Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details)

v3.3.1.900
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