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Exploration in non-operating areas (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units $ 36,307 $ 18,262 $ 26,589
Services provided by third parties      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 22,764 5,401 13,629
Personnel expenses      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 4,830 4,064 3,908
Lands      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 1,867 1,781 0
Rentals      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 1,524 1,171 578
Consumption of materials and supplies      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units 1,420 582 768
Other expenses      
Disclosure Of Exploration In Non-Operating Areas [Line Items]      
Expense arising from exploration for and evaluation of non operating units $ 3,902 $ 5,263 $ 7,706