FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets And Liabilities Components [Line Items]
Management Estimate of 62% Fe	62.00%clf_ManagementEstimateOf62Fe
Goodwill, Impairment Loss	$ 0us-gaap_GoodwillImpairmentLoss	$ 73,500,000us-gaap_GoodwillImpairmentLoss
Other Asset Impairment Charges		562,000,000us-gaap_OtherAssetImpairmentCharges
Global Exploration Investment in Venture [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Equity Method Investments, Fair Value Disclosure		0
Equity Method Investment, Other than Temporary Impairment		9,200,000
Asia Pacific Iron Ore [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Goodwill, Impairment Loss		73,500,000
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Debt Instrument, Par Value	544,200,000
Debt Instrument, Fair Value Disclosure	397,200,000
Stated interest rate	7.75%
Debt discount	0.27
Debt Instrument, Unamortized Discount	147,000,000
Fair Value, Inputs, Level 2 [Member] \| Global Exploration Investment in Venture [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Equity Method Investments, Fair Value Disclosure		0
Fair Value, Inputs, Level 2 [Member] \| Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Debt Instrument, Fair Value Disclosure	$ 397,200,000	$ 0

Debt discount

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Debt Instrument, Par Value

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- Definition

Management Estimate of 62% Fe

+ References

No definition available.

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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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-Name Accounting Standards Codification

-Topic 210

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-Section S99

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-Subparagraph (SX 210.5-02.22(a)(1))

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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-Publisher FASB

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-Topic 323

-SubTopic 10

-Section 35

-Paragraph 32

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Fair value portion of investments accounted under the equity method.

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-SubTopic 10

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Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

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-Paragraph 28

-Subparagraph (b)

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The charge against earnings resulting from the write down of long lived assets other than goodwill due to the difference between the carrying value and lower fair value.

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