PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 4,973,400		$ 4,973,400		$ 4,838,171
Less—Accumulated Depreciation, Depletion and Amortization	2,866,649		2,866,649		2,731,643
Total Property, Plant and Equipment—Net	2,106,751		2,106,751		2,106,528
Gross assets under finance lease	49,899		49,899
Gross assets under capital lease					49,775
Accumulated amortization for finance leases	27,458		27,458
Accumulated amortization for capital leases					15,973
Amortization of right of use assets	3,919		11,753
Amortization expense for assets under capital lease		$ 3,927		$ 9,236
Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	3,008,945		3,008,945		2,890,970
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	863,191		863,191		858,153
Airshafts
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	430,592		430,592		419,100
Mine Development
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	342,706		342,706		342,405
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 327,966		$ 327,966		$ 327,543