COMPONENTS OF COAL WORKERS’ PNEUMOCONIOSIS (CWP) AND WORKERS’ COMPENSATION NET PERIODIC BENEFIT COSTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 1,662	$ 1,280
Interest Cost	1,311	1,013
Amortization of Actuarial Gain	(213)	(1,908)
State Administrative Fees and Insurance Bond Premiums	0	0
Net Periodic Benefit (Credit) Cost	2,760	385
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,558	1,568
Interest Cost	571	580
Amortization of Actuarial Gain	(20)	(149)
State Administrative Fees and Insurance Bond Premiums	593	783
Net Periodic Benefit (Credit) Cost	$ 2,702	$ 2,782