DERIVATIVE INSTRUMENTS - Narrative (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) MMBTU MWh MW	Mar. 31, 2018 USD ($)	Dec. 31, 2018 MMBTU
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	787
Amount of natural gas to be purchased under futures contracts | MMBTU	6.2		12.5
Gains (losses) on deferred as regulatory costs | $	$ (5.2)	$ (36.1)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	3.70%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts	15.10%
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