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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) gal MMcf | Dec. 31, 2017 USD ($) gal MMcf | Dec. 31, 2016 USD ($)
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | $ 422.6 | $ 119.3 | $ 57.8
Derivative asset, fair value | 91.5 | 37.9 |
Derivative liability, fair value | 3.7 | 2.0 |
Not Designated as Hedging Instrument [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | 91.4 | 37.9 |
Derivative liability, fair value | 0.0 | 1.7 |
Provisional Pricing Arrangements [Member] | Product Revenues [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | $ (3.2) | $ (42.7) | $ 14.2
Natural Gas [Member] |  |  |
Derivative [Line Items] |  |  |
Natural Gas Hedges | MMcf | 1,800,000.0 | 3,500,000.0 |
Energy Related Derivative [Member] |  |  |
Derivative [Line Items] |  |  |
Natural Gas Hedges | gal | 11,000,000.0 | 0 |
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | $ 89.3 | $ 37.9 |
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative asset, fair value | 2.1 | 0.0 |
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative liability, fair value | $ 0.0 | $ 1.7 |

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