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Property, Plant, Equipment and Mine Development
 9 Months Ended
Sep. 30, 2024
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, 2024 and December 31, 2023 is set forth in the table below:
September 30, 2024December 31, 2023
(Dollars in millions)
Land and coal interests$2,624.4 $2,475.2 
Buildings and improvements658.1 647.6 
Machinery and equipment2,021.4 1,787.6 
Less: Accumulated depreciation, depletion and amortization(2,290.4)(2,066.3)
Property, plant, equipment and mine development, net$3,013.5 $2,844.1 
At-Risk Assets
The Company identified certain assets with an aggregate carrying value of approximately $207 million at September 30, 2024 in its Other U.S. Thermal segment whose recoverability is most sensitive to customer concentration risk.