COMPONENTS OF COAL WORKERS’ PNEUMOCONIOSIS (CWP) AND WORKERS’ COMPENSATION NET PERIODIC BENEFIT COSTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 948	$ 1,662	$ 1,896	$ 3,325
Interest Cost	1,750	1,311	3,500	2,623
Amortization of Actuarial Loss (Gain)	254	(213)	508	(427)
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit (Credit) Cost	2,952	2,760	5,904	5,521
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,421	1,558	2,842	3,115
Interest Cost	646	571	1,293	1,141
Amortization of Actuarial Loss (Gain)	(193)	(20)	(387)	(39)
State Administrative Fees and Insurance Bond Premiums	574	718	1,161	1,311
Net Periodic Benefit (Credit) Cost	$ 2,448	$ 2,827	$ 4,909	$ 5,528