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Production costs - Summary of Product Cost (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Expenses by nature [abstract]    
Labour $ 74,023 $ 52,670
Fuel 14,910 23,241
Reagents 40,587 40,839
Electricity 14,288 12,132
Mining contractors 17,107 12,575
Operating and maintenance supplies and services 31,772 56,342
Site general and administrative costs 33,667 23,621
Inventory change 33,459 (36,501)
Royalties, production taxes and selling expenses 8,167 7,821
Production costs $ 267,980 $ 192,740