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Property, Plant, Equipment and Mine Development
 3 Months Ended
Mar. 31, 2021
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, 2021 and December 31, 2020 is set forth in the table below:
March 31, 2021December 31, 2020
(Dollars in millions)
Land and coal interests$2,484.1 $2,482.9 
Buildings and improvements484.7 481.0 
Machinery and equipment1,442.5 1,408.5 
Less: Accumulated depreciation, depletion and amortization(1,385.9)(1,321.3)
Property, plant, equipment and mine development, net$3,025.4 $3,051.1 
Asset Impairment and Other At-Risk Assets
The Company has identified certain assets with an aggregate carrying value of approximately $1.2 billion at March 31, 2021 in its Seaborne Metallurgical Mining, Powder River Basin Mining, Other U.S. Thermal Mining and Corporate and Other segments whose recoverability is most sensitive to coal pricing, cost pressures, customer demand, customer concentration risk and future economic viability. The Company conducted a review of those assets for recoverability as of March 31, 2021 and determined that no impairment charges were necessary as of that date.