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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 1,662	$ 1,280	$ 3,325	$ 2,562
Interest Cost	1,311	1,013	2,623	2,025
Amortization of Actuarial Gain	(213)	(1,908)	(427)	(3,816)
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit (Credit) Cost	2,760	385	5,521	771
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,558	1,568	3,115	3,137
Interest Cost	571	580	1,141	1,160
Amortization of Actuarial Gain	(20)	(149)	(39)	(299)
State Administrative Fees and Insurance Bond Premiums	718	577	1,311	1,360
Net Periodic Benefit (Credit) Cost	$ 2,827	$ 2,576	$ 5,528	$ 5,358