OTHER ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Subsidence Liability	$ 89,055	$ 83,532
Accrued Payroll and Benefits	15,985	12,978
Accrued Interest	12,705	6,850
Litigation	9,673	8,235
Accrued Other Taxes	4,782	5,050
Short-Term Incentive Compensation	2,608	6,024
Other	13,648	15,588
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	32,351	32,345
Asset Retirement Obligations	31,017	31,017
Operating Lease Liability	19,600
Workers' Compensation	12,135	12,628
Pneumoconiosis Benefits	11,720	12,187
Total Other Accrued Liabilities	$ 255,279	$ 226,434