OTHER ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Subsidence Liability	$ 89,735	$ 88,027
Accrued Payroll and Benefits	19,121	14,689
Accrued Interest	9,949	10,039
Litigation	9,200	8,197
Accrued Other Taxes	8,575	7,510
Short-Term Incentive Compensation	3,354	4,729
Deferred Revenue	175	6,807
Longwall Equipment Buyout	0	22,631
Equipment Lease Rental	0	9,865
Other	17,642	23,900
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	37,313	37,464
Asset Retirement Obligations	31,823	30,480
Workers' Compensation	12,822	13,317
Pneumoconiosis Benefits	11,518	12,972
Total Other Accrued Liabilities	$ 251,227	$ 290,627