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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2021
Accounting Policies [Abstract] |
Property, Plant and Equipment | The estimated useful lives by category of assets are as follows:
| Years
Building and improvements | up to 29
Machinery and equipment | 1 - 15
Leasehold improvements | Shorter of Useful Life or Remaining Life of Lease
Property, plant, equipment and mine development, net, as of December 31, 2021 and December 31, 2020 consisted of the following:
| December 31,
| 2021 |  | 2020
| (Dollars in millions)
Land and coal interests | $ | 2,494.1 |  |  | $ | 2,482.9 |
Buildings and improvements | 550.8 |  |  | 481.0 |
Machinery and equipment | 1,386.5 |  |  | 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 |

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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