Asset Retirement Obligations (ARO Activity) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period		$ 36,859
Liabilities incurred during the period	$ 9,800	9,809
Liabilities settled during the period		(6,519)
Accretion expense		2,347
Carrying amount of the asset retirement obligations, balance at end of period	43,096	43,096
Centennial
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows		600
Operating segments \| Coal Mining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period		19,015
Liabilities incurred during the period		9,809
Liabilities settled during the period		(5,957)
Accretion expense		1,296
Carrying amount of the asset retirement obligations, balance at end of period	24,763	24,763
Operating segments \| Coal Mining \| Centennial
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows		600
Operating segments \| NAMining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period		604
Liabilities incurred during the period		0
Liabilities settled during the period		0
Accretion expense		28
Carrying amount of the asset retirement obligations, balance at end of period	632	632
Operating segments \| NAMining \| Centennial
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows		0
Unallocated Items
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period		17,240
Liabilities incurred during the period		0
Liabilities settled during the period		(562)
Accretion expense		1,023
Carrying amount of the asset retirement obligations, balance at end of period	$ 17,701	17,701
Unallocated Items \| Centennial
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows		$ 0

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ 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 (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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Amount of asset retirement obligations incurred during the period.

+ References

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

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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
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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