Asset Retirement Obligations (ARO Activity) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	$ 37,703	$ 40,097
Liabilities incurred during the period	91	189
Liabilities settled during the period	(9,017)	(1,667)
Accretion expense	2,611	2,579
Revision of estimated cash flows	3,145	(3,495)
Carrying amount of the asset retirement obligations, balance at end of period	36,859	37,703
Operating Segments \| Coal Mining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	20,396	22,589
Liabilities incurred during the period	0	0
Liabilities settled during the period	(8,265)	(920)
Accretion expense	1,260	1,504
Revision of estimated cash flows	3,145	(2,777)
Carrying amount of the asset retirement obligations, balance at end of period	19,015	20,396
Operating Segments \| NAMining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	485	1,085
Liabilities incurred during the period	91	189
Liabilities settled during the period	0	0
Accretion expense	28	31
Revision of estimated cash flows	0	(820)
Carrying amount of the asset retirement obligations, balance at end of period	604	485
Unallocated Items
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	16,822	16,423
Liabilities incurred during the period	0	0
Liabilities settled during the period	(752)	(747)
Accretion expense	1,323	1,044
Revision of estimated cash flows	0	102
Carrying amount of the asset retirement obligations, balance at end of period	17,240	$ 16,822
Centennial
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows	2,326
Centennial \| Operating Segments \| Coal Mining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows	2,479
Centennial \| Operating Segments \| NAMining
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows	0
Centennial \| Unallocated Items
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revision of estimated cash flows	$ (153)

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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- Definition

Amount of asset retirement obligations incurred during the period.

+ References

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- Definition

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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- Definition

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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