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, 2019 USD ($) MMBTU MWh gal	Jun. 30, 2018 USD ($) MMBTU MWh gal	Jun. 30, 2019 USD ($) MMBTU MWh gal	Jun. 30, 2018 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.1)	$ 1.9	$ 1.7	$ 0.9
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (3.1)	$ (2.3)	$ (3.6)	$ (7.5)
Notional Sales Volumes
Notional sales volumes | MMBTU	43.8	39.9	99.9	88.0
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 0.0	$ 0.3	$ 0.0	$ 0.8
Notional Sales Volumes
Notional sales volumes (gallons) | gal	0.0	1.7	0.0	3.8
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 3.0	$ 3.9	$ 5.3	$ 7.6
Notional Sales Volumes
Notional sales volumes | MWh	8.0	6.8	16.1	15.0