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Property, Plant, Equipment and Mine Development
 3 Months Ended
Mar. 31, 2022
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, 2022 and December 31, 2021 is set forth in the table below:
March 31, 2022December 31, 2021
(Dollars in millions)
Land and coal interests$2,494.1 $2,494.1 
Buildings and improvements590.2 550.8 
Machinery and equipment1,370.9 1,386.2 
Less: Accumulated depreciation, depletion and amortization(1,551.9)(1,480.5)
Property, plant, equipment and mine development, net$2,903.3 $2,950.6 
Asset Impairment and Other At-Risk Assets
The Company has identified certain assets with an aggregate carrying value of approximately $0.5 billion at March 31, 2022 in its Other U.S. Thermal Mining and Corporate and Other segments whose recoverability is most sensitive to customer demand, customer concentration risk and future economic viability. The Company conducted a review of those assets as of March 31, 2022 and determined that no impairment charges were necessary as of that date.