DERIVATIVE INSTRUMENTS - Narrative (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) MMBTU MWh MW	Sep. 30, 2019 USD ($)	Dec. 31, 2019 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	676		676		676
Amount of natural gas to be purchased under futures contracts | MMBTU			11.4		9.6
Gains (losses) on deferred as regulatory costs | $	$ (2.6)	$ (9.2)	$ (20.6)	$ (19.5)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			1.00%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			12.00%
Level 3 | Weighted Average
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			10.90%
CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts			80.00%
Amount of energy to be purchased under renewable energy contract (mwh) | MWh			0.1