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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Investments In Unconsolidated Ventures) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]    
Investment $ 3.4us-gaap_EquityMethodInvestments $ 4.1us-gaap_EquityMethodInvestments
Other Noncurrent Liabilities [Member] | Hibbing [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment   3.1us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentLiabilitiesMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember
Other Noncurrent Assets [Member] | Hibbing [Member]    
Schedule of Equity Method Investments [Line Items]    
Ownership interest, equity method investment 23.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember
 
Investment 2.5us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember
 
Other Noncurrent Assets [Member] | Other Equity Investees [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment $ 0.9us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_OtherEquityInvesteesMember
$ 1.0us-gaap_EquityMethodInvestments
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherNoncurrentAssetsMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_OtherEquityInvesteesMember