Annual report pursuant to Section 13 and 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)

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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