DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) MMBTU MWh	Dec. 31, 2021 USD ($) MMBTU MWh	Dec. 31, 2020 USD ($) MMBTU MWh
Realized gains (losses) on derivatives
Gains (losses)	$ 45.6	$ 30.5	$ (7.3)
Realized gains and losses on derivatives income statement location	Cost of sales	Cost of sales	Cost of sales
Natural gas contracts
Realized gains (losses) on derivatives
Gains (losses)	$ 43.1	$ 21.8	$ (8.7)
Notional sales volumes
Notional sales volumes | MMBTU	33.4	37.5	38.1
FTRs
Realized gains (losses) on derivatives
Gains (losses)	$ 2.5	$ 8.7	$ 1.4
Notional sales volumes
Notional sales volumes | MWh	7.8	7.0	8.3