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 ($) MMBTU MWh	Sep. 30, 2020 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ 7.8	$ (1.3)	$ 11.9	$ (7.9)
Natural gas contracts
Notional sales volumes
Notional sales volumes | MMBTU	6.4	6.4	28.9	27.6
Realized gains (losses)
Gains (losses)	$ 6.0	$ (1.7)	$ 4.7	$ (8.8)
FTRs
Notional sales volumes
Notional sales volumes | MWh	0.9	2.3	5.3	6.2
Realized gains (losses)
Gains (losses)	$ 1.8	$ 0.4	$ 7.2	$ 0.9