DERIVATIVE INSTRUMENTS - Narrative (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) MW	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) MMBTU MWh MW	Jun. 30, 2018 USD ($)	Dec. 31, 2018 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	679		679		787
Amount of natural gas to be purchased under futures contracts | MMBTU			7.1		12.5
Gains (losses) on deferred as regulatory costs | $	$ (5.1)	$ 8.6	$ (10.3)	$ (27.5)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			3.10%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			14.60%
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