Annual report pursuant to Section 13 and 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2015
USD ($)
Facility
Dec. 31, 2014
USD ($)
Site Contingency [Line Items]    
Payments for Environmental Liabilities $ 2.6 $ 3.1
Accrual for Environmental Loss Contingencies 3.6 5.5
Mine Reclamation and Closing Liability, current and noncurrent 230.4 165.3
Total environmental and mine closure obligations $ 234.0 170.8
Current Fiscal Year End Date --12-31  
U.S. Iron Ore [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) | Facility 5  
Asia Pacific Iron Ore [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) | Facility 1  
LTV Steel Mining Company [Member]    
Site Contingency [Line Items]    
Mine closure obligation, number of mines (in number of facilities) | Facility 1  
Rio Tinto Mine Site [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies $ 0.0 2.5
Loss Contingency, Range of Possible Loss, Maximum   12.2
Rio Tinto Mine Site [Member] | Rio Tinto Working Group RTWG [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies   $ 29.0