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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the changes in black lung benefit obligations
Benefit obligations at beginning of year	$ 72,095	$ 74,859
Service cost	2,593	2,525	$ 2,255
Interest cost	3,044	2,542	2,555
Actuarial (gain) loss	23,298	(4,599)
Benefits and expenses paid	(3,347)	(3,232)
Benefit obligations at end of year	$ 97,683	$ 72,095	$ 74,859

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- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of cost recognized for passage of time related to defined benefit plan.

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