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Summary of Significant Accounting Policies (Tables)
12 Months Ended
Dec. 31, 2023
Accounting Policies [Abstract]  
Property, Plant and Equipment estimated useful lives of the building and improvements and machinery and equipment asset categories range from 1 to 30 years. The estimated life of leasehold improvements is the shorter of useful life or remaining life of the lease.
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