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 ($) MMBTU MWh gal	Mar. 31, 2017 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (1.0)	$ 2.2
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (5.2)	$ (0.3)
Notional Sales Volumes
Notional sales volumes | MMBTU	48.1	34.1
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 0.5	$ (0.5)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	2.1	4.9
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 3.7	$ 3.0
Notional Sales Volumes
Notional sales volumes | MWh	8.2	9.2