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EXPLORATION AND EVALUATION ASSETS - Narrative (Details) - CAD ($) $ in Millions | 12 Months Ended
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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 |

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