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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Percentage of qualifying income for tax purposes	90.00%
Assets and Liabilities, Lessee
Operating lease liabilities	$ 14,932
Offsetting right-of-use assets	$ 15,004	$ 17,660
Minimum
Revenue Recognition
Coal shipments, payment period	14 days
Maximum
Revenue Recognition
Coal shipments, payment period	28 days
Defined benefit pension plan
Employee Benefit Plans
Threshold for amortization of unrecognized actuarial gains and losses (as a percent)	10.00%

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- Definition

Represents the minimum percentage of the entity's income consisting of qualifying income, per which the entity is not taxable for federal or state income tax purposes.

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- Definition

Period of time within which payments are due once performance obligation is met.

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- Definition

Unrecognized actuarial gains and losses as a percentage of the greater of the benefit obligation or the market-related value of plan assets, above which such gains and losses are amortized over the participants' average remaining future years of service.

+ References

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- References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

+ Details

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Balance Type:	credit
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- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

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