DERIVATIVE INSTRUMENTS - NOTIONAL VOLUMES and GAINS (LOSSES) (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) MWh MMBTU gal	Jun. 30, 2018 USD ($) MWh MMBTU gal	Jun. 30, 2019 USD ($) MWh MMBTU gal	Jun. 30, 2018 USD ($) MWh MMBTU gal
Realized Gain (Loss)
Gain (Loss)	$ 1.8	$ 2.8	$ 2.0	$ 4.1
Natural gas contracts
Realized Gain (Loss)
Gain (Loss)	$ (0.7)	$ (0.3)	$ (1.2)	$ (2.0)
Notional Volumes
Notional sales volumes | MMBTU	8.7	8.3	20.8	21.3
Petroleum products contracts
Realized Gain (Loss)
Gain (Loss)	$ 0.0	$ 0.1	$ 0.0	$ 0.2
Notional Volumes
Notional sales volumes (gallons) | gal	0.0	0.6	0.0	1.3
FTRs
Realized Gain (Loss)
Gain (Loss)	$ 2.5	$ 3.0	$ 3.2	$ 5.9
Notional Volumes
Notional sales volumes | MWh	2.4	2.0	5.0	4.4