DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) MMBTU MWh MW	Dec. 31, 2018 USD ($) MMBTU MW	Dec. 31, 2017 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	676	787
Amount of natural gas to be purchased under futures contracts (in MMBtu) | MMBTU	9.6	12.5
Loss deferred as regulatory costs | $	$ 20.7	$ 25.0	$ 29.0
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%
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	2.10%
Minimum | Level 3 | CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	13.60%