Income taxes - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019 CAD ($)	Jun. 30, 2019 CAD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 14, 2021 CAD ($)	Dec. 14, 2021 USD ($)
Income Tax Disclosure [Abstract]
Deferred tax liability, outside basis difference			$ 7,079		$ 0		$ 7,200	$ 5,600
Deferred tax recovery, Kansas City Southern outside basis difference			33	$ 26
Deferred revaluation recovery amount due to change in tax rate					29
Effect of tax rate decrease		$ 88	11		32	$ 95
Income tax operating losses carried forward			15		15
Minimum tax credits carried forward			0
Capital losses carried forward			2		2
Unrecognized tax benefits from capital losses			0		0
Tax benefits related to prior years	$ 24		0		10	34
Interest and penalties accrual (recovery)			4		(1)	(1)
Accrued interest and penalties associated with unrecognized tax benefits	$ 10		$ 13		$ 9	$ 10