Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS (Narrative) (Details)

v3.4.0.3
DERIVATIVE INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ (1.6) $ (15.6)  
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 (5.9)  
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 (0.1) 10.1  
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Derivative Financial Instruments, Assets [Member]      
Derivative [Line Items]      
Derivative assets 5.7   5.8
Provisional Pricing Arrangements [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (1.5) (16.2)  
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 (1.5) $ (16.2)  
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 3.3   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 $ 6.2   $ 3.4