DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) MWh MMBTU	Sep. 30, 2020 USD ($) MMBTU MWh	Sep. 30, 2021 USD ($) MWh MMBTU	Sep. 30, 2020 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ 17.3	$ (3.3)	$ 22.2	$ (13.3)
Natural gas contracts
Notional sales volumes
Notional sales volumes | MMBTU	15.9	12.7	51.5	45.8
Realized gains (losses)
Gains (losses)	$ 16.2	$ (4.1)	$ 14.6	$ (15.4)
FTRs
Notional sales volumes
Notional sales volumes | MWh	5.4	5.8	16.5	16.0
Realized gains (losses)
Gains (losses)	$ 1.1	$ 0.8	$ 7.6	$ 2.1