PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,722,123		$ 4,676,353
Less: Accumulated Depreciation, Depletion and Amortization	2,599,775		2,554,056
Total Property, Plant and Equipment—Net	2,122,348		2,122,297
Gross assets under capital lease	26,255		3,559
Accumulated amortization for capital leases	4,215		2,839
Amortization expense for assets under capital lease	1,376	$ 113
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,799,723		2,757,062
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	858,218		857,031
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	393,347		392,266
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	344,139		344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 326,696		$ 325,855