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Property, Plant, Equipment and Mine Development | 12 Months Ended
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Dec. 31, 2023
Property, Plant and Equipment [Abstract] |
Property, Plant and Equipment | Property, Plant, Equipment and Mine Development Property, plant, equipment and mine development, net, as of December 31, 2023 and 2022 consisted of the following:
| December 31,
| 2023 |  | 2022
| (Dollars in millions)
Land and coal interests | $ | 2,475.2 |  |  | $ | 2,514.7 |
Buildings and improvements | 647.6 |  |  | 594.2 |
Machinery and equipment | 1,787.6 |  |  | 1,543.1 |
Less: Accumulated depreciation, depletion and amortization | (2,066.3) |  |  | (1,787.0) |
Property, plant, equipment and mine development, net | $ | 2,844.1 |  |  | $ | 2,865.0 |
Land and coal interests included coal reserves and resources with a net book value of $1.2 billion and $1.3 billion as of December 31, 2023 and 2022, respectively. Such coal reserves and resources were comprised of mineral rights for leased coal interests and advance royalties that had a net book value of $0.7 billion as of both December 31, 2023 and 2022, and coal reserves and resources held by fee ownership of $0.5 billion and $0.6 billion as of December 31, 2023 and 2022, respectively. The amount of coal reserves and resources unassigned to active mining operations, and thus not subject to current depletion, including certain exploratory properties, was $0.1 billion as of both December 31, 2023 and 2022.

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- Definition The entire disclosure for long-lived, physical asset used in normal conduct of business and not intended for resale. Includes, but is not limited to, work of art, historical treasure, and similar asset classified as collections. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 360 -Name Accounting Standards Codification -Publisher FASB -URI https://asc.fasb.org//360/tableOfContent Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 6 -SubTopic 360 -Topic 958 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480321/958-360-50-6 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d) -SubTopic 360 -Topic 958 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480321/958-360-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 50 -Paragraph 7 -SubTopic 360 -Topic 958 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480321/958-360-50-7 + Details
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