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SEGMENT REPORTING (Narrative) (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) customer Facility | Dec. 31, 2014 USD ($) customer | Dec. 31, 2013 USD ($) customer
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |  |  | $ 3.3 | $ 635.5 | $ 14.3
Revenues | $ 476.0 | $ 593.2 | $ 498.1 | $ 446.0 | $ 1,030.3 | $ 979.7 | $ 747.7 | $ 615.5 | $ 2,013.3 | 3,373.2 | 3,890.8
U.S. Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines | Facility |  |  |  |  |  |  |  |  | 5 |  |
Revenues |  |  |  |  |  |  |  |  | $ 1,525.4 | 2,506.5 | 2,667.9
Asia Pacific Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines | Facility |  |  |  |  |  |  |  |  | 1 |  |
Revenues |  |  |  |  |  |  |  |  | $ 487.9 | $ 866.7 | $ 1,224.3
North American Coal [Member] | Low Volatile Metallurgical Coal Mines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number Of Mines | Facility |  |  |  |  |  |  |  |  | 2 |  |
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of Customers | customer |  |  |  |  |  |  |  |  | 3 | 1,000,000 | 1,000,000
Concentration Risk, Percentage |  |  |  |  |  |  |  |  | 10.00% | 10.00% | 10.00%
Revenues |  |  |  |  |  |  |  |  | $ 1,300.0 | $ 1,900.0 | $ 1,900.0

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