Accounts Receivable, Net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	$ (40)	$ (43)
Current period credit loss provision, net	0	3
Allowance for credit losses, closing balance	(40)	(40)
Freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(26)	(27)
Current period credit loss provision, net	0	1
Allowance for credit losses, closing balance	(26)	(26)
Non-freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(14)	(16)
Current period credit loss provision, net	0	2
Allowance for credit losses, closing balance	$ (14)	$ (14)