INVENTORIES (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Coal stockpiles	$ 69,918	$ 38,145
Materials and supplies	71,610	73,517
Reserve for obsolete inventory	(4,288)	(4,867)
Total	$ 137,240	$ 106,795