DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($) MMBTU MWh	Jun. 30, 2024 USD ($) MMBTU MWh	Jun. 30, 2025 USD ($) MMBTU MWh	Jun. 30, 2024 USD ($) MMBTU MWh
Realized gains and losses
Realized gains and losses on derivatives income statement location	Cost of sales	Cost of sales	Cost of sales	Cost of sales
Gains (losses)	$ 1.7	$ (4.2)	$ 3.0	$ (12.6)
Natural gas contracts
Realized gains and losses
Gains (losses)	$ 0.1	$ (4.6)	$ 0.8	$ (12.9)
Notional sales volumes
Notional sales volumes | MMBTU	6.5	9.7	18.8	24.1
FTRs
Realized gains and losses
Gains (losses)	$ 1.6	$ 0.4	$ 2.2	$ 0.3
Notional sales volumes
Notional sales volumes | MWh	1.9	2.1	4.2	4.2