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Commitments and Contingencies - Restricted Investments (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
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Long-term Purchase Commitment [Line Items] |  |
Total restricted investments | $ 38,055 | $ 40,597
Workers’ compensation and black lung obligations |  |
Long-term Purchase Commitment [Line Items] |  |
Total restricted investments | 3,279 | 2,514
Reclamation-related obligations |  |
Long-term Purchase Commitment [Line Items] |  |
Total restricted investments | 30,305 | 33,173
Financial payments and other performance obligations |  |
Long-term Purchase Commitment [Line Items] |  |
Total restricted investments | $ 4,471 | $ 4,910

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- Definition Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 954 -SubTopic 210 -Name Accounting Standards Codification -Section 45 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480632/954-210-45-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 954 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480602/954-210-50-2 + Details
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