Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENTS (Narrative) (Details)

v3.7.0.1
FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Assets And Liabilities Components [Line Items]    
Cash equivalents $ 160.0 $ 177.0
Management Estimate of 62% Fe 62.00%  
Derivative Asset $ 72.5 33.1
Derivative Liability 20.8 0.5
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Cash equivalents 40.0 177.0
Derivative Asset 0.0 0.0
Derivative Liability 0.0 0.0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Cash equivalents 120.0 0.0
Derivative Asset 0.0 1.5
Derivative Liability 0.0 0.0
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Cash equivalents 0.0 0.0
Derivative Asset 72.5 31.6
Derivative Liability 20.8 0.5
Not Designated as Hedging Instrument [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Derivative Asset 72.5 33.1
Derivative Liability 20.8 0.5
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Customer Supply Agreement [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Derivative Asset 66.4 21.3
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Provisional Pricing Arrangements [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Derivative Asset 6.1 10.3
Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Market Approach Valuation Technique [Member] | Provisional Pricing Arrangements [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Derivative Liability $ 20.8 $ 0.5