Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS (Schedule Of Derivative Instruments In Statement Of Financial Position, Fair Value) (Details)

v3.8.0.1
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