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INCOME TAXES - Summary of Movements in Deferred Tax Assets and Liabilities (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) $ 557 $ 640 $ (241)
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) 281 541 37
Timing of partnership items      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) 0 0 (261)
Unrealized foreign exchange (gain) loss on long-term debt      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) (75) 120 63
Unrealized risk management activities      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) 18 (46) (44)
Asset retirement obligations      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) 175 (88) (20)
Loss carryforwards      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) (61) 48 (221)
Investments      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) (50) (2) 38
Investment in North West Redwater Partnership      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) 162 30 81
Deferred PRT      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) 17 54 (135)
PRT deduction for corporate income tax      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) (7) (21) 61
Other      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax expense (recovery) $ 97 $ 4 $ 160