DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) MMBTU MWh MW	Mar. 31, 2017 USD ($)	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 | MW	787
Amount of natural gas to be purchased under futures contracts | MMBTU	5.9		9.5
Gains (losses) on deferred as regulatory costs | $	$ (36.1)	$ (26.5)
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 (in mwh) | MWh	0.1