Annual report pursuant to Section 13 and 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)

v3.8.0.1
DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative [Line Items]      
Natural Gas Hedges $ 3.5    
Unrealized net gain (loss) on derivative financial instruments, net of tax $ 0.5 $ 2.6 $ (20.7)
Current Fiscal Year End Date --12-31    
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 104.7 92.6 5.5
Derivative asset, fair value 39.4 33.1  
Derivative liability, fair value 2.7 0.5  
Derivative assets 39.4 33.1  
Accounts receivable, net 140.6 128.7  
Not Designated as Hedging Instrument [Member]      
Derivative [Line Items]      
Derivative asset, fair value 39.4 33.1  
Derivative liability, fair value 2.4 0.5  
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member]      
Derivative [Line Items]      
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments     (3.6)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 0.0  
Foreign Exchange Contract [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 0.0 0.0 (12.6)
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative, Gain (Loss) on Derivative, Net 163.3 41.7 27.1
Provisional Pricing Arrangements [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (58.6) 49.0 (1.4)
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]      
Derivative, Gain (Loss) on Derivative, Net 58.6 49.0 $ (1.4)
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]      
Derivative [Line Items]      
Derivative asset, fair value 37.9 21.3  
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]      
Derivative [Line Items]      
Derivative asset, fair value 1.5 10.3  
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]      
Derivative [Line Items]      
Derivative liability, fair value $ 2.4 $ 0.5