Annual report pursuant to Section 13 and 15(d)

GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Finite-Lived Intangible Assets By Major Class) (Details)

v2.4.1.9
GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Finite-Lived Intangible Assets By Major Class) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]    
Definite lived intangible assets - Gross Carrying Amount $ 79.2us-gaap_FiniteLivedIntangibleAssetsGross $ 184.5us-gaap_FiniteLivedIntangibleAssetsGross
Definite lived intangible assets - Accumulated Amortization (16.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization (89.1)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Definite lived intangible assets - Net Carrying Amount 62.7us-gaap_FiniteLivedIntangibleAssetsNet 95.4us-gaap_FiniteLivedIntangibleAssetsNet
Permits [Member] | Other Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Definite lived intangible assets - Gross Carrying Amount 79.2us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
127.4us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
Definite lived intangible assets - Accumulated Amortization (16.5)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
(35.9)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
Definite lived intangible assets - Net Carrying Amount 62.7us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
91.5us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_UseRightsMember
Utility Contracts [Member] | Other Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Definite lived intangible assets - Gross Carrying Amount   54.7us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_UtilityContractsMember
Definite lived intangible assets - Accumulated Amortization   (53.1)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_UtilityContractsMember
Definite lived intangible assets - Net Carrying Amount   1.6us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_UtilityContractsMember
Leases [Member] | Other Assets [Member]    
Finite-Lived Intangible Assets [Line Items]    
Definite lived intangible assets - Gross Carrying Amount   2.4us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeaseAgreementsMember
Definite lived intangible assets - Accumulated Amortization   (0.1)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeaseAgreementsMember
Definite lived intangible assets - Net Carrying Amount   2.3us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeaseAgreementsMember
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
159.7us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_BelowMarketSalesContractsMember
Definite lived intangible assets - Net Carrying Amount (46.1)us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_BelowMarketSalesContractsMember
(69.2)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
159.7us-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
$ (46.2)us-gaap_FiniteLivedIntangibleAssetsNet
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= clf_BelowMarketSalesContractsMember