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Property, Plant, Equipment and Mine Development
 12 Months Ended
Dec. 31, 2023
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment Property, Plant, Equipment and Mine Development
Property, plant, equipment and mine development, net, as of December 31, 2023 and 2022 consisted of the following:
December 31,
20232022
(Dollars in millions)
Land and coal interests$2,475.2 $2,514.7 
Buildings and improvements647.6 594.2 
Machinery and equipment1,787.6 1,543.1 
Less: Accumulated depreciation, depletion and amortization(2,066.3)(1,787.0)
Property, plant, equipment and mine development, net$2,844.1 $2,865.0 
Land and coal interests included coal reserves and resources with a net book value of $1.2 billion and $1.3 billion as of December 31, 2023 and 2022, respectively. Such coal reserves and resources were comprised of mineral rights for leased coal interests and advance royalties that had a net book value of $0.7 billion as of both December 31, 2023 and 2022, and coal reserves and resources held by fee ownership of $0.5 billion and $0.6 billion as of December 31, 2023 and 2022, respectively. The amount of coal reserves and resources unassigned to active mining operations, and thus not subject to current depletion, including certain exploratory properties, was $0.1 billion as of both December 31, 2023 and 2022.