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Property, Plant, Equipment and Mine Development
 9 Months Ended
Sep. 30, 2025
Property, Plant and Equipment [Abstract]  
Property, Plant, Equipment and Mine Development Property, Plant, Equipment and Mine Development
The composition of property, plant, equipment and mine development, net, as of September 30, 2025 and December 31, 2024 is set forth in the table below:
September 30, 2025December 31, 2024
(Dollars in millions)
Land and coal interests$2,645.1 $2,648.5 
Buildings and improvements754.1 726.7 
Machinery and equipment2,295.7 2,078.8 
Less: Accumulated depreciation, depletion and amortization(2,619.4)(2,372.5)
Property, plant, equipment and mine development, net$3,075.5 $3,081.5 
At-Risk Assets
The Company identified certain assets with an aggregate carrying value of approximately $68 million at September 30, 2025 in its Other U.S. Thermal segment whose recoverability is most sensitive to customer concentration risk.