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Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Settlement proceeds	$ 1,800,000
Throughput obligation		$ 0		$ 0		$ 0
Coal royalty expense		$ 15,700,000	$ 13,600,000	$ 28,400,000	$ 27,800,000

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- Definition

Amount awarded from other party in judgment or settlement of litigation.

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- Definition

Amount of the recorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=123406679&loc=d3e25383-109308

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Amount of expense related to royalty payments under a contractual arrangement such as payment for mineral and drilling rights and use of technology or intellectual property.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

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