Income taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.11%	26.12%	26.12%
Expected income tax (recovery) expense at Canadian enacted statutory tax rates	$ (797)	$ 1,083	$ 946
(Decrease) increase in taxes resulting from:
Reversal of outside basis deferred tax (Note 11)	(7,832)	0	0
Remeasurement loss of Kansas City Southern	1,873	0	0
Losses (gains) not subject to tax	10	(9)	(116)
Canadian tax rate differentials	(14)	(12)	(22)
Foreign tax rate differentials	(62)	(94)	(37)
Effect of tax rate decrease	(72)	(25)	(11)
Deduction for dividends taxed on outside basis	(68)	(270)	0
Unrecognized tax benefits	(10)	(24)	(2)
Inflation in Mexico	(31)	0	0
Valuation allowance	1	0	0
Other	26	(21)	10
Total income tax (recovery) expense	$ (6,976)	$ 628	$ 768