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ACQUISITION OF AK STEEL - Pro Forma Information (Details) - AK Steel Holding Corporation [Member] - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] |  |  |  |
Business Acquisition, Pro Forma Revenue | $ 979.1 | $ 2,233.8 | $ 2,427.7 | $ 4,021.1
Business Acquisition, Pro Forma Net Income (Loss) | (125.3) | 184.9 | $ (163.7) | 128.3
Business Acquisition, Pro Forma Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent |  |  | 24.30% |
Business Acquisition, Pro Forma Income Tax Expense (Benefit) | 1.6 | (8.0) | $ (3.3) | 2.8
Fair Value Adjustment to Inventory [Member] |  |  |  |
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] |  |  |  |
Business Acquisition, Pro Forma Net Income (Loss) | 36.2 | 18.5 | 59.4 | 74.2
Acquisition-related Costs [Member] |  |  |  |
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] |  |  |  |
Business Acquisition, Pro Forma Net Income (Loss) | 1.8 |  | 28.4 |
Other Adjustments [Member] |  |  |  |
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] |  |  |  |
Business Acquisition, Pro Forma Net Income (Loss) | (12.3) | (4.0) | (1.0) | (6.9)
Intersegment Eliminations [Member] |  |  |  |
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] |  |  |  |
Business Acquisition, Pro Forma Revenue | $ 113.6 | $ 189.9 | $ 259.2 | $ 257.3

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