DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) MMBTU MWh gal	Mar. 31, 2016 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 2.2	$ (31.6)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.3)	$ (33.5)
Notional Sales Volumes
Notional sales volumes (Dth or MWh) | MMBTU	34.1	50.1
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.5)	$ (1.1)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	4.9	3.0
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 3.0	$ 3.0
Notional Sales Volumes
Notional sales volumes (Dth or MWh) | MWh	9.2	7.6