PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Property, plant and equipment
Total cost		$ 308,321	$ 292,124
Accumulated depreciation and depletion		74,691	53,467
Net property, plant and equipment		233,630	238,657
Machinery and equipment
Property, plant and equipment
Total cost	[1]	160,203	146,951
Buildings and improvements
Property, plant and equipment
Total cost	[1]	58,036	51,027
Land and improvements
Property, plant and equipment
Total cost	[1]	47,901	37,461
Mineral reserves
Property, plant and equipment
Total cost		30,181	30,181
Construction in progress
Property, plant and equipment
Total cost		9,555	24,172
Capitalized reclamation costs
Property, plant and equipment
Total cost		$ 2,445	$ 2,332

[1] Includes assets under capital lease