DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MMBTU MWh MW	Sep. 30, 2017 USD ($)	Dec. 31, 2017 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			10.6		9.5
Gains (losses) on deferred as regulatory costs | $	$ 1.6	$ 0.6	$ (25.8)	$ (30.3)
The Connecticut Light and Power Company
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