Income Taxes - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Business Acquisition [Line Items]
Effective tax rate	12.67%	24.05%
Effective tax rate, excluding discrete items	24.25%	24.60%
Acquisition-related costs	$ 0	$ 3
Effective Income Tax Rate Reconciliation, Basis Difference On Equity Method Investment	32
Foreign exchange gain on debt and lease liabilities	0	(33)
KCS
Business Acquisition [Line Items]
Acquisition-related costs	20	36
KCS | Purchased services and other
Business Acquisition [Line Items]
Acquisition-related costs	$ 20	$ 33