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Property, Plant, Equipment and Mine Development - (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Property, Plant, Equipment and Mine Development[Line Items] |  |
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
Coal reserves | 1,500.0 | 2,800.0
Coal reserves not subject to depletion | 100.0 | 100.0
Land and coal interests |  |
Property, Plant, Equipment and Mine Development[Line Items] |  |
Property, plant equipment and mine development, gross | 2,482.9 | 4,022.4
Buildings and improvements |  |
Property, Plant, Equipment and Mine Development[Line Items] |  |
Property, plant equipment and mine development, gross | 481.0 | 547.9
Machinery and equipment |  |
Property, Plant, Equipment and Mine Development[Line Items] |  |
Property, plant equipment and mine development, gross | 1,408.5 | 1,518.6
Mining Properties and Mineral Rights |  |
Property, Plant, Equipment and Mine Development[Line Items] |  |
Mineral rights and advanced royalties | 800.0 | 2,000.0
Coal Reserves Held by Fee Ownership |  |
Property, Plant, Equipment and Mine Development[Line Items] |  |
Coal reserves | $ 700.0 | $ 800.0

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- Definition Net book value of coal reserves. + References No definition available. + Details
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- Definition Coal reserves not subject to depletion. + References No definition available. + Details
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- Definition Property, Plant, And Equipment And Finance Lease Right-Of-Use Asset, Accumulated Depreciation And Amortization + References No definition available. + Details
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- Definition Property, Plant And Equipment And Finance Lease Right-Of-Use Asset, After Accumulated Depreciation and Amortization + References No definition available. + Details
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- Definition Property, Plant, And Equipment And Finance Lease Right-Of-Use Asset, Before Accumulated Depreciation and Amortization + References No definition available. + Details
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- Definition Carrying amount at the balance sheet date of mineral rights, or rights to extract a mineral from the earth or to receive payment in the form of a royalty for the extraction of minerals, net of amortization. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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