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DERIVATIVE INSTRUMENTS (Narrative) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016
Customer Supply Agreement [Member] | Product Revenues [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Amount of gain/(loss) recognized in income on derivative | $ 51.4 | $ 19.5 | $ 69.2 | $ 19.9 |
Customer Supply Agreement [Member] | Derivative Financial Instruments, Assets [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets | 66.4 |  | 66.4 |  | $ 21.3
Provisional Pricing Arrangements [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Amount of gain/(loss) recognized in income on derivative | (35.4) | $ 1.8 | (19.8) | $ 0.3 |
Provisional Pricing Arrangements [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Assets [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets | 6.1 |  | 6.1 |  | 10.3
Provisional Pricing Arrangements [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Liabilities [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative liabilities | $ 20.8 |  | $ 20.8 |  | $ 0.5

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