PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 4,796,141		$ 4,796,141		$ 4,676,353
Less: Accumulated Depreciation, Depletion and Amortization	2,692,450		2,692,450		2,554,056
Total Property, Plant and Equipment—Net	2,103,691		2,103,691		2,122,297
Gross assets under capital lease	49,619		49,619		3,559
Accumulated amortization for capital leases	12,058		12,058		2,839
Amortization expense for assets under capital lease	3,927	$ 104	9,236	$ 315
Plant and Equipment
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	2,860,449		2,860,449		2,757,062
Coal Properties and Surface Lands
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	857,814		857,814		857,031
Airshafts
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	406,726		406,726		392,266
Mine Development
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	344,147		344,147		344,139
Advance Mining Royalties
Property, Plant and Equipment [Line Items]
Total Property, Plant and Equipment	$ 327,005		$ 327,005		$ 325,855