Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Impairment Charges on Financial and Nonfinancial Assets) (Details)

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FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Impairment Charges on Financial and Nonfinancial Assets) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Impairment Loss $ 80.9 $ 1,000.0
Assets, Fair Value Disclosure, Nonrecurring 47.9 72.5
Impairment Charges 318.4 1,415.3
Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Nonrecurring 0 0
Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Nonrecurring 0 0
Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Nonrecurring 47.9 72.5
Amapa [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity Method Investments, Fair Value Disclosure 0 72.5
Equity Method Investment, Other than Temporary Impairment 67.6 365.4
Amapa [Member] | Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity Method Investments, Fair Value Disclosure 0 0
Amapa [Member] | Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity Method Investments, Fair Value Disclosure 0 0
Amapa [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity Method Investments, Fair Value Disclosure 0 72.5
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Impairment Loss 80.9  
Wabush [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure   0
Goodwill, Impairment Loss   2.7
Property, Plant, and Equipment, Fair Value Disclosure   0
Impairment of Long-Lived Assets Held-for-use 140.1 49.9
Finite-lived Intangible Assets, Fair Value Disclosure 1.6  
Other Asset Impairment Charges 14.5  
Wabush [Member] | Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure   0
Property, Plant, and Equipment, Fair Value Disclosure   0
Finite-lived Intangible Assets, Fair Value Disclosure 0  
Wabush [Member] | Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure   0
Property, Plant, and Equipment, Fair Value Disclosure   0
Finite-lived Intangible Assets, Fair Value Disclosure 0  
Wabush [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure   0
Property, Plant, and Equipment, Fair Value Disclosure   0
Finite-lived Intangible Assets, Fair Value Disclosure 1.6  
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Property, Plant, and Equipment, Fair Value Disclosure 46.3  
Impairment of Long-Lived Assets Held-for-use 155.4  
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Property, Plant, and Equipment, Fair Value Disclosure 0  
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Property, Plant, and Equipment, Fair Value Disclosure 0  
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Property, Plant, and Equipment, Fair Value Disclosure 46.3  
CQIM Reporting Unit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure 0 0
Goodwill, Impairment Loss   997.3
CQIM Reporting Unit [Member] | Fair Value, Inputs, Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure 0 0
CQIM Reporting Unit [Member] | Fair Value, Inputs, Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure 0 0
CQIM Reporting Unit [Member] | Fair Value, Inputs, Level 3 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Goodwill, Fair Value Disclosure $ 0 $ 0