DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions		12 Months Ended
	Oct. 31, 2025	Dec. 31, 2025 USD ($) MWh MW	Dec. 31, 2024 USD ($) MW	Dec. 31, 2023 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company (as a percentage)		20.00%
Gain (loss) on deferred regulatory costs | $		$ (662.9)	$ (3.8)	$ (3.9)
NSTAR Electric
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Power purchase agreement	20 years
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW		579
NSTAR Electric | Minimum | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Energy price		24.65
NSTAR Electric | Maximum | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Energy price		145.33
NSTAR Electric | Weighted Average | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Energy price		53.44
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW		3	610
Percentage of costs or benefits borne under capacity-related contracts (as a percentage)		80.00%