Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS (Narrative) (Details)

v3.5.0.2
DERIVATIVE INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 11.6 $ 16.0 $ 49.7 $ 3.1  
Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]          
Derivative [Line Items]          
Derivative, Notional Amount         $ 0.0
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 (1.1) 0.0 (3.6)  
Foreign Exchange Contract [Member] | Product Revenues [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 (2.1) 0.0 (11.8)  
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 7.1 11.6 26.8 22.1  
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Derivative Financial Instruments, Assets [Member]          
Derivative [Line Items]          
Derivative assets 28.0   28.0   5.8
Provisional Pricing Arrangements [Member] | Product Revenues [Member]          
Derivative [Line Items]          
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 4.5 7.6 22.9 (0.2)  
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 4.5 $ 7.6 22.9 $ (0.2)  
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 0.4   0.4   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 $ 2.7   $ 2.7   $ 3.4