Accounts Receivable, Net - Allowance for Credit Loss (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 CAD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses, opening balance	$ (42)
Current period credit loss provision, net	3
Allowance for credit loss, closing balance	(40)
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses, opening balance	(26)
Current period credit loss provision, net	2
Allowance for credit loss, closing balance	(25)
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses, opening balance	(16)
Current period credit loss provision, net	1
Allowance for credit loss, closing balance	$ (15)