| GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Finite-Lived Intangible Assets By Major Class) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | 
|---|---|---|
| Finite-Lived Intangible Assets [Line Items] | ||
| Definite lived intangible assets - Gross Carrying Amount | $ 78.7us-gaap_FiniteLivedIntangibleAssetsGross | $ 79.2us-gaap_FiniteLivedIntangibleAssetsGross | 
| Definite lived intangible assets - Accumulated Amortization | (17.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization | (16.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization | 
| Definite lived intangible assets - Net Carrying Amount | 61.2us-gaap_FiniteLivedIntangibleAssetsNet | 62.7us-gaap_FiniteLivedIntangibleAssetsNet | 
| Permits [Member] | Other Assets [Member] | ||
| Finite-Lived Intangible Assets [Line Items] | ||
| Definite lived intangible assets - Gross Carrying Amount | 78.7us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | 79.2us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | 
| Definite lived intangible assets - Accumulated Amortization | (17.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | (16.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | 
| Definite lived intangible assets - Net Carrying Amount | 61.2us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | 62.7us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UseRightsMember | 
| Below Market Sales Contracts [Member] | ||
| Finite-Lived Intangible Assets [Line Items] | ||
| Definite lived intangible assets - Gross Carrying Amount | (228.9)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | (228.9)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Definite lived intangible assets - Accumulated Amortization | 182.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 182.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Definite lived intangible assets - Net Carrying Amount | (46.1)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | (46.1)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Below Market Sales Contracts [Member] | Other Current Liabilities [Member] | ||
| Finite-Lived Intangible Assets [Line Items] | ||
| Definite lived intangible assets - Gross Carrying Amount | (23.0)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | (23.0)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Definite lived intangible assets - Net Carrying Amount | (23.0)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | (23.0)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Below Market Sales Contracts [Member] | Other Liabilities [Member] | ||
| Finite-Lived Intangible Assets [Line Items] | ||
| Definite lived intangible assets - Gross Carrying Amount | (205.9)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | (205.9)us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Definite lived intangible assets - Accumulated Amortization | 182.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 182.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
| Definite lived intangible assets - Net Carrying Amount | $ (23.1)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | $ (23.1)us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherLiabilitiesMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = clf_BelowMarketSalesContractsMember | 
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- Definition Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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