Accounts Receivable, Net - Schedule of Accounts Receivable (Details) - CAD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 880
Allowance for credit losses	(40)	$ (40)	$ (43)
Total accounts receivable, net	840
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	705
Allowance for credit losses	(26)	(26)	(27)
Total accounts receivable, net	679
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	175
Allowance for credit losses	(14)	$ (14)	$ (16)
Total accounts receivable, net	$ 161