DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) MMBTU MWh gal	Mar. 31, 2018 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 1.8	$ (1.0)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.5)	$ (5.2)
Notional Sales Volumes
Notional sales volumes | MMBTU	56.1	48.1
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 0.0	$ 0.5
Notional Sales Volumes
Notional sales volumes (gallons) | gal	0.0	2.1
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 2.3	$ 3.7
Notional Sales Volumes
Notional sales volumes | MWh	8.1	8.2