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DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2018 USD ($) MMBTU MWh gal | Jun. 30, 2017 USD ($) MMBTU MWh gal | Jun. 30, 2018 USD ($) MMBTU MWh gal | Jun. 30, 2017 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ 1.9 | $ 3.1 | $ 0.9 | $ 5.3
Natural gas contracts |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ (2.3) | $ 1.3 | $ (7.5) | $ 1.0
Notional Sales Volumes |  |  |  |
Notional sales volumes | MMBTU | 39.9 | 25.2 | 88.0 | 59.3
Petroleum products contracts |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ 0.3 | $ (0.4) | $ 0.8 | $ (0.9)
Notional Sales Volumes |  |  |  |
Notional sales volumes (gallons) | gal | 1.7 | 4.9 | 3.8 | 9.8
FTRs |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ 3.9 | $ 2.2 | $ 7.6 | $ 5.2
Notional Sales Volumes |  |  |  |
Notional sales volumes | MWh | 6.8 | 9.4 | 15.0 | 18.6

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