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ACCOUNTING POLICIES - Narrative (Details)
 12 Months Ended
Dec. 31, 2018
Bottom of range | Midstream  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful life 5 years
Bottom of range | Other equipment | Oil Sands Mining and Upgrading  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful life 2 years
Top of range | Midstream  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful life 30 years
Top of range | Other equipment | Oil Sands Mining and Upgrading  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful life 18 years
Performance Share Units  
Disclosure of detailed information about property, plant and equipment [line items]  
Vesting term 3 years