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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | 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 | $ (0.3) | $ 2.4 | $ (1.2) | $ 15.2 |
Short-term intercompany loan [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax |  | 0.1 |  | 11.1 |
Cash and Cash Equivalents [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | (1.1) | $ 1.3 | 0.3 | $ 2.0 |
Liability [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Foreign Currency Transaction Gain (Loss), before Tax | 0.6 |  | (2.0) |  |
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 | $ (1.9) |  | $ (1.9) |  | $ (2.4)

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