Accounts Receivable, Net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	$ (39)	$ (41)	$ (40)	$ (43)
Current period credit loss provision, net	1	1	2	3
Allowance for credit losses, closing balance	(38)	(40)	(38)	(40)
Freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(24)	(27)	(25)	(27)
Current period credit loss provision, net	1	1	2	1
Allowance for credit losses, closing balance	(23)	(26)	(23)	(26)
Non-freight
Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(15)	(14)	(15)	(16)
Current period credit loss provision, net	0	0	0	2
Allowance for credit losses, closing balance	$ (15)	$ (14)	$ (15)	$ (14)