Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value Measurements, Nonrecurring) (Details)

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FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value Measurements, Nonrecurring) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2015
Jun. 30, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Impairment Loss   $ 0.0 $ 73.5
Assets, Fair Value Disclosure, Nonrecurring $ 397.2   79.4
Impairment of goodwill and other long-lived assets 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
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]      
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] | 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] | 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] | 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
Other Asset Impairment Charges     24.2
Finite-lived Intangible Assets, Fair Value Disclosure     7.0
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 $ 348.2 $ 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