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
	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