PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 5,053,698		$ 5,008,180
Less—Accumulated Depreciation, Depletion and Amortization	2,965,903		2,916,015
Total Property, Plant and Equipment—Net	2,087,795		2,092,165
Gross assets under finance lease	75,435		52,729
Accumulated amortization for finance leases	37,861		31,373
Amortization of right of use assets	4,964	$ 3,914
Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	3,066,656		3,028,514
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	873,597		872,909
Airshafts
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	443,600		437,003
Mine Development
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	342,707		342,706
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 327,138		$ 327,048