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Inventories - Summary of Inventories (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Classes of current inventories [abstract]    
Ore stockpiles $ 1,620 $ 3,297
In-process inventory and finished goods 59,974 96,651
Materials and supplies 76,291 68,896
Inventories $ 137,885 $ 168,844