DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) MW	Mar. 31, 2021 USD ($)	Dec. 31, 2021 $ / KWmo MW
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company	20.00%
Amount of power to be purchased under capacity-related contract (in MW) | MW	675		675
Gains (losses) on deferred as regulatory costs | $	$ 6.2	$ (11.1)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	4.50%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	8.80%
Level 3 | Weighted Average
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	7.70%
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts	80.00%
CL&P | Weighted Average | Measurement Input, Capacity Prices
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input | $ / KWmo			2.61