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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2016 USD ($) | Jun. 30, 2015 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) Facility
Transaction Gains and Losses Resulting from Remeasurement [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | $ 0.2 | $ (0.8) | $ (0.9) | $ 12.7 |
Short-term intercompany loan [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  |  |  | 12.4 |
Cash and Cash Equivalents [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | 0.5 | $ (0.7) | 1.5 | $ 0.7 |
Liability [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | (0.2) |  | (2.6) |  |
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Number of mines (in number of facilities) | Facility |  |  |  |  | 2
Hibbing [Member] | Other Noncurrent Liabilities [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Equity Method Investments | $ 3.7 |  | $ 3.7 |  | $ (2.4)

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