Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)

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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Mar. 31, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Jun. 30, 2014
Derivative [Line Items]              
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 16.0   $ 64.9 $ 3.1 $ 103.4    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (4.4)   0.6 (18.5) (18.8)    
Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member]              
Derivative [Line Items]              
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net       0.0 (0.5)    
Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member]              
Derivative [Line Items]              
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net     0.1   0.3    
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]              
Derivative [Line Items]              
Derivative, Notional Amount             $ 250.0
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Product Revenues [Member]              
Derivative [Line Items]              
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (0.1) $ (6.3) 1.6 (7.4) (11.2)    
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member]              
Derivative [Line Items]              
Derivative, Notional Amount 0.0     0.0   $ 220.0  
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member]              
Derivative [Line Items]              
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts 0.0     0.0      
Australian Dollar Foreign Exchange Contract Prior To De Designation [Member] | Not Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member]              
Derivative [Line Items]              
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts (0.4)     (0.4)      
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] | Cost of Sales [Member]              
Derivative [Line Items]              
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0.0   (1.0) 0.0 (7.1)    
Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]              
Derivative [Line Items]              
Derivative, Notional Amount 0.0     0.0   0.0  
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member]              
Derivative [Line Items]              
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [1] (1.1)   0.0 (3.6) (3.3)    
Foreign Exchange Contract [Member] | Product Revenues [Member]              
Derivative [Line Items]              
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (2.1)   0.0 (11.8) 0.0    
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 11.6   62.6 22.1 124.6    
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Derivative Financial Instruments, Assets [Member]              
Derivative [Line Items]              
Derivative assets 9.8     9.8   63.2  
Provisional Pricing Arrangements [Member] | Product Revenues [Member]              
Derivative [Line Items]              
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 7.6   2.3 (0.2) (17.9)    
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 7.6   2.3 (0.2) (17.9)    
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 0.6     0.6   0.0  
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 0.8     0.8   9.5  
Corporate [Member] | Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member]              
Derivative [Line Items]              
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net     0.0   (0.5)    
Corporate [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]              
Derivative [Line Items]              
Derivative, Notional Amount $ 0.0     $ 0.0   183.0  
Corporate [Member] | Foreign Exchange Contract [Member] | Cost of Sales [Member]              
Derivative [Line Items]              
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net     $ 0.0   $ (3.3) $ (7.3)  
[1] At September 30, 2014, the location of the Gain (Loss) Recognized in Income on Derivative for Foreign Exchange Contracts was Cost of goods sold and operating expenses.