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Property, Plant, Equipment and Mine Development
 3 Months Ended
Mar. 31, 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 March 31, 2023 and December 31, 2022 is set forth in the table below:
March 31, 2023December 31, 2022
(Dollars in millions)
Land and coal interests$2,514.8 $2,514.7 
Buildings and improvements617.3 594.2 
Machinery and equipment1,578.5 1,543.1 
Less: Accumulated depreciation, depletion and amortization(1,862.7)(1,787.0)
Property, plant, equipment and mine development, net$2,847.9 $2,865.0