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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative [Line Items] |  |  |
Natural Gas Hedges | $ 3.5 |  |
Unrealized net gain (loss) on derivative financial instruments, net of tax | $ 0.5 | $ 2.6 | $ (20.7)
Current Fiscal Year End Date | --12-31 |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | $ 104.7 | 92.6 | 5.5
Derivative asset, fair value | 39.4 | 33.1 |
Derivative liability, fair value | 2.7 | 0.5 |
Derivative assets | 39.4 | 33.1 |
Accounts receivable, net | 140.6 | 128.7 |
Not Designated as Hedging Instrument [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | 39.4 | 33.1 |
Derivative liability, fair value | 2.4 | 0.5 |
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member] |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments |  |  | (3.6)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 0.0 | 0.0 |
Foreign Exchange Contract [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 0.0 | 0.0 | (12.6)
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | 163.3 | 41.7 | 27.1
Provisional Pricing Arrangements [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | (58.6) | 49.0 | (1.4)
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | 58.6 | 49.0 | $ (1.4)
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | 37.9 | 21.3 |
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | 1.5 | 10.3 |
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative liability, fair value | $ 2.4 | $ 0.5 |

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