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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Total adjustments | $ (31,122) | $ 5,069 | $ (13,400)
Pneumoconiosis benefits |  |  |
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss: |  |  |
Net actuarial loss | (23,298) | 4,599 | (7,938)
Reversal of amortization item: net actuarial gain | (4,582) | 2 | (2,092)
Total adjustments | $ (27,880) | $ 4,601 | $ (10,030)
Estimated present value of future obligations and other information |  |  |
Pneumoconiosis discount rate | 3.12% | 4.13% | 3.49%
Amount recognized in accumulated other comprehensive income consists of: |  |  |
Net actuarial loss | $ 31,927 | $ 4,047 | $ 8,648

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