PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 4,927,492		$ 4,927,492		$ 4,838,171
Less—Accumulated Depreciation, Depletion and Amortization	2,824,348		2,824,348		2,731,643
Total Property, Plant and Equipment—Net	2,103,144		2,103,144		2,106,528
Gross assets under finance lease	49,903		49,903
Gross assets under capital lease					49,775
Accumulated amortization for finance leases	23,571		23,571
Accumulated amortization for capital leases					15,973
Amortization of right of use assets	3,920		7,834
Amortization expense for assets under capital lease		$ 3,934		$ 5,309
Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	2,971,535		2,971,535		2,890,970
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	858,153		858,153		858,153
Airshafts
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	427,254		427,254		419,100
Mine Development
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	342,685		342,685		342,405
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 327,865		$ 327,865		$ 327,543