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REVENUE (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2017
Contract with Customer, Liability, Revenue Recognized |  |  | $ 47.1 |  | $ 51.3 |  |
Take or Pay Contracts [Member] |  |  |  |  |  |  |
Contract with Customer, Liability | $ 2.7 |  | 2.7 |  | 8.2 |  |
Not Designated as Hedging Instrument [Member] |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 74.8 | $ 154.7 | 80.3 | $ 198.5 |  |  |
Other Current Liabilities [Member] |  |  |  |  |  |  |
Contract with Customer, Liability | 12.8 |  | 12.8 |  | 12.9 |  |
Other Liabilities [Member] |  |  |  |  |  |  |
Contract with Customer, Liability, Noncurrent | $ 34.3 |  | $ 34.3 |  | $ 38.5 | $ 47.1 | $ 51.4

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