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Exploration in operating units (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Exploration In Operating Units [Line Items]      
Beginning balance of finished goods and products in process, net of depreciation and amortization $ 2,837 $ 4,478 $ 5,309
Beginning balance of finished goods and products in process, net     5,157
Total exploration in operating units 41,770 87,410 88,480
Final balance of products in process and finished goods, net of depreciation and amortization 444 2,837 4,478
Exploration in operating units 44,163 89,730 89,311
Services provided by third parties [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 33,591 71,513 75,743
Consumption of materials and supplies [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 3,712 8,594 7,673
Direct labor [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 1,747 2,349 2,142
Short-term and low-value lease [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 1,186 2,065 1,405
Electricity and water [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 905 1,337 820
Maintenance and repair [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 10 450 98
Transport [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units 71 192 543
Others [Member]      
Disclosure Of Exploration In Operating Units [Line Items]      
Exploration in operating units $ 548 $ 910 $ 56