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
Sep. 30, 2018 USD ($) MWh MMBTU gal	Sep. 30, 2017 USD ($) MWh MMBTU gal	Sep. 30, 2018 USD ($) MWh MMBTU gal	Sep. 30, 2017 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 8.0	$ 1.6	$ 8.9	$ 6.9
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 0.4	$ (2.1)	$ (7.1)	$ (1.1)
Notional Sales Volumes
Notional sales volumes \| MMBTU	36.7	24.9	124.7	84.2
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 0.5	$ (0.5)	$ 1.3	$ (1.4)
Notional Sales Volumes
Notional sales volumes (gallons) \| gal	1.3	4.4	5.1	14.2
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 7.1	$ 4.2	$ 14.7	$ 9.4
Notional Sales Volumes
Notional sales volumes \| MWh	7.9	9.4	22.9	28.0