Income Taxes - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2022 CAD ($)	Jun. 30, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 CAD ($)	Jun. 30, 2021 CAD ($)	Jun. 30, 2021 USD ($)
Business Acquisition [Line Items]
Effective tax rate	24.21%	17.10%	17.10%	19.59%	19.50%	19.50%
Effective tax rate, excluding discrete items	24.25%	24.60%	24.60%	24.25%	24.60%	24.60%
Acquisition-related costs	$ 0	$ 209		$ 0	$ 212
Effective Income Tax Rate Reconciliation, Basis Difference On Equity Method Investment	49			17
Foreign exchange gain on debt and lease liabilities	0	(52)		0	(85)
Merger termination fee	0	845	$ 700	0	845	$ 700
KCS
Business Acquisition [Line Items]
Acquisition-related costs	19	308		39	344
Merger termination fee					845	$ 700
KCS | Purchased services and other
Business Acquisition [Line Items]
Acquisition-related costs	$ 19	$ 99		$ 39	$ 132