Disposition of properties Disposition of properties - Proceeds and Gains on Sale (Details) $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets			$ 26	$ 78	$ 42
Bass Lake Spur [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets	$ 37	$ 27
Gain on sale of properties and other assets	35
Gain after tax on sale of properties and other assets	$ 26