Accounts Receivable, Net - Schedule of Accounts Receivable (Details) - CAD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 793		$ 865
Allowance for credit losses	(38)	$ (39)	(40)	$ (40)	$ (41)	$ (43)
Total accounts receivable, net	755		825
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	614		662
Allowance for credit losses	(23)	(24)	(25)	(26)	(27)	(27)
Total accounts receivable, net	591		637
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	179		203
Allowance for credit losses	(15)	$ (15)	(15)	$ (14)	$ (14)	$ (16)
Total accounts receivable, net	$ 164		$ 188