PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 3,044,260	$ 2,815,620
Less accumulated depreciation, depletion and amortization	(1,243,985)	(1,150,414)
Total property, plant and equipment, net	1,800,275	1,665,206
Equipment under capital leases	105,800
Accumulated amortization related to capital leases	7,100	5,600	$ 5,800
Amortization expense related to capital leases	5,700	1,600	$ 2,000
Mining equipment and processing facilities
Property, Plant and Equipment
Property, plant and equipment, at cost	1,923,310	1,757,772
Land and mineral rights
Property, Plant and Equipment
Property, plant and equipment, at cost	418,668	376,937
Buildings, office equipment and improvements
Property, Plant and Equipment
Property, plant and equipment, at cost	291,106	278,283
Construction and mine development in progress
Property, Plant and Equipment
Property, plant and equipment, at cost	94,482	82,530
Mine development costs
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 316,694	$ 320,098