DERIVATIVE INSTRUMENTS - Narrative (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) MW	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) MMBTU MWh MW	Sep. 30, 2018 USD ($)	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			11.6		12.5
Gains (losses) on deferred as regulatory costs | $	$ (9.2)	$ 1.6	$ (19.5)	$ (25.8)
Level 3 | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			2.60%
Level 3 | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of exit price premiums related to derivative contracts			14.10%
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