Accounts Receivable, Net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	$ (34)	$ (40)
Current period credit loss provision, net	(2)	1
Allowance for credit losses, closing balance	(36)	(39)
Freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(20)	(25)
Current period credit loss provision, net	(2)	1
Allowance for credit losses, closing balance	(22)	(24)
Non-freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(14)	(15)
Current period credit loss provision, net	0	0
Allowance for credit losses, closing balance	$ (14)	$ (15)