SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION (Revenues from Product Sales and Services) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Realization of deferred revenue		$ 34.6		$ 0.0
Revenues	[1]	359.1		157.0
Steel and Manufacturing [Member]
Disaggregation of Revenue [Line Items]
Revenues		217.5		0.0
Steel and Manufacturing [Member] | Carbon Steel [Member]
Disaggregation of Revenue [Line Items]
Revenues		138.6		0.0
Steel and Manufacturing [Member] | Stainless and Electrical Steel [Member]
Disaggregation of Revenue [Line Items]
Revenues		59.4		0.0
Steel and Manufacturing [Member] | Other Product Lines [Member]
Disaggregation of Revenue [Line Items]
Revenues		19.5		0.0
Mining and Pelletizing [Member]
Disaggregation of Revenue [Line Items]
Revenues		141.6		157.0
Mining and Pelletizing [Member] | Iron Ore [Member]
Disaggregation of Revenue [Line Items]
Revenues		131.3	[2]	145.4
Mining and Pelletizing [Member] | Freight [Member]
Disaggregation of Revenue [Line Items]
Revenues		10.3		11.6
Automotive [Member] | Steel and Manufacturing [Member]
Disaggregation of Revenue [Line Items]
Revenues		120.2		0.0
Infrastructure and Manufacturing [Member] | Steel and Manufacturing [Member]
Disaggregation of Revenue [Line Items]
Revenues		44.0		0.0
Distributors and Converters [Member] | Steel and Manufacturing [Member]
Disaggregation of Revenue [Line Items]
Revenues		53.3		0.0
Steel Producers [Member] | Mining and Pelletizing [Member]
Disaggregation of Revenue [Line Items]
Revenues		141.6	[2]	157.0
Non-US [Member]
Disaggregation of Revenue [Line Items]
Revenues		$ 46.7		$ 43.0

[1] ¹ Includes Realization of deferred revenue of $34.6 million for the three months ended March 31, 2020.

[2] ¹ Includes Realization of deferred revenue of $34.6 million for the three months ended March 31, 2020.