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FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value Assets And Liabilities Components [Line Items] |  |
Derivative assets | $ 118.3 | $ 91.5
Derivative liability | 2.6 | 3.7
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 | 0.1
Derivative liability | 2.6 | 3.7
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Components [Line Items] |  |
Derivative assets | 118.1 | 91.4
Derivative liability |  | 0.0
Not Designated as Hedging Instrument [Member] |  |
Fair Value Assets And Liabilities Components [Line Items] |  |
Derivative assets | 118.1 | 91.4
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Customer Supply Agreement [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Components [Line Items] |  |
Derivative assets | 102.4 | 89.3
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Provisional Pricing Arrangements [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Components [Line Items] |  |
Derivative assets | 15.7 | $ 2.1
Derivative liability | $ 0.0 |

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