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FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Mar. 31, 2015 USD ($)
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Debt Instrument, Par Value | $ 3,448.2 | $ 3,980.0 |  |
Management Estimate of 62% Fe | 62.00% |  |  |
Goodwill, Impairment Loss | $ 0.0 | 73.5 |  |
Other Asset Impairment Charges |  | 562.0 |  |
Tangible Asset Impairment Charges |  | 537.8 |  |
Global Exploration Investment in Venture [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0.0 |  |
Equity Method Investment, Other than Temporary Impairment |  | 9.2 |  |
Amapa [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Equity Method Investment, Other than Temporary Impairment |  |  | $ 67.6 |
Asia Pacific Iron Ore [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Goodwill, Impairment Loss |  | 73.5 |  |
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Debt Instrument, Par Value | $ 544.2 |  |  |
Stated interest rate | 7.75% |  |  |
Debt Instrument, Fair Value Disclosure |  |  |  | $ 397.2
Debt discount |  |  |  | 0.27
Debt Instrument, Unamortized Discount | $ 131.5 |  |  | $ 147.0
Fair Value, Inputs, Level 2 [Member] | Global Exploration Investment in Venture [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0.0 |  |
Fair Value, Inputs, Level 2 [Member] | Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |  |
Debt Instrument, Fair Value Disclosure | $ 134.7 | $ 0.0 |  | $ 397.2

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