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Property, Plant, Equipment and Mine Development
 9 Months Ended
Sep. 30, 2023
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, 2023 and December 31, 2022 is set forth in the table below:
September 30, 2023December 31, 2022
(Dollars in millions)
Land and coal interests$2,511.9 $2,514.7 
Buildings and improvements633.5 594.2 
Machinery and equipment1,671.0 1,543.1 
Less: Accumulated depreciation, depletion and amortization(2,010.9)(1,787.0)
Property, plant, equipment and mine development, net$2,805.5 $2,865.0 
Asset Impairment and Other At-Risk Assets
The Company identified certain assets with an aggregate carrying value of approximately $388 million at September 30, 2023 in its Powder River Basin and Other U.S. Thermal segments whose recoverability is most sensitive to customer demand and customer concentration risk. The Company conducted a review of those assets as of September 30, 2023 and determined that no impairment charges were necessary as of that date.