DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) MWh MMBTU gal	Mar. 31, 2017 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ (0.6)	$ 2.5
Natural gas contracts
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ (1.8)	$ 0.5
Notional Sales Volumes
Notional sales volumes | MMBTU	11.7	8.2
Petroleum products contracts
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ 0.4	$ (0.5)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	1.4	4.9
FTRs
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ 0.8	$ 2.5
Notional Sales Volumes
Notional sales volumes | MWh	5.8	7.0