PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 4,878,730		$ 4,838,171
Less—Accumulated Depreciation, Depletion and Amortization	2,778,448		2,731,643
Total Property, Plant and Equipment—Net	2,100,282		2,106,528
Gross assets under finance lease	49,865
Gross assets under capital lease			49,775
Accumulated amortization for finance leases	19,700
Accumulated amortization for capital leases			15,973
Amortization of right of use assets	3,914
Amortization expense for assets under capital lease		$ 1,376
Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	2,922,041		2,890,970
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	858,153		858,153
Airshafts
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	428,387		419,100
Mine Development
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	342,489		342,405
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 327,660		$ 327,543