Income taxes (Details) $ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2025 CAD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 CAD ($)	Jun. 30, 2025 CAD ($)	Jun. 30, 2024 CAD ($)
Business Combination [Line Items]
Effective tax rate	22.45%	22.45%	24.40%	23.26%	24.72%
Gain on sale of equity investment	$ 333		$ 0	$ 333	$ 0
Effective tax rate, excluding discrete items	24.50%	24.50%	25.00%	24.50%	25.00%
Other tax expense (recovery)			$ (3)		$ (3)
Settlement of Mexican tax audits	$ 1		0	$ 12	0
Mexican Tax Authority
Business Combination [Line Items]
Other tax expense (recovery)					10
Settlement of Mexican tax audits				11
Panama Canal Railway Company
Business Combination [Line Items]
Gain on sale of equity investment	333	$ 232		333
KCS
Business Combination [Line Items]
Amortization of fair value adjustments	96		88	190	174
Acquisition-related costs	$ 19		$ 28	$ 39	$ 54