Accounts Receivable, Net - Schedule of Accounts Receivable (Details) - CAD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 879		$ 926
Allowance for credit losses	(39)	$ (40)	(41)	$ (43)
Total accounts receivable, net	840		885
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	681		724
Allowance for credit losses	(24)	(25)	(27)	(27)
Total accounts receivable, net	657		697
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	198		202
Allowance for credit losses	(15)	$ (15)	(14)	$ (16)
Total accounts receivable, net	$ 183		$ 188