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 ($) MWh MMBTU	Jun. 30, 2020 USD ($) MWh MMBTU	Jun. 30, 2021 USD ($) MWh MMBTU	Jun. 30, 2020 USD ($) MWh MMBTU
Realized gains (losses)
Gains (losses)	$ 6.5	$ (3.9)	$ 4.9	$ (10.0)
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ 1.1	$ (4.4)	$ (1.6)	$ (11.3)
Notional sales volumes
Notional sales volumes | MMBTU	16.5	14.0	35.6	33.1
FTRs
Realized gains (losses)
Gains (losses)	$ 5.4	$ 0.5	$ 6.5	$ 1.3
Notional sales volumes
Notional sales volumes | MWh	5.5	5.1	11.1	10.2