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Property, Plant, Equipment and Mine Development
 12 Months Ended
Dec. 31, 2020
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, 2020 and December 31, 2019 consisted of the following:
December 31,
20202019
(Dollars in millions)
Land and coal interests$2,482.9 $4,022.4 
Buildings and improvements481.0 547.9 
Machinery and equipment1,408.5 1,518.6 
Less: Accumulated depreciation, depletion and amortization(1,321.3)(1,409.8)
Property, plant, equipment and mine development, net$3,051.1 $4,679.1 
Land and coal interests included coal reserves with a net book value of $1.5 billion as of December 31, 2020 and $2.8 billion as of December 31, 2019. Such coal reserves were comprised of mineral rights for leased coal interests and advance royalties that had a net book value of $0.8 billion and $2.0 billion as of December 31, 2020 and 2019, respectively, and coal reserves held by fee ownership of $0.7 billion and $0.8 billion at December 31, 2020 and 2019, respectively. The amount of coal reserves 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, 2020 and 2019.