Accounts receivable, net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	$ (40)	$ (43)
Current period credit loss provision, net	6	3
Allowance for credit loss, closing balance	(34)	(40)
Freight
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(25)	(27)
Current period credit loss provision, net	5	2
Allowance for credit loss, closing balance	(20)	(25)
Non-freight
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, opening balance	(15)	(16)
Current period credit loss provision, net	1	1
Allowance for credit loss, closing balance	$ (14)	$ (15)