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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 | 9 Months Ended
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Sep. 30, 2017 USD ($) MMBTU MWh gal | Sep. 30, 2016 USD ($) MMBTU MWh gal | Sep. 30, 2017 USD ($) MMBTU MWh gal | Sep. 30, 2016 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ 1.6 | $ 3.3 | $ 6.9 | $ (47.7)
Natural gas contracts |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ (2.1) | $ (3.4) | $ (1.1) | $ (56.9)
Notional Sales Volumes |  |  |  |
Notional sales volumes | MMBTU | 24.9 | 30.5 | 84.2 | 113.3
Petroleum products contracts |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ (0.5) | $ (0.4) | $ (1.4) | $ (2.5)
Notional Sales Volumes |  |  |  |
Notional sales volumes (gallons) | gal | 4.4 | 4.3 | 14.2 | 10.9
FTRs |  |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |  |
Gains (Losses) | $ 4.2 | $ 7.1 | $ 9.4 | $ 11.7
Notional Sales Volumes |  |  |  |
Notional sales volumes | MWh | 9.4 | 9.9 | 28.0 | 24.9

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