DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($) MW	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) MW	Sep. 30, 2023 USD ($)	Dec. 31, 2023 MW
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company (in percent)			20.00%
Amount of power to be purchased under capacity-related contract (in MW) | MW	615		615		682
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts (in percent)			80.00%
Derivative instruments losses recognized in regulatory asset liability | $	$ 0.5	$ 1.5	$ 3.4	$ 1.6