Income taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.11%	26.11%	26.12%
Expected income tax expense (recovery) at Canadian enacted statutory tax rates	$ 1,246	$ (797)	$ 1,083
(Decrease) increase in taxes resulting from:
Reversal of outside basis deferred income tax (Note 10)	0	(7,832)	0
Remeasurement loss of Kansas City Southern	0	1,873	0
(Gains) losses not subject to tax	(10)	10	(9)
Canadian tax rate differentials	(17)	(14)	(12)
Foreign tax rate differentials	(41)	(62)	(94)
Effect of tax rate decrease	(70)	(72)	(25)
Deduction for dividends taxed on outside basis	0	(68)	(270)
Unrecognized tax benefits	3	(10)	(24)
Inflation in Mexico	(33)	(31)	0
Valuation allowance	5	1	0
Other	(24)	26	(21)
Total income tax expense (recovery)	$ 1,059	$ (6,976)	$ 628