COMPONENTS OF COAL WORKERS’ PNEUMOCONIOSIS (CWP) AND WORKERS’ COMPENSATION NET PERIODIC BENEFIT COSTS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 948	$ 1,662	$ 2,844	$ 4,987
Interest Cost	1,750	1,311	5,250	3,934
Amortization of Actuarial Loss (Gain)	254	(213)	762	(640)
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit (Credit) Cost	2,952	2,760	8,856	8,281
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,421	1,558	4,264	4,673
Interest Cost	646	571	1,939	1,712
Amortization of Actuarial Loss (Gain)	(193)	(20)	(580)	(59)
State Administrative Fees and Insurance Bond Premiums	510	675	1,671	1,986
Net Periodic Benefit (Credit) Cost	$ 2,384	$ 2,784	$ 7,294	$ 8,312