DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) MMBTU MWh	Jun. 30, 2020 USD ($) MWh MMBTU	Jun. 30, 2021 USD ($) MWh MMBTU	Jun. 30, 2020 USD ($) MWh MMBTU
Realized gains (losses)
Gains (losses)	$ 4.9	$ (2.6)	$ 4.1	$ (6.6)
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ 0.4	$ (2.7)	$ (1.3)	$ (7.1)
Notional sales volumes
Notional sales volumes | MMBTU	8.9	8.5	22.5	21.2
FTRs
Realized gains (losses)
Gains (losses)	$ 4.5	$ 0.1	$ 5.4	$ 0.5
Notional sales volumes
Notional sales volumes | MWh	1.7	2.0	4.4	3.9