PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 2,934,188	$ 2,920,988
Less accumulated depreciation, depletion and amortization	(1,457,532)	(1,335,145)
Total property, plant and equipment, net	1,476,656	1,585,843
Mining Equipment And Processing Facilities
Property, Plant and Equipment
Property, plant and equipment, at cost	1,847,037	1,854,001
Land And Mineral Rights
Property, Plant and Equipment
Property, plant and equipment, at cost	449,152	439,236
Buildings, Office Equipment And Improvements
Property, Plant and Equipment
Property, plant and equipment, at cost	310,167	304,696
Construction and mine development in progress
Property, Plant and Equipment
Property, plant and equipment, at cost	47,223	26,025
Mine development costs
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 280,609	$ 297,030