OTHER ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Subsidence Liability	$ 90,311	$ 88,027
Accrued Payroll and Benefits	16,045	14,689
Accrued Interest	13,305	10,039
Litigation	9,270	8,197
Accrued Other Taxes	5,164	7,510
Short-Term Incentive Compensation	4,862	4,729
Deferred Revenue	155	6,807
Longwall Equipment Buyout	0	22,631
Equipment Lease Rental	0	9,865
Other	18,424	23,900
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	37,238	37,464
Asset Retirement Obligations	31,823	30,480
Workers' Compensation	12,369	13,317
Pneumoconiosis Benefits	10,792	12,972
Total Other Accrued Liabilities	$ 249,758	$ 290,627