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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Accrued Workers Compensation And Pneumoconiosis Benefits |  |  |  |
Interest cost | $ 1,030 | $ 1,217 | $ 3,138 | $ 3,649
Net amortization | 969 | 980 | 3,096 | 2,941
Net periodic benefit cost | 633 | 1,010 | 1,978 | 3,030
Pneumoconiosis benefits |  |  |  |
Accrued Workers Compensation And Pneumoconiosis Benefits |  |  |  |
Service cost | 882 | 656 | 2,643 | 1,947
Interest cost | 750 | 761 | 2,249 | 2,283
Net amortization | (172) | (1,145) | (515) | (3,436)
Net periodic benefit cost | $ 1,460 | $ 272 | $ 4,377 | $ 794

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- Definition Amount of cost recognized for passage of time related to defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + Details
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