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DERIVATIVE INSTRUMENTS - Narrative (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2025
USD ($)
MW
Mar. 31, 2024
USD ($)
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 610   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 | $ $ 0.3 $ 1.0