WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accrued Workers Compensation And Pneumoconiosis Benefits
Interest cost	$ 1,054	$ 1,216
Net amortization	1,064	979
Net periodic benefit cost	672	1,010
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost	878	645
Interest cost	750	760
Net amortization	(172)	(1,145)
Net periodic benefit cost	$ 1,456	$ 260