OTHER ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Subsidence Liability	$ 87,633	$ 88,027
Longwall Equipment Buyout	23,348	22,631
Accrued Interest	16,410	10,039
Accrued Payroll and Benefits	14,511	14,689
Litigation	10,190	8,197
Accrued Other Taxes	8,486	7,510
Equipment Lease Rental	7,839	9,865
Deferred Revenue	1,967	6,807
Short-Term Incentive Compensation	1,764	4,729
Other	19,959	23,900
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	37,388	37,464
Asset Retirement Obligations	30,480	30,480
Workers' Compensation	12,912	13,317
Pneumoconiosis Benefits	12,245	12,972
Total Other Accrued Liabilities	$ 285,132	$ 290,627