Accounts Receivable, Net - Allowance for Credit Loss (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Jan. 01, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 926
Allowance for Credit Loss [Roll Forward]
Current period credit loss provision, net	2
Allowance for credit losses, Closing	(41)
Accounts receivable, net	885	$ 804
Freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	724
Allowance for Credit Loss [Roll Forward]
Current period credit loss provision, net	0
Allowance for credit losses, Closing	(27)
Accounts receivable, net	697	610
Non-freight
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	202
Allowance for Credit Loss [Roll Forward]
Current period credit loss provision, net	2
Allowance for credit losses, Closing	(14)
Accounts receivable, net	$ 188	$ 194