DERIVATIVE INSTRUMENTS (Schedule Of Derivative Instruments In Statement Of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	$ 59.4	$ 33.1
Derivative liability, fair value	9.1	0.5
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	35.9	21.3
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value	0.0	0.0
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	23.5	10.3
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value	9.1	0.5
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	0.0	1.5
Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, fair value	$ 0.0	$ 0.0