Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details)

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FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 616.9 $ 177.0
Derivative Asset 39.4 33.1
Derivative Liability 2.7 0.5
Derivative asset, fair value 39.4 33.1
Derivative liability, fair value $ 2.7 $ 0.5
Management Estimate of 62% Fe 62.00% 62.00%
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 66.3 $ 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 Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 550.6 0.0
Derivative Asset 0.0 1.5
Derivative Liability 0.3 0.0
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0.0 0.0
Derivative Asset 39.4 31.6
Derivative Liability 2.4 0.5
Not Designated as Hedging Instrument [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 39.4 33.1
Derivative liability, fair value 2.4 0.5
Market Approach Valuation Technique [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Customer Supply Agreement [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 37.9 21.3
Market Approach Valuation Technique [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Provisional Pricing Arrangements [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 1.5 10.3
Market Approach Valuation Technique [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member] | Provisional Pricing Arrangements [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative liability, fair value $ 2.4 $ 0.5