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Property, Plant, Equipment and Mine Development
 12 Months Ended
Dec. 31, 2021
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, 2021 and December 31, 2020 consisted of the following:
December 31,
20212020
(Dollars in millions)
Land and coal interests$2,494.1 $2,482.9 
Buildings and improvements550.8 481.0 
Machinery and equipment1,386.5 1,408.5 
Less: Accumulated depreciation, depletion and amortization(1,480.5)(1,321.3)
Property, plant, equipment and mine development, net$2,950.6 $3,051.1 
Land and coal interests included coal reserves and resources with a net book value of $1.4 billion as of December 31, 2021 and $1.5 billion as of December 31, 2020. Such coal reserves and resources were comprised of mineral rights for leased coal interests and advance royalties that had a net book value of $0.8 billion as of both December 31, 2021 and 2020, and coal reserves and resources held by fee ownership of $0.6 billion and $0.7 billion at December 31, 2021 and 2020, 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, 2021 and 2020.