DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) MMBTU MWh gal	Sep. 30, 2016 USD ($) MMBTU MWh gal	Sep. 30, 2017 USD ($) MMBTU MWh gal	Sep. 30, 2016 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ 1.4	$ 3.6	$ 5.7	$ (7.6)
Natural gas contracts
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ (0.5)	$ (0.5)	$ 0.2	$ (12.5)
Notional Sales Volumes
Notional sales volumes | MMBTU	4.6	6.8	17.8	27.0
Petroleum products contracts
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ (0.5)	$ (0.4)	$ (1.4)	$ (1.9)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	4.1	3.3	13.9	7.5
FTRs
Realized Gain (Loss) on Derivatives
Gains (Losses)	$ 2.4	$ 4.5	$ 6.9	$ 6.8
Notional Sales Volumes
Notional sales volumes | MWh	6.9	7.7	21.2	18.6