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Exploration in non-operating areas (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units $ 11,879 $ 36,307 $ 18,262
Personnel expenses [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 3,632 4,830 4,064
Services provided by third parties [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 3,611 22,764 5,401
Lands [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 1,528 1,867 1,781
Short-term and low-value lease [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 415 1,524 1,171
Consumption of materials and supplies [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 328 1,420 582
Other expenses [Member]      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units $ 2,365 $ 3,902 $ 5,263