Accounts Receivable, Net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	$ (40)	$ (43)
Current period credit loss provision, net	1	2
Allowance for credit losses, closing balance	(39)	(41)
Freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(25)	(27)
Current period credit loss provision, net	1	0
Allowance for credit losses, closing balance	(24)	(27)
Non-freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(15)	(16)
Current period credit loss provision, net	0	2
Allowance for credit losses, closing balance	$ (15)	$ (14)