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INCOME TAXES - Summary of Movements in Deferred Tax Assets and Liabilities (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | $ 640 | $ (241) | $ 231
Property, plant and equipment and exploration and evaluation assets |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 541 | 37 | (7)
Timing of partnership items |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 0 | (261) | (176)
Unrealized foreign exchange loss (gain) on long-term debt |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 120 | 63 | (222)
Unrealized risk management activities |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | (46) | (44) | (5)
Asset retirement obligations |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | (88) | (20) | 522
Loss carryforwards |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 48 | (221) | (53)
Investments |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | (2) | 38 | 60
Investment in North West Redwater Partnership |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 30 | 81 | 106
Deferred PRT |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | 54 | (135) | 15
PRT deduction for corporate income tax |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | (21) | 61 | (5)
Other |  |  |
Disclosure of temporary difference, unused tax losses and unused tax credits [line items] |  |  |
Deferred income tax expense (recovery) | $ 4 | $ 160 | $ (4)

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- Definition The amount of tax expense or income relating to changes in deferred tax liabilities and deferred tax assets, recognised in profit or loss. [Refer: Deferred tax assets; Deferred tax expense (income); Deferred tax liabilities] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 12 -IssueDate 2017-01-01 -Paragraph 81 -Subparagraph g -Clause ii -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2017-03-01&anchor=para_81_g_ii&doctype=Standard -URIDate 2017-03-09 + Details
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