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
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 543.4 $ 616.9
Derivative Asset 91.5 37.9
Derivative Liability 3.7 2.0
Derivative asset, fair value 91.5 37.9
Derivative liability, fair value 3.7 2.0
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0.8 66.3
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 542.6 550.6
Derivative Asset 0.1 0.0
Derivative Liability 3.7 0.3
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 91.4 37.9
Derivative Liability 0.0 1.7
Not Designated as Hedging Instrument [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset, fair value 91.4 37.9
Derivative liability, fair value 0.0 1.7
Valuation, Market Approach [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 89.3 37.9
Valuation, Market Approach [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 2.1 0.0
Valuation, Market Approach [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 $ 0.0 $ 1.7