Annual report pursuant to Section 13 and 15(d)

GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Narrative) (Details)

v2.4.1.9
GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Goodwill [Line Items]      
Goodwill, Impairment Loss $ 73.5us-gaap_GoodwillImpairmentLoss $ 80.9us-gaap_GoodwillImpairmentLoss  
Product 4,230.8us-gaap_SalesRevenueGoodsNet 5,346.6us-gaap_SalesRevenueGoodsNet 5,520.9us-gaap_SalesRevenueGoodsNet
Below-market sales contracts, remaining contract life 2 years    
Cost of Sales [Member]      
Goodwill [Line Items]      
Amortization expense relating to intangible assets 10.4us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		19.9us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		22.5us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Product Revenues [Member] | Sales Revenue, Goods, Net [Member]      
Goodwill [Line Items]      
Product 23.1us-gaap_SalesRevenueGoodsNet
/ clf_BelowMarketSalesContractsAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		45.9us-gaap_SalesRevenueGoodsNet
/ clf_BelowMarketSalesContractsAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		46.3us-gaap_SalesRevenueGoodsNet
/ clf_BelowMarketSalesContractsAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]      
Goodwill [Line Items]      
Goodwill, Impairment Loss   80.9us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember 		 
Permits [Member]      
Goodwill [Line Items]      
Impairment of Intangible Assets, Finite-lived 15.5us-gaap_ImpairmentOfIntangibleAssetsFinitelived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember 		   
Utility Contracts [Member]      
Goodwill [Line Items]      
Impairment of Intangible Assets, Finite-lived   $ 9.5us-gaap_ImpairmentOfIntangibleAssetsFinitelived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_UtilityContractsMember 		$ 0us-gaap_ImpairmentOfIntangibleAssetsFinitelived
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_UtilityContractsMember