DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) MMBTU MWh	Jun. 30, 2020 USD ($) MMBTU MWh	Jun. 30, 2021 USD ($) MMBTU MWh	Jun. 30, 2020 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ 15.0	$ (16.6)	$ 9.6	$ (39.9)
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ 4.8	$ (17.2)	$ (2.7)	$ (41.9)
Notional sales volumes
Notional sales volumes | MMBTU	47.9	44.7	107.7	103.1
FTRs
Realized gains (losses)
Gains (losses)	$ 10.2	$ 0.6	$ 12.3	$ 2.0
Notional sales volumes
Notional sales volumes | MWh	7.4	7.2	15.8	14.4