Income taxes - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2022 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 14, 2021 CAD ($)	Dec. 14, 2021 USD ($)
Income Tax Disclosure [Abstract]
Deferred tax recovery, Kansas City Southern outside basis difference			$ 19	$ 15	$ 33	$ 26
Deferred revaluation recovery amount due to change in tax rate			12				$ 29
Deferred tax liability, outside basis difference	$ 7,526		7,526		7,079			$ 7,200	$ 5,600
Income tax operating losses carried forward	22		22		15
Minimum tax credits carried forward	0		0
Capital losses carried forward	2		2		2
Unrecognized tax benefits from capital losses	0		0		0
Tax benefits related to prior years		$ 24	0		0		10
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	24		30		6		9
Interest and penalties accrual (recovery)			5		4		(1)
Accrued interest and penalties associated with unrecognized tax benefits	$ 18		$ 18		$ 13		$ 9