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 ($)	May 30, 2014 CAD	May 30, 2014 USD ($)	Dec. 31, 2016 CAD	Mar. 31, 2016 CAD	Sep. 30, 2015 CAD	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD	Dec. 31, 2014 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 116	CAD 114	CAD 52
Proceeds from sales of property, plant and equipment									281	236
Gain on sale								0	68	0
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					37
Gain after tax					CAD 33
Arbutus Corridor [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sales of property, plant and equipment						CAD 55
Gain on sale						50
Gain after tax						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 sales of property, plant and equipment	CAD 281	$ 214						0	281	0
Gain on sale							CAD 68
Gain after tax							CAD 42
Dakota Minnesota and Eastern Railroad [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sales of property, plant and equipment			CAD 236	$ 218				CAD 0	CAD 0	CAD 236