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SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Major Asset Categories (Details)
 12 Months Ended
Dec. 31, 2017
Minimum | Buildings, plant and equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 7 years
Minimum | Underground mobile equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 5 years
Minimum | Light vehicles and other mobile equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 2 years
Minimum | Furniture, computer and office equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 2 years
Maximum | Buildings, plant and equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 27 years
Maximum | Underground mobile equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 7 years
Maximum | Light vehicles and other mobile equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 3 years
Maximum | Furniture, computer and office equipment  
Disclosure of detailed information about property, plant and equipment [line items]  
Useful life 3 years