Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENTS (Narrative) (Details)

v3.10.0.1
FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets $ 190.8 $ 49.5 $ 37.9
Derivative liability $ 5.8   2.0
Management Estimate of 62% Fe 62.00%    
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets $ 0.0   0.0
Derivative liability 0.0   0.0
Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets 0.2   0.0
Derivative liability 0.1   0.3
Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets 190.6   37.9
Derivative liability 5.7   1.7
Not Designated as Hedging Instrument [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets 190.6   37.9
Derivative liability 5.7   1.7
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets 186.0   37.9
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative assets 4.6   0.0
Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative liability 0.0   0.0
Other Current Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Derivative liability $ 5.7   $ 1.7