| DISCONTINUED OPERATIONS Schedule of Disposal Groups, including Discontinued Operations, Income Statement - North American Coal (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | |
|---|---|---|
| Mar. 31, 2015 | Mar. 31, 2014 | |
| Schedule of Disposal Groups, Including Discontinued Operations, Assets and Liabilities [Line Items] | ||
| LOSS FROM DISCONTINUED OPERATIONS, net of tax | $ (928.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | $ (140.4)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | 
| North American Coal [Member] | ||
| Schedule of Disposal Groups, Including Discontinued Operations, Assets and Liabilities [Line Items] | ||
| Disposal Group, Including Discontinued Operation, Revenue | 116.6us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 166.2us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Disposal Group, Including Discontinued Operation, Costs of Goods Sold | (107.3)us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | (214.6)us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Disposal Group, Including Discontinued Operation, Gross Profit (Loss) | 9.3us-gaap_DisposalGroupIncludingDiscontinuedOperationGrossProfitLoss / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | (48.4)us-gaap_DisposalGroupIncludingDiscontinuedOperationGrossProfitLoss / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Disposal Group, Including Discontinued Operation, Operating Expense | (11.3)us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | (4.5)us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Disposal Group, Including Discontinued Operation, Other Expense | (0.4)us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherExpense / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | (0.6)us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherExpense / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax | (2.4)us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | (53.5)us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Impairment of Long-Lived Assets to be Disposed of | (73.4)us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 0us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| Discontinued Operation, Tax Effect of Discontinued Operation | 0.1us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 7.2us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
| LOSS FROM DISCONTINUED OPERATIONS, net of tax | $ (75.7)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | $ (46.3)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 
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- Definition Overall income (loss) from a disposal group that is classified as a component of the entity, before income tax, reported as a separate component of income before extraordinary items. Includes the following (before income tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Tax effect allocated to a disposal group that is classified as a component of the entity reported as a separate component of income before extraordinary items. Includes the tax effects of the following: income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of costs of goods sold attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of gross profit attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of operating expenses attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of other expense attributable to disposal group. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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