Asset Retirement Obligations (ARO Activity) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	$ 36,859	$ 37,703
Liabilities incurred during the period	9,809	91
Liabilities settled during the period	(6,709)	(9,017)
Accretion expense	2,815	2,611
Revision of estimated cash flows	(1,042)	5,471
Carrying amount of the asset retirement obligations, balance at end of period	41,732	36,859
Operating Segments \| Coal Mining customer
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	19,015	20,396
Liabilities incurred during the period	9,809	0
Liabilities settled during the period	(5,977)	(8,265)
Accretion expense	1,793	1,260
Revision of estimated cash flows	400	5,624
Carrying amount of the asset retirement obligations, balance at end of period	25,040	19,015
Operating Segments \| NAMining customer
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	604	485
Liabilities incurred during the period	0	91
Liabilities settled during the period	0	0
Accretion expense	0	28
Revision of estimated cash flows	(604)	0
Carrying amount of the asset retirement obligations, balance at end of period	0	604
Unallocated Items
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Carrying amount of the asset retirement obligations, balance at beginning of period	17,240	16,822
Liabilities incurred during the period	0	0
Liabilities settled during the period	(732)	(752)
Accretion expense	1,022	1,323
Revision of estimated cash flows	(838)	(153)
Carrying amount of the asset retirement obligations, balance at end of period	$ 16,692	$ 17,240

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

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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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