discrete items		25.00%	25.18%		25.00%	24.88%
Deferred tax expense (recovery)		$ 18	$ (7,953)		$ 35	$ (7,928)
Remeasurement loss of Kansas City Southern		0	7,175		0	7,175
Re-evaluation of deferred income tax			51			51
Equity earnings of Kansas City Southern	$ 26	0	26	$ (230)	0	230
Outside basis deferred tax recovery						23
Mexican Tax Authority
Business Acquisition [Line Items]
Other tax expense					10
KCS
Business Acquisition [Line Items]
Amortization of fair value adjustments		88	75		174	75
Acquisition-related costs		$ 28	119		$ 54	134
Deferred tax expense (recovery)			(7,832)			(7,832)
Remeasurement loss of Kansas City Southern			$ 7,175			$ 7,175