PROPERTY, PLANT AND EQUIPMENT (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 2,645,872,000	$ 2,361,863,000
Less accumulated depreciation, depletion and amortization	(1,031,493,000)	(832,293,000)
Total property, plant and equipment, net	1,614,379,000	1,529,570,000
Equipment under capital leases	22,800,000
Accumulated amortization related to capital leases	5,800,000	3,800,000
Amortization expense related to capital leases	2,000,000	1,900,000	800,000
Mining Equipment And Processing Facilities
Property, Plant and Equipment
Property, plant and equipment, at cost	1,583,329,000	1,434,674,000
Land And Mineral Rights
Property, Plant and Equipment
Property, plant and equipment, at cost	369,347,000	303,725,000
Buildings, Office Equipment And Improvements
Property, Plant and Equipment
Property, plant and equipment, at cost	226,672,000	208,351,000
Construction and mine development in progress
Property, Plant and Equipment
Property, plant and equipment, at cost	194,221,000	162,331,000
Mine development costs
Property, Plant and Equipment
Property, plant and equipment, at cost	$ 272,303,000	$ 252,782,000