Accounts Receivable, Net - Schedule of Accounts Receivable (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 865		$ 847
Allowance for credit losses	(40)	$ (43)	(42)
Total accounts receivable, net	825		805
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	662		637
Allowance for credit losses	(25)	(27)	(26)
Total accounts receivable, net	637		611
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	203		210
Allowance for credit losses	(15)	$ (16)	(16)
Total accounts receivable, net	$ 188		$ 194