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Property, Plant, Equipment and Mine Development | 12 Months Ended
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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,
| 2020 |  | 2019
| (Dollars in millions)
Land and coal interests | $ | 2,482.9 |  |  | $ | 4,022.4 |
Buildings and improvements | 481.0 |  |  | 547.9 |
Machinery and equipment | 1,408.5 |  |  | 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.

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- Definition The entire disclosure for long-lived, physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, accounting policies and methodology, roll forwards, depreciation, depletion and amortization expense, including composite depreciation, accumulated depreciation, depletion and amortization expense, useful lives and method used, income statement disclosures, assets held for sale and public utility disclosures. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -URI http://asc.fasb.org/topic&trid=2155823 + Details
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