Annual report pursuant to Section 13 and 15(d)

SEGMENT REPORTING (Narrative) (Details)

v2.4.1.9
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                 $ 9,029,900,000us-gaap_AssetImpairmentCharges $ 250,800,000us-gaap_AssetImpairmentCharges $ 1,049,900,000us-gaap_AssetImpairmentCharges
Revenues 1,284,700,000us-gaap_Revenues 1,298,200,000us-gaap_Revenues 1,100,800,000us-gaap_Revenues 940,000,000us-gaap_Revenues 1,515,800,000us-gaap_Revenues 1,546,600,000us-gaap_Revenues 1,488,500,000us-gaap_Revenues 1,140,500,000us-gaap_Revenues 4,623,700,000us-gaap_Revenues 5,691,400,000us-gaap_Revenues 5,872,700,000us-gaap_Revenues
U.S. Iron Ore [Member]                      
Segment Reporting Information [Line Items]                      
Number Of Mines                 5clf_NumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		   
Revenues                 2,506,500,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		2,667,900,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		2,723,300,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
Eastern Canadian Iron Ore [Member]                      
Segment Reporting Information [Line Items]                      
Number Of Mines                 2clf_NumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		   
Revenues                 563,400,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		978,700,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		1,008,900,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
Asia Pacific Iron Ore [Member]                      
Segment Reporting Information [Line Items]                      
Number Of Mines                 1clf_NumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		   
Revenues                 866,700,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		1,224,300,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		1,259,300,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember
North American Coal [Member]                      
Segment Reporting Information [Line Items]                      
Revenues                 687,100,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember 		821,900,000us-gaap_Revenues
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember 		881,100,000us-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                 2clf_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                 2clf_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                 1clf_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                 2clf_NumberOfCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember 		1clf_NumberOfCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember 		1,000,000clf_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                 $ 1,600,000,000us-gaap_Revenues
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember 		$ 1,000,000,000us-gaap_Revenues
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember 		$ 900,000,000us-gaap_Revenues
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember