PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,758,267		$ 4,758,267		$ 4,676,353
Less: Accumulated Depreciation, Depletion and Amortization	2,648,812		2,648,812		2,554,056
Total Property, Plant and Equipment—Net	2,109,455		2,109,455		2,122,297
Gross assets under capital lease	49,619		49,619		3,559
Accumulated amortization for capital leases	8,128		8,128		2,839
Amortization expense for assets under capital lease	3,934	$ 99	5,309	$ 212
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,829,805		2,829,805		2,757,062
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	858,833		858,833		857,031
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	398,573		398,573		392,266
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	344,146		344,146		344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 326,910		$ 326,910		$ 325,855