Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS (Narrative) (Details)

v3.7.0.1
DERIVATIVE INSTRUMENTS (Narrative) (Details) - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Customer Supply Agreement [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Amount of gain/(loss) recognized in income on derivative $ 17.8 $ 0.1  
Customer Supply Agreement [Member] | Derivative Financial Instruments, Assets [Member]      
Derivative [Line Items]      
Derivative assets 35.9   $ 21.3
Provisional Pricing Arrangements [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 15.7 $ (1.5)  
Provisional Pricing Arrangements [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Assets [Member]      
Derivative [Line Items]      
Derivative assets 23.5   10.3
Provisional Pricing Arrangements [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Liabilities [Member]      
Derivative [Line Items]      
Derivative liabilities $ 9.1   $ 0.5