DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) MMBTU MWh	Dec. 31, 2020 USD ($) MMBTU MWh	Dec. 31, 2019 USD ($) MWh MMBTU
Realized gains (losses) on derivatives
Gains (losses)	$ 30.5	$ (7.3)	$ 1.9
Natural gas contracts
Realized gains (losses) on derivatives
Gains (losses)	$ 21.8	$ (8.7)	$ (5.3)
Notional sales volumes
Notional sales volumes | MMBTU	37.5	38.1	35.4
FTRs
Realized gains (losses) on derivatives
Gains (losses)	$ 8.7	$ 1.4	$ 7.2
Notional sales volumes
Notional sales volumes | MWh	7.0	8.3	9.1