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.