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EXPLORATION AND EVALUATION ASSETS - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Acquisition of exploration and evaluation assets $ 290    
Disposals of exploration and evaluation assets 1 $ 21  
Proceeds from disposal of exploration and evaluation assets 35 35 $ 416
North America      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Acquisition of exploration and evaluation assets 31    
Disposals of exploration and evaluation assets 1 3  
Proceeds from disposal of exploration and evaluation assets 36 35  
Gain on sale of exploration and evaluation assets 35 32  
Offshore Africa      
Disclosure Of Detailed Information About Exploration For And Evaluation Of Mineral Resources [Line Items]      
Acquisition of exploration and evaluation assets 0    
Disposals of exploration and evaluation assets $ 0 $ 18