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Significant accounting policies (Details)
 12 Months Ended
Dec. 31, 2017
Administrative facilities [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 35 and 48
Main production structures [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 30 and 49
Minor production structures [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 20 and 35
Mills and horizontal furnaces [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 42 and 49
Vertical furnaces, crushers and grinders [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 23 and 36
Electricity facilities and other minors [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 12 and 35
Furniture and fixtures [member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 10
Heavy units [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 11 and 21
Light units [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 8 and 11
Computer equipment [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 4
Tools [Member]  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives Between 5 and 10