COMPONENTS OF COAL WORKERS’ PNEUMOCONIOSIS (CWP) AND WORKERS’ COMPENSATION NET PERIODIC BENEFIT COSTS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 948	$ 1,662
Interest Cost	1,750	1,311
Amortization of Actuarial Loss (Gain)	254	(213)
State Administrative Fees and Insurance Bond Premiums	0	0
Net Periodic Benefit (Credit) Cost	2,952	2,760
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1,421	1,558
Interest Cost	646	571
Amortization of Actuarial Loss (Gain)	(193)	(20)
State Administrative Fees and Insurance Bond Premiums	587	593
Net Periodic Benefit (Credit) Cost	$ 2,461	$ 2,702