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Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Increase (decrease) in asset retirement obligation	$ (5,400)
Company transferred	3,400
Escrow account	2,400
Net loss on the transactions	400
Revision of estimated cash flows	3,145	$ (3,495)
Bellaire
Segment Reporting Information [Line Items]
Fair value of trust assets	10,100	5,000
Operating Segments | Coal Mining
Segment Reporting Information [Line Items]
Revision of estimated cash flows	3,145	$ (2,777)
Centennial
Segment Reporting Information [Line Items]
Increase (decrease) in asset retirement obligation	2,000
Revision of estimated cash flows	2,326
Centennial | Operating Segments | Coal Mining
Segment Reporting Information [Line Items]
Revision of estimated cash flows	$ 2,479

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- Definition

Asset Retirement Obligation, Amount Transferred To Escrow

+ References

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- Definition

Asset Retirement Obligation, Gain (Loss) Recognized On Transaction

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- Definition

Amount of cash paid during the period to settle an asset retirement obligation. Amounts paid to settle an asset retirement obligation are generally included in the operating section of the Statement of Cash Flows.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 17
-Subparagraph (e)
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Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
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- Definition

Amount of increase (decrease) in asset retirement obligations.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
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