Annual report pursuant to Section 13 and 15(d)

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

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