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 ($) MWh MMBTU	Sep. 30, 2020 USD ($) MWh MMBTU	Sep. 30, 2021 USD ($) MWh MMBTU	Sep. 30, 2020 USD ($) MWh MMBTU
Realized gains (losses)
Gains (losses)	$ 44.2	$ (10.9)	$ 53.8	$ (50.8)
Natural gas contracts
Notional sales volumes
Notional sales volumes | MMBTU	39.4	36.2	147.1	139.3
Realized gains (losses)
Gains (losses)	$ 41.2	$ (12.0)	$ 38.5	$ (53.9)
FTRs
Notional sales volumes
Notional sales volumes | MWh	6.3	8.3	22.1	22.7
Realized gains (losses)
Gains (losses)	$ 3.0	$ 1.1	$ 15.3	$ 3.1