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FAIR VALUE OF FINANCIAL INSTRUMENTS (Schedule Of Quantitative Inputs And Assumptions For Level 3 Assets And Liabilities) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value $ 63,200,000us-gaap_DerivativeFairValueOfDerivativeAsset $ 59,200,000us-gaap_DerivativeFairValueOfDerivativeAsset
Derivative liability, fair value 43,300,000us-gaap_DerivativeFairValueOfDerivativeLiability 39,300,000us-gaap_DerivativeFairValueOfDerivativeLiability
Management Estimate of 62% Fe 62.00%clf_ManagementEstimateOf62Fe  
Not Designated as Hedging Instrument [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 63,200,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
58,900,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative liability, fair value 21,700,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
11,400,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Managements Estimate Of 62% Fe [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Management Estimate of 62% Fe 62.00%clf_ManagementEstimateOf62Fe
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Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Provisional Pricing Arrangements [Member] | Managements Estimate Of 62% Fe [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value measurement with unobservable inputs derivative asset range 72clf_FairValueMeasurementWithUnobservableInputsDerivativeAssetRange
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Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Customer Supply Agreement [Member] | Hot-Rolled Steel Estimate [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value measurement with unobservable inputs derivative asset range 640clf_FairValueMeasurementWithUnobservableInputsDerivativeAssetRange
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Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Customer Supply Agreement [Member] | Hot-Rolled Steel Estimate [Member] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value measurement with unobservable inputs derivative asset range 590clf_FairValueMeasurementWithUnobservableInputsDerivativeAssetRange
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Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Customer Supply Agreement [Member] | Hot-Rolled Steel Estimate [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value measurement with unobservable inputs derivative asset range 610clf_FairValueMeasurementWithUnobservableInputsDerivativeAssetRange
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Other Current Liabilities [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Provisional Pricing Arrangements [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative liability, fair value 11,800,000us-gaap_DerivativeFairValueOfDerivativeLiability
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10,300,000us-gaap_DerivativeFairValueOfDerivativeLiability
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Other Current Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Provisional Pricing Arrangements [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 0us-gaap_DerivativeFairValueOfDerivativeAsset
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3,100,000us-gaap_DerivativeFairValueOfDerivativeAsset
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Other Current Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Customer Supply Agreement [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value $ 63,200,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentAssetsMember
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$ 55,800,000us-gaap_DerivativeFairValueOfDerivativeAsset
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