Quarterly report pursuant to Section 13 or 15(d)

PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details)

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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
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.