COMPONENTS OF COAL WORKERS’ PNEUMOCONIOSIS (CWP) AND WORKERS’ COMPENSATION NET PERIODIC BENEFIT COSTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 1,151	$ 948
Interest Cost	1,551	1,750
Amortization of Actuarial Loss (Gain)	1,401	254
State Administrative Fees and Insurance Bond Premiums	0	0
Net Periodic Benefit (Credit) Cost	4,103	2,952
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,569	1,421
Interest Cost	461	646
Amortization of Actuarial Loss (Gain)	(122)	(193)
State Administrative Fees and Insurance Bond Premiums	621	587
Net Periodic Benefit (Credit) Cost	$ 2,529	$ 2,461