Mineral Rights, Net - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Net Book Value	$ 451,610,000		$ 460,373,000
Asset impairments	4,043,000	$ 0
Depletion and amortization expense on coal and other mineral rights	4,700,000	1,900,000
Depletion and amortization expense on coal and other mineral rights	4,700,000	$ 1,900,000
Coal Mineral Rights
Property, Plant and Equipment [Line Items]
Net Book Value	430,355,000		$ 438,850,000
Asset impairments	$ 4,000,000.0

Depletion and amortization expense on coal and other mineral rights.

No definition available.

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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- Definition

Mineral properties, net of adjustments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 360
-Section 25
-Paragraph 7
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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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