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Property, Plant, Equipment and Mine Development
 12 Months Ended
Dec. 31, 2025
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, 2025 and 2024 consisted of the following:
December 31,
20252024
(Dollars in millions)
Land and coal interests$2,691.2 $2,648.5 
Buildings and improvements758.8 726.7 
Machinery and equipment2,377.1 2,078.8 
Less: Accumulated depreciation, depletion and amortization(2,673.8)(2,372.5)
Property, plant, equipment and mine development, net$3,153.3 $3,081.5 
Land and coal interests included coal reserves and resources with a net book value of $1.1 billion as of both December 31, 2025 and 2024. Such coal reserves and resources were comprised of mineral rights for leased coal interests and advance royalties that had a net book value of $0.6 billion as of both December 31, 2025 and 2024 and coal reserves and resources held by fee ownership of $0.5 billion as of both December 31, 2025 and 2024. The amount of coal reserves and resources unassigned to active mining operations, and thus not subject to current depletion, including certain exploratory properties, of $0.1 billion as of both at December 31, 2025 and 2024.
The Company identified certain assets with an aggregate carrying value of approximately $64 million at December 31, 2025 in its Other U.S. Thermal reportable segment whose recoverability is most sensitive to customer concentration risk.