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DERIVATIVE INSTRUMENTS (Schedule Of Derivative Instruments In Statement Of Financial Position, Fair Value) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Current | $ 89.5 | $ 33.1
Derivative Liability, Current | 9.3 | 0.5
Derivative Asset | 89.5 | 33.1
Derivative Liability | 9.3 | 0.5
Not Designated as Hedging Instrument [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset | 89.5 | 33.1
Derivative Liability | 9.3 | 0.5
Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] | Derivative Financial Instruments, Assets [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Current | 84.8 | 21.3
Fair Value, Inputs, Level 3 [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset | 89.5 | 31.6
Derivative Liability | 9.3 | 0.5
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] | Derivative Financial Instruments, Liabilities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Liability | 0.0 | 0.0
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract [Member] | Other Current Assets [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset | 0.0 | 1.5
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract [Member] | Other Current Liabilities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Liability | 0.0 | 0.0
U S Iron Ore And Asia Pacific Iron Ore [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member] | Derivative Financial Instruments, Assets [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Asset, Current | 4.7 | 10.3
U S Iron Ore And Asia Pacific Iron Ore [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member] | Derivative Financial Instruments, Liabilities [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative Liability, Current | $ 9.3 | $ 0.5

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