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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2023
Accounting Policies [Abstract] |
Property, Plant and Equipment | estimated useful lives of the building and improvements and machinery and equipment asset categories range from 1 to 30 years. The estimated life of leasehold improvements is the shorter of useful life or remaining life of the lease. 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 |

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 10 -Topic 360 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482099/360-10-50-1 + Details
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