OTHER ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Subsidence Liability	$ 96,022	$ 90,645
Accrued Payroll and Benefits	15,674	21,102
Accrued Interest	9,280	6,281
Accrued Other Taxes	5,407	4,753
Litigation	2,685	2,565
Short-Term Incentive Compensation	1,456	3,997
Other	12,139	9,719
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	31,359	31,833
Asset Retirement Obligations	21,741	21,741
Operating Lease Liability	18,249	19,479
Pneumoconiosis Benefits	12,251	12,331
Workers' Compensation	10,998	11,323
Total Other Accrued Liabilities	$ 237,261	$ 235,769