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Property, Plant, Equipment and Mine Development
 3 Months Ended
Mar. 31, 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 March 31, 2024 and December 31, 2023 is set forth in the table below:
March 31, 2024December 31, 2023
(Dollars in millions)
Land and coal interests$2,475.1 $2,475.2 
Buildings and improvements645.4 647.6 
Machinery and equipment1,842.0 1,787.6 
Less: Accumulated depreciation, depletion and amortization(2,132.3)(2,066.3)
Property, plant, equipment and mine development, net$2,830.2 $2,844.1 
Asset Impairment and Other At-Risk Assets
The Company identified certain assets with an aggregate carrying value of approximately $219 million at March 31, 2024 in its Other U.S. Thermal segment whose recoverability is most sensitive to customer concentration risk.