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Inventories - Summary of Inventories (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Classes of current inventories [abstract]    
Ore stockpiles $ 3,297 $ 2,715
In-process inventory and finished goods 96,651 50,195
Materials and supplies 68,896 67,920
Inventories $ 168,844 $ 120,830