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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 USD ($)
Income Taxes |
Percentage of qualifying income for tax purposes | 90.00%
Assets and Liabilities, Lessee |
Operating lease liabilities | $ 17,567
Offsetting right-of-use assets | $ 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 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=77888419&loc=SL77918627-209977 + Details
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