PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,604,067		$ 4,604,067		$ 4,593,395
Less: Accumulated Depreciation, Depletion and Amortization	2,508,143		2,508,143		2,413,125
Total Property, Plant and Equipment—Net	2,095,924		2,095,924		2,180,270
Gross assets under capital lease	3,529		3,529		3,547
Accumulated amortization for capital leases	2,717		2,717		2,399
Amortization expense for assets under capital lease	104	$ 116	315	$ 304
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,685,707		2,685,707		2,680,453
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	388,299		388,299		381,755
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	859,030		859,030		861,048
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	344,139		344,139		344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 326,892		$ 326,892		$ 326,000