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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2016 USD ($) T | Dec. 31, 2015 USD ($)
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | $ 9.2 | $ (45.2)
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) | T | 115,000,000 |
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent | $ 206.8 | 234.0
Payments for Environmental Liabilities | 2.4 | 2.6
Accrual for Environmental Loss Contingencies | 2.8 | 3.6
Mine Reclamation and Closing Liability, current and noncurrent | 204.0 | 230.4
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member] |  |
Site Contingency [Line Items] |  |
Mine Reclamation and Closing Liability, current and noncurrent | 25.5 | $ 24.1
Selenium [Domain] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) | $ 29.6 |

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