Commitments and Contingencies - Schedule of Restricted Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Long-term Purchase Commitment [Line Items]
Total restricted investments	$ 35,332	$ 23,768
Restricted investments, trading	34,573	22,498
Restricted investments, held-to-maturity	759	1,270
Workers' compensation and black lung obligations
Long-term Purchase Commitment [Line Items]
Total restricted investments	50	51
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted investments	33,224	22,233
Financial payments and other performance obligations
Long-term Purchase Commitment [Line Items]
Total restricted investments	$ 2,058	$ 1,484

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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- Definition

Amount of restricted investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(f))
-URI http://asc.fasb.org/extlink&oid=120401367&loc=d3e600178-122990

+ Details

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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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

No definition available.

+ Details

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