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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Periodic Benefit Cost (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Accrued Workers Compensation And Pneumoconiosis Benefits |  |  |  |
Interest cost | $ 1,054 | $ 1,216 | $ 2,108 | $ 2,432
Net amortization | 1,063 | 982 | 2,127 | 1,961
Net periodic benefit cost | 673 | 1,010 | 1,345 | 2,020
Pneumoconiosis benefits |  |  |  |
Accrued Workers Compensation And Pneumoconiosis Benefits |  |  |  |
Service cost | 883 | 646 | 1,761 | 1,291
Interest cost | 749 | 762 | 1,499 | 1,522
Net amortization | (171) | (1,146) | (343) | (2,291)
Net periodic benefit cost | $ 1,461 | $ 262 | $ 2,917 | $ 522

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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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