DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) MMBTU MWh	Mar. 31, 2021 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ 32.6	$ (5.4)
Natural gas contracts
Notional sales volumes
Notional sales volumes | MMBTU	59.5	59.8
Realized gains (losses)
Gains (losses)	$ 31.6	$ (7.5)
FTRs
Notional sales volumes
Notional sales volumes | MWh	7.0	8.4
Realized gains (losses)
Gains (losses)	$ 1.0	$ 2.1