DERIVATIVE INSTRUMENTS - Narrative (Details) MMBTU in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) MMBTU MW	Mar. 31, 2020 USD ($)	Dec. 31, 2020 MMBTU 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
Amount of natural gas to be purchased under futures contracts | MMBTU	1.2		8.9
Gains (losses) on deferred as regulatory costs | $	$ (11.1)	$ (18.0)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	6.60%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	10.90%
Level 3 | Weighted Average
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	9.80%
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts	80.00%