Accounts Receivable, Net - Schedule of Accounts Receivable (Details) - CAD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 849		$ 865
Allowance for credit losses	(38)	$ (38)	(40)	$ (40)	$ (40)	$ (43)
Total accounts receivable, net	811		825
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	655		662
Allowance for credit losses	(22)	(23)	(25)	(26)	(26)	(27)
Total accounts receivable, net	633		637
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	194		203
Allowance for credit losses	(16)	$ (15)	(15)	$ (14)	$ (14)	$ (16)
Total accounts receivable, net	$ 178		$ 188