WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Liability (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of changes in the workers' compensation liability
Beginning balance	$ 52,059	$ 54,478	$ 54,739	$ 53,384	$ 54,739	$ 53,384	$ 53,384
Accruals increase	1,674	1,599			4,911	3,539
Payments	(2,939)	(2,123)			(7,914)	(6,686)
Interest accretion	232	319			695	958
Valuation loss (gain)					(1,405)	3,078
Ending balance	51,026	$ 54,273	$ 52,059	$ 54,478	51,026	$ 54,273	$ 54,739	$ 53,384
Workers' compensation discount rate			2.34%	2.05%			1.95%	2.81%
Receivables for traumatic injury claims	$ 7,100				$ 7,100

Represents the change in the liability for workers' compensation due to payments made during the period.

No definition available.

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- Definition

Represents the change in the liability for workers' compensation due to an adjustment for valuation gain or loss.

+ References

No definition available.

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- Definition

Represents the change in the liability for workers' compensation due to additional amounts accrued during the period.

+ References

No definition available.

+ Details

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- References

No definition available.

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- Definition

Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

+ References

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- Definition

Carrying amount due after one year of the balance sheet date (or one operating cycle, if longer) of amount due in settlement of a claim for reimbursement from an insurance company when the Company has suffered a loss covered under an insurance policy.

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- Definition

Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value.

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- Definition

Carrying value as of the balance sheet date of obligations and payables pertaining to claims incurred of a workers compensation nature.

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