DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MWh MMBTU MW	Dec. 31, 2019 USD ($) MMBTU MW	Dec. 31, 2018 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	675	676
Amount of natural gas to be purchased under futures contracts (in MMBtu) | MMBTU	8.9	9.6
Loss deferred as regulatory costs | $	$ 21.2	$ 20.7	$ 25.0
The Connecticut Light and Power Company
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts (as a percentage)	80.00%
Amount of energy to be purchased under renewable energy contract (in mwh) | MWh	0.1
Minimum | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	7.10%
Minimum | Level 3 | The Connecticut Light and Power Company
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	11.40%
Weighted Average | Level 3
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	10.30%