Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS (Narrative) (Details)

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DERIVATIVE INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 21.3 $ (4.1) $ 20.2 $ (7.9)  
Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]          
Derivative [Line Items]          
Derivative, Notional Amount         $ 0.0
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 (3.4) 0.0 2.5  
Foreign Exchange Contract [Member] | Product Revenues [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 (9.7) 0.0 (9.7)  
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member]          
Derivative [Line Items]          
Amount of gain/(loss) recognized in income on derivative 19.5 0.4 19.9 10.5  
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Derivative Financial Instruments, Assets [Member]          
Derivative [Line Items]          
Derivative assets 23.7   23.7   5.8
Provisional Pricing Arrangements [Member] | Product Revenues [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 1.8 8.4 0.3 (7.8)  
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]          
Amount of gain/(loss) recognized in income on derivative 1.8 $ 8.4 0.3 $ (7.8)  
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Assets [Member]          
Derivative [Line Items]          
Derivative assets 2.1   2.1   2.0
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Liabilities [Member]          
Derivative [Line Items]          
Derivative liabilities $ 2.6   $ 2.6   $ 3.4