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FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | $ 93.6 | $ 51.0 | $ 39.4
Derivative Liability | $ 4.4 |  | 2.7
Management Estimate of 62% Fe | 62.00% |  |
Impairment Charges, CIP | $ 2.6 |  |
Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 0.0 |  | 0.0
Derivative Liability | 0.0 |  | 0.0
Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 0.0 |  | 0.0
Derivative Liability | 0.2 |  | 0.3
Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 93.6 |  | 39.4
Derivative Liability | 4.2 |  | 2.4
Not Designated as Hedging Instrument [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 93.6 |  | 39.4
Derivative Liability | 4.2 |  | 2.4
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 91.2 |  | 37.9
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative assets | 2.4 |  | 1.5
Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative Liability | 0.0 |  | 0.0
Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Derivative Liability | $ 4.2 |  | $ 2.4

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