DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) MWh MMBTU	Mar. 31, 2022 USD ($) MMBTU MWh
Realized gains and losses
Realized gains and losses on derivatives income statement location	Cost of sales	Cost of sales
Gains (losses)	$ (14.0)	$ 6.0
Natural gas contracts
Realized gains and losses
Gains (losses)	$ (14.8)	$ 5.2
Notional sales volumes
Notional sales volumes | MMBTU	11.7	11.6
FTRs
Realized gains and losses
Gains (losses)	$ 0.8	$ 0.8
Notional sales volumes
Notional sales volumes | MWh	2.1	2.0