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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Total defined benefit pension plan adjustments | $ (9,681) | $ (31,122) | $ 5,069
Pneumoconiosis benefits |  |  |
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Net actuarial gain (loss) | (7,787) | (23,298) | 4,599
Reversal of amortization item: Net actuarial (gain) loss | (686) | (4,582) | 2
Total defined benefit pension plan adjustments | $ (8,473) | $ (27,880) | $ 4,601
Estimated present value of future obligations and other information |  |  |
Pneumoconiosis discount rate | 2.38% | 3.12% | 4.13%
Amount recognized in accumulated other comprehensive income consists of: |  |  |
Net actuarial loss | $ 40,399 | $ 31,927 | $ 4,047

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