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DERIVATIVE INSTRUMENTS - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
MW
Dec. 31, 2023
USD ($)
MW
Dec. 31, 2022
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 610 682  
Gain (loss) on deferred regulatory costs | $ $ (3.8) $ (3.9) $ 10.1
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%