Disposition of properties Disposition of properties - Proceeds and Gains on Sale (Details) CAD in Millions, $ in Millions			3 Months Ended			12 Months Ended
	Sep. 18, 2015 CAD	Sep. 18, 2015 USD ($)	Dec. 31, 2016 CAD	Mar. 31, 2016 CAD	Sep. 30, 2015 CAD	Dec. 31, 2017 CAD	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets						CAD 42	CAD 116	CAD 114
Proceeds from the sale of property, plant and equipment						0	0	281
Gain on sale of property, pant and equipment						CAD 0	CAD 0	CAD 68
Obico rail yard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of properties and other assets			CAD 38
Gain on sale of properties and other assets			37
Gain after tax on sale of properties and other assets			CAD 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				CAD 55
Gain on sale of property, pant and equipment				50
Gain after tax on sale of property, plant and equipment				CAD 43
Delaware and Hudson Railway Company [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	CAD 281	$ 214
Gain on sale of property, pant and equipment					CAD 68
Gain after tax on sale of property, plant and equipment					CAD 42