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
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Goodwill, Impairment Loss   $ 0.0 $ 73.5  
Tangible Asset Impairment Charges     537.8  
Impairment of Intangible Assets (Excluding Goodwill)     13.8  
Other Asset Impairment Charges     562.0  
Assets, Fair Value Disclosure, Nonrecurring $ 397.2   79.4  
Impairment Charges 269.5   644.7  
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   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 397.2   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 0.0   79.4  
Global Exploration Investment in Venture [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity Method Investments, Fair Value Disclosure     0.0  
Equity Method Investment, Other than Temporary Impairment     9.2  
Global Exploration Investment in Venture [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  
Global Exploration Investment in Venture [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  
Global Exploration Investment in Venture [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     0.0  
Amapa [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity Method Investment, Other than Temporary Impairment       $ 67.6
Asia Pacific Iron Ore [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Goodwill, Fair Value Disclosure     0.0  
Goodwill, Impairment Loss     73.5  
Asia Pacific Iron Ore [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  
Asia Pacific Iron Ore [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  
Asia Pacific Iron Ore [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  
Other Reporting Units [Member] | Other Reporting Units [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Property, Plant, and Equipment, Fair Value Disclosure     0.0  
Tangible Asset Impairment Charges     11.3  
Other Reporting Units [Member] | Other Reporting Units [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     0.0  
Other Reporting Units [Member] | Other Reporting Units [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     0.0  
Other Reporting Units [Member] | Other Reporting Units [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     0.0  
Asia Pacific [Member] | Asia Pacific Iron Ore [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     0.0  
Finite-lived Intangible Assets, Fair Value Disclosure     0.0  
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Property, Plant, and Equipment, Fair Value Disclosure     72.4  
Tangible Asset Impairment Charges     526.5  
Finite-lived Intangible Assets, Fair Value Disclosure     7.0  
Impairment of Intangible Assets (Excluding Goodwill)     24.2  
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [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     0.0  
Finite-lived Intangible Assets, Fair Value Disclosure     0.0  
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [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     72.4  
Finite-lived Intangible Assets, Fair Value Disclosure     7.0  
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument, Fair Value Disclosure 397.2      
Gain on new debt issued 269.5      
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 1 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument, Fair Value Disclosure 0.0      
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument, Fair Value Disclosure 397.2 $ 134.7 $ 0.0  
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 3 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument, Fair Value Disclosure $ 0.0