ACQUISITIONS AND DISPOSITIONS - Dispositions (Narrative) (Details) CAD in Millions, $ in Millions						12 Months Ended
	Jul. 31, 2017 CAD	Jul. 31, 2017 USD ($)	Mar. 01, 2017 CAD	Mar. 01, 2017 USD ($)	Dec. 01, 2016 CAD	Dec. 31, 2017 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal of assets						CAD 120		CAD 848	CAD 94
Dispositions | Liquids Pipelines | Olympic Pipeline
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of interest	CAD 203	$ 160
Dispositions | Liquids Pipelines | Sandpiper
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of assets						148	$ 111
Gain on disposal of assets						CAD 83	$ 63
Dispositions | Liquids Pipelines | Ozark Pipeline
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of assets			CAD 294	$ 220
Gain on disposal of assets			CAD 14	$ 10
Dispositions | Liquids Pipelines | South Prairie Region
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash proceeds from sale of assets					CAD 1,100
Other income/(expense) | Dispositions | Liquids Pipelines | Olympic Pipeline
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal of interest	CAD 27	$ 21
Other income/(expense) | Dispositions | Liquids Pipelines | South Prairie Region
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal of interest					CAD 850