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DISCONTINUED OPERATIONS Schedule of Disposal Groups, including Discontinued Operations, Income Statement, Balance Sheet and Cash Flows (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
LOSS FROM DISCONTINUED OPERATIONS, net of tax | $ (928.5)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax | $ (140.4)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax |
Short-term assets of discontinued operations | 197.2 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent |  | 330.6 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent
Long-term assets of discontinued operations | 0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent |  | 400.1 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent
Short-term liabilities of discontinued operations | 265.0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent |  | 400.6 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent
LONG-TERM LIABILITIES OF DISCONTINUED OPERATIONS | 125.2 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent |  | 436.1 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent
Depreciation, depletion and amortization | 33.0 us-gaap_DepreciationDepletionAndAmortization | 141.1 us-gaap_DepreciationDepletionAndAmortization |
Payments to Acquire Property, Plant, and Equipment | 15.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 103.3 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment |
Asset Impairment Charges, Cash Flows | 76.6 clf_AssetImpairmentChargesCashFlows | 0 clf_AssetImpairmentChargesCashFlows |
North American Coal [Member] |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
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 |
Short-term assets of discontinued operations | 188.6 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |  | 143.8 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember
Long-term assets of discontinued operations | 0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |  | 130.4 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember
Short-term liabilities of discontinued operations | 197.7 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |  | 81.3 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember
LONG-TERM LIABILITIES OF DISCONTINUED OPERATIONS | 0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |  | 125.9 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember
Depreciation, depletion and amortization | 3.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 29.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |
Payments to Acquire Property, Plant, and Equipment | 2.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 10.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |
Asset Impairment Charges, Cash Flows | 73.4 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_NorthAmericanCoalMember |
Eastern Canadian Iron Ore [Member] |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
LOSS FROM DISCONTINUED OPERATIONS, net of tax | (852.7)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember | (91.2)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |
Short-term assets of discontinued operations | 8.6 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |  | 183.5 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember
Long-term assets of discontinued operations | 0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |  | 256.0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember
Short-term liabilities of discontinued operations | 67.3 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |  | 316.3 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember
LONG-TERM LIABILITIES OF DISCONTINUED OPERATIONS | 125.2 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |  | 304.6 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember
Depreciation, depletion and amortization | 0 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember | 41.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |
Payments to Acquire Property, Plant, and Equipment | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember | 75.6 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |
Asset Impairment Charges, Cash Flows | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_EasternCanadianIronOreMember |
Other Canadian Operations [Member] |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
LOSS FROM DISCONTINUED OPERATIONS, net of tax | (0.1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember | (2.9)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |
Short-term assets of discontinued operations | 0 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |  | 3.3 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember
Long-term assets of discontinued operations | 0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |  | 13.7 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember
Short-term liabilities of discontinued operations | 0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |  | 3.0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember
LONG-TERM LIABILITIES OF DISCONTINUED OPERATIONS | 0 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |  | 5.6 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember
Depreciation, depletion and amortization | 0 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember | 0.1 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |
Payments to Acquire Property, Plant, and Equipment | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |
Asset Impairment Charges, Cash Flows | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_OtherCanadianOperationsMember |
Canadian Entities [Member] |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
LOSS FROM DISCONTINUED OPERATIONS, net of tax | (852.8)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember | (94.1)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |
Short-term assets of discontinued operations | 8.6 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |  | 186.8 us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember
Long-term assets of discontinued operations | 0 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |  | 269.7 us-gaap_DisposalGroupIncludingDiscontinuedOperationAssetsNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember
Short-term liabilities of discontinued operations | 67.3 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |  | 319.3 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationCurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember
LONG-TERM LIABILITIES OF DISCONTINUED OPERATIONS | 125.2 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |  | 310.2 us-gaap_LiabilitiesOfDisposalGroupIncludingDiscontinuedOperationNoncurrent / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember
Depreciation, depletion and amortization | 0 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember | 41.3 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |
Payments to Acquire Property, Plant, and Equipment | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember | 75.6 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |
Asset Impairment Charges, Cash Flows | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember | 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = clf_CanadianEntitiesMember |
Discontinued Operations [Member] |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Depreciation, depletion and amortization | 3.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember | 71.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember |
Payments to Acquire Property, Plant, and Equipment | 2.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember | 85.6 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember |
Asset Impairment Charges, Cash Flows | $ 73.4 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember | $ 0 clf_AssetImpairmentChargesCashFlows / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = us-gaap_SegmentDiscontinuedOperationsMember |

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