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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

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