DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) MWh MMBTU	Sep. 30, 2019 USD ($) MWh MMBTU	Sep. 30, 2020 USD ($) MWh MMBTU	Sep. 30, 2019 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ (10.9)	$ (5.6)	$ (50.8)	$ (3.9)
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ (12.0)	$ (13.2)	$ (53.9)	$ (16.8)
Notional sales volumes
Notional sales volumes | MMBTU	36.2	37.8	139.3	137.7
FTRs
Realized gains (losses)
Gains (losses)	$ 1.1	$ 7.6	$ 3.1	$ 12.9
Notional sales volumes
Notional sales volumes | MWh	8.3	7.8	22.7	23.9