Business combination - Fair values of the acquired assets and liabilities (Details) $ in Millions, $ in Millions	Dec. 30, 2019 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 30, 2019 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)
Business Acquisition [Line Items]
Goodwill		$ 194		$ 194	$ 178
CMQ
Business Acquisition [Line Items]
Accounts receivable, net	$ 7
Properties	42
Intangible assets	5
Accounts payable and accrued liabilities	(2)
Long-term debt maturing within one year	(11)		$ (8)
Other long-term liabilities	(4)
Total Identifiable Assets and Liabilities	37
Goodwill	10
Fair value of net assets acquired	47
Cash, net of cash acquired	$ 47