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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Derivative [Line Items] |  |  |
Current Fiscal Year End Date | --12-31 |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | $ 92,600,000 | $ 5,500,000 | $ 178,300,000
Accounts receivable, net | 128,700,000 | 40,200,000 |
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member] |  |  |
Derivative [Line Items] |  |  |
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments |  |  | 0
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 0 | (3,600,000) |
Foreign Exchange Contract [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 0 | (12,600,000) | 0
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | 41,700,000 | 27,100,000 | 187,800,000
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative assets | 21,300,000 | 5,800,000 |
Provisional Pricing Arrangements [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 49,000,000 | (1,400,000) | (9,500,000)
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, Gain (Loss) on Derivative, Net | 49,000,000 | (1,400,000) | $ (9,500,000)
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative assets | 10,300,000 | 2,000,000 |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Liabilities [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative liabilities | 500,000 | 3,400,000 |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |
Derivative [Line Items] |  |  |
Debt Instrument, Face Amount | $ 500,000,000 | $ 500,000,000.0 |

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