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INCOME TAXES - Summary of Movements in Deferred Tax Assets and Liabilities (Details) - CAD ($)
$ in Millions
 12 Months Ended
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)