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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Related Party Transaction [Line Items] |  |  |
Unamortized Debt Issuance Expense | $ 46.2 us-gaap_UnamortizedDebtIssuanceExpense |  | $ 25.6 us-gaap_UnamortizedDebtIssuanceExpense
Derivative, Policy, Percentage of Amount Permitted to be Hedged | 75.00%clf_DerivativePolicyPercentageOfAmountPermittedToBeHedged |  |
Current Fiscal Year End Date | --12-31 |  |
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Twelve Months | 40.00%clf_DerivativePolicyMinimumPercentageOfAmountPermittedToBeHedgedForDerivativesWithPeriodsOfUpToTwelveMonths |  |
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Fifteen Months | 10.00%clf_DerivativePolicyMinimumPercentageOfAmountPermittedToBeHedgedForDerivativesWithPeriodsOfUpToFifteenMonths |  |
Transaction Gains and Losses Resulting from Remeasurement [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | 13.5 us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = clf_TransactionGainsandLossesResultingfromRemeasurementMember | (11.4)us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = clf_TransactionGainsandLossesResultingfromRemeasurementMember |
Short-term intercompany loan [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | 12.4 us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = clf_ShorttermintercompanyloanMember | (8.8)us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = clf_ShorttermintercompanyloanMember |
Cash and Cash Equivalents [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | $ 1.5 us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = us-gaap_CashAndCashEquivalentsMember | $ (3.1)us-gaap_ForeignCurrencyTransactionGainLossBeforeTax / us-gaap_IntercompanyForeignCurrencyBalanceByDescriptionAxis = us-gaap_CashAndCashEquivalentsMember |

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- Definition Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Twelve Months + References No definition available. + Details
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- Definition Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30690-110894 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6450189&loc=d3e30700-110894 + Details
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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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