DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU MWh MW	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company (as a percentage)	20.00%
Amount of power to be purchased under capacity-related contract (up to for 35 mw) (in mw) | MW	787
Amount of natural gas to be purchased under futures contracts (in MMBtu) | MMBTU	9.5	9.2
Gain (loss) deferred as regulatory costs | $	$ (29.0)	$ (125.5)	$ 60.2
The Connecticut Light And Power Company
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of costs or benefits borne under capacity-related contracts (as a percentage)	80.00%
Amount of energy to be purchased under renewable energy contract (in mwh) | MWh	0.1