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SEGMENT REPORTING (Narrative) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |  |  | $ 9,029,900,000 us-gaap_AssetImpairmentCharges | $ 250,800,000 us-gaap_AssetImpairmentCharges | $ 1,049,900,000 us-gaap_AssetImpairmentCharges
Revenues | 1,284,700,000 us-gaap_Revenues | 1,298,200,000 us-gaap_Revenues | 1,100,800,000 us-gaap_Revenues | 940,000,000 us-gaap_Revenues | 1,515,800,000 us-gaap_Revenues | 1,546,600,000 us-gaap_Revenues | 1,488,500,000 us-gaap_Revenues | 1,140,500,000 us-gaap_Revenues | 4,623,700,000 us-gaap_Revenues | 5,691,400,000 us-gaap_Revenues | 5,872,700,000 us-gaap_Revenues
U.S. Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 5 clf_NumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  |
Revenues |  |  |  |  |  |  |  |  | 2,506,500,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 2,667,900,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 2,723,300,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember
Eastern Canadian Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 2 clf_NumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  |
Revenues |  |  |  |  |  |  |  |  | 563,400,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | 978,700,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | 1,008,900,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember
Asia Pacific Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 1 clf_NumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  |
Revenues |  |  |  |  |  |  |  |  | 866,700,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 1,224,300,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 1,259,300,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember
North American Coal [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 687,100,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 821,900,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 881,100,000 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember
North American Coal [Member] | Low Volatile Metallurgical Coal Mines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 2 clf_NumberOfMines / clf_ProductSegmentAxis = clf_LowVolatileMetallurgicalCoalMinesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  |
North American Coal [Member] | High volatile metallurgical coal mines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 2 clf_NumberOfMines / clf_ProductSegmentAxis = clf_HighvolatilemetallurgicalcoalminesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  |
North American Coal [Member] | Thermal Coal Mines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines |  |  |  |  |  |  |  |  | 1 clf_NumberOfMines / clf_ProductSegmentAxis = clf_ThermalCoalMinesMember / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  |
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of Customers |  |  |  |  |  |  |  |  | 2 clf_NumberOfCustomers / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | 1 clf_NumberOfCustomers / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | 1,000,000 clf_NumberOfCustomers / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Concentration Risk, Percentage |  |  |  |  |  |  |  |  | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Revenues |  |  |  |  |  |  |  |  | $ 1,600,000,000 us-gaap_Revenues / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | $ 1,000,000,000 us-gaap_Revenues / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember | $ 900,000,000 us-gaap_Revenues / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember

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