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

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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