Income taxes (Details) - CAD ($) $ in Millions	1 Months Ended	3 Months Ended				9 Months Ended
	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Apr. 13, 2023	Sep. 30, 2024	Sep. 30, 2023
Business Acquisition [Line Items]
Deferred tax recovery			$ 14			$ 3	$ 14
Effective tax rate		23.88%	24.88%			24.44%	166.83%
Effective tax rate, excluding discrete items		24.24%	24.96%			24.75%	24.91%
Deferred tax expense (recovery)		$ 5	$ 3			$ 40	$ (7,925)
Remeasurement loss of Kansas City Southern		0	0			0	7,175
Equity earnings of Kansas City Southern		0	0		$ 230	0	230
Re-evaluation of deferred income tax							51
Outside basis deferred tax recovery							23
Settlement of Mexican tax audits		2	75			2	75
Mexican Tax Authority
Business Acquisition [Line Items]
Reserve for potential future audits			15				15
Other tax expense (recovery)		(7)				3
Settlement of Mexican tax audits	$ 75
Iowa & Arkansas Tax Authorities
Business Acquisition [Line Items]
Deferred tax recovery			14
Deferred tax expense (recovery)							(14)
KCS
Business Acquisition [Line Items]
Amortization of fair value adjustments		90	87			264	162
Acquisition-related costs		$ 36	$ 24			$ 90	158
Deferred tax expense (recovery)				$ (7,832)			(7,832)
Remeasurement loss of Kansas City Southern				$ 7,175			$ 7,175