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PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019
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Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross | [1] | $ 5,812.2 | $ 3,027.4
Allowance for depreciation and depletion |  | (1,262.4) | (1,098.4)
Property, plant and equipment, net |  | 4,549.8 | 1,929.0
Finance Lease, Right-of-Use Asset |  | 84.2 | 49.0
Land, land improvements and mineral rights |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | 652.7 | 582.2
Buildings |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | 452.5 | 157.8
Mining and Pelletizing equipment |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | 1,431.8 | 1,413.6
Steel and Manufacturing equipment |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | 2,140.9 | 42.0
Other |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | 123.0 | 101.5
Construction-in-progress |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Property, plant and equipment, gross |  | $ 1,011.3 | $ 730.3
[1] | 1 Includes right-of-use assets related to finance leases of $84.2 million and $49.0 million as of March 31, 2020 and December 31, 2019, respectively.

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- Definition Amount of lessee's right to use underlying asset under finance lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
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