DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) MW	Dec. 31, 2022 USD ($) $ / KWmo MW	Dec. 31, 2021 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%
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW	682	674
Gain (loss) on deferred regulatory costs | $	$ (3.9)	$ 10.1	$ (7.1)
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts (as a percentage)	80.00%
Minimum | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts		2.90%
Minimum | Level 3 | CL&P | Measurement Input, Forward Reserve
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input		0.44
Weighted Average | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts		6.10%
Weighted Average | Level 3 | CL&P | Measurement Input, Forward Reserve
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input		0.47
Maximum | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts		7.10%
Maximum | Level 3 | CL&P | Measurement Input, Forward Reserve
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input		0.50