DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) MMBTU MWh	Jun. 30, 2019 USD ($) MMBTU MWh	Jun. 30, 2020 USD ($) MMBTU MWh	Jun. 30, 2019 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ (16.6)	$ (0.1)	$ (39.9)	$ 1.7
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ (17.2)	$ (3.1)	$ (41.9)	$ (3.6)
Notional sales volumes
Notional sales volumes | MMBTU	44.7	43.8	103.1	99.9
FTRs
Realized gains (losses)
Gains (losses)	$ 0.6	$ 3.0	$ 2.0	$ 5.3
Notional sales volumes
Notional sales volumes | MWh	7.2	8.0	14.4	16.1