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Other Long-Term Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Other Liabilities Disclosure [Abstract] |  |
Black lung obligations | $ 34,229 | $ 34,567
Other | 2,985 | 2,258
Total other long-term liabilities | $ 37,214 | $ 36,825

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