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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - AOCL (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Total recognized in accumulated other comprehensive loss | $ (20,471) | $ 23,175 | $ 23,445
Pneumoconiosis benefits |  |  |
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Net actuarial gain (loss) | (25,615) | 9,840 | (161)
Reversal of amortization item: Net actuarial loss (gain) | 1,382 | 1,038 | 4,172
Total recognized in accumulated other comprehensive loss | $ (24,233) | $ 10,878 | $ 4,011
Estimated present value of future obligations and other information |  |  |
Pneumoconiosis discount rate | 4.81% | 5.00% | 2.73%
Amount recognized in accumulated other comprehensive income consists of: |  |  |
Net actuarial loss | $ 49,745 | $ 25,510 | $ 36,388

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