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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.

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