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DERIVATIVE INSTRUMENTS (Narrative) (Details) gal in Millions, MMcf in Millions, $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 USD ($) gal MMcf | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) gal MMcf
Derivative [Line Items] |  |  |
Derivative assets | $ 107.4 |  | $ 91.5
Derivative liability | $ 10.5 |  | $ 3.7
Natural Gas [Member] |  |  |
Derivative [Line Items] |  |  |
Investment Contract Volume | MMcf | 4.0 |  | 1.8
Diesel [Member] |  |  |
Derivative [Line Items] |  |  |
Investment Contract Volume | gal | 7.5 |  | 11.0
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Revenue from Contract with Customer [Member] |  |  |
Derivative [Line Items] |  |  |
Amount of gain/(loss) recognized in income on derivative | $ 17.1 | $ 41.9 |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Revenue from Contract with Customer [Member] |  |  |
Derivative [Line Items] |  |  |
Amount of gain/(loss) recognized in income on derivative | $ (11.6) | $ 1.9 |

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