PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 2,920,988	$ 3,044,260
Less accumulated depreciation, depletion and amortization	(1,335,145)	(1,243,985)
Total property, plant and equipment, net	1,585,843	1,800,275
Mining Equipment And Processing Facilities
Property, Plant and Equipment
Property, plant and equipment, at cost	1,854,001	1,923,310
Land And Mineral Rights
Property, Plant and Equipment
Property, plant and equipment, at cost	439,236	418,668
Buildings, Office Equipment And Improvements
Property, Plant and Equipment
Property, plant and equipment, at cost	304,696	291,106
Construction and mine development in progress
Property, Plant and Equipment
Property, plant and equipment, at cost	26,025	94,482
Mine development costs
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 297,030	$ 316,694