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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