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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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