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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 1,662	$ 1,280	$ 4,987	$ 3,842
Interest Cost	1,311	1,013	3,934	3,038
Amortization of Actuarial Gain	(213)	(1,908)	(640)	(5,724)
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit (Credit) Cost	2,760	385	8,281	1,156
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,558	1,569	4,673	4,706
Interest Cost	571	580	1,712	1,740
Amortization of Actuarial Gain	(20)	(150)	(59)	(449)
State Administrative Fees and Insurance Bond Premiums	675	609	1,986	1,969
Net Periodic Benefit (Credit) Cost	$ 2,784	$ 2,608	$ 8,312	$ 7,966