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INVENTORIES (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
U.S. Iron Ore [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory, LIFO Reserve | $ 119.0 us-gaap_InventoryLIFOReserve / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | $ 115.3 us-gaap_InventoryLIFOReserve / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Effect of LIFO Inventory Liquidation on Income | 44.8 us-gaap_EffectOfLIFOInventoryLiquidationOnIncome / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  |
Increase in Inventories due to build up of LIFO layers |  | 7.4 clf_IncreaseInInventoriesDueToBuildUpOfLifoLayers / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Cost of Sales [Member] | Eastern Canadian Iron Ore [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 38.9 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 0 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember
Cost of Sales [Member] | North American Coal [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 44.5 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 11.1 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 24.4 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember
Work in process inventory [Member] | Bloom Lake [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 0 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_WorkinprocessinventoryMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |
Work in process inventory [Member] | Wabush [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 0 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_WorkinprocessinventoryMember / clf_ReportingUnitAxis = clf_WabushMember |  |
Finished goods inventory [Member] | Bloom Lake [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 0 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_FinishedgoodsinventoryMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |
Finished goods inventory [Member] | Wabush [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down | 0 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_FinishedgoodsinventoryMember / clf_ReportingUnitAxis = clf_WabushMember |  |
Pellets [Member] | Cost of Sales [Member] | Eastern Canadian Iron Ore [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down |  | 11.1 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_PelletsMember / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Pellets [Member] | Wabush [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Unsaleable Inventory Impairment Charge |  | 10.6 clf_UnsaleableInventoryImpairmentCharge / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_PelletsMember / clf_ReportingUnitAxis = clf_WabushMember |
Concentrate [Member] | Cost of Sales [Member] | Eastern Canadian Iron Ore [Member] |  |  |
Inventory [Line Items] |  |  |
Inventory Write-down |  | 13.2 us-gaap_InventoryWriteDown / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_ConcentrateMember / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Concentrate [Member] | Wabush [Member] | Cost of Sales [Member] |  |  |
Inventory [Line Items] |  |  |
Unsaleable Inventory Impairment Charge |  | $ 7.9 clf_UnsaleableInventoryImpairmentCharge / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / us-gaap_PublicUtilitiesInventoryAxis = clf_ConcentrateMember / clf_ReportingUnitAxis = clf_WabushMember |

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- Definition The effect of liquidating LIFO (last in first out) inventory layers on income. + 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 210.5-02.6b) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100079-122729 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6a) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6386917&loc=d3e99609-122727 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6c) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SAB TOPIC 11.F) -URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100079-122729 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 11 -Section F + Details
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- Definition Amount of difference between inventory at lower of LIFO cost or market and inventory at lower of other inventory accounting method cost or market, for example, but not limited to, FIFO or average cost. + 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 210.5-02.6(c)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph c -Article 5 + Details
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- Definition Charge to cost of goods sold that represents the reduction of the carrying amount of inventory, generally attributable to obsolescence or market conditions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28360613&loc=d3e4542-108314 + Details
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