DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions		3 Months Ended		9 Months Ended
	Oct. 31, 2025 USD ($)	Sep. 30, 2025 USD ($) MW	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) MW	Sep. 30, 2024 USD ($)	Dec. 31, 2024 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		609		609		610
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		$ 0.1	$ 0.5	$ 0.7	$ 3.4
NSTAR Electric | Subsequent Event
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, term of contract	20 years
Derivative notional amount	$ 6,700.0
Derivative contracts at fair value, resources payment, term	20 years
Derivative liability	$ 350.0