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, 2016 CAD ($)	Mar. 31, 2016 CAD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2016 CAD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets					$ 78	$ 42	$ 116
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
Obico [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets			$ 38
Gain on sale of properties and other assets			37
Gain after tax on sale of properties and other assets			$ 33
Arbutus Corridor [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from the sale of property, plant and equipment				$ 55
Gain on sale of property, pant and equipment				50
Gain after tax on sale of property, plant and equipment				$ 43