DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) MMBTU MWh	Mar. 31, 2020 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ (5.4)	$ (23.3)
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ (7.5)	$ (24.7)
Notional sales volumes
Notional sales volumes | MMBTU	59.8	58.4
FTRs
Realized gains (losses)
Gains (losses)	$ 2.1	$ 1.4
Notional sales volumes
Notional sales volumes | MWh	8.4	7.2