ACQUISITIONS AND DISPOSITIONS - Assets Held for Sale (Narrative) (Details) - Disposal group, held-for-sale, not discontinued operations - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Montana-Alberta Tie Line (MATL)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of long-lived assets to be disposed of	$ 297
Asset Impairment \| Line 10 Crude Oil
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, provision for loss (gain) on disposal, before income tax		$ 154

Amount before tax of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a discontinued operation.

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