SEGMENT INFORMATION - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment company	Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of companies that transmit hydro electricity imported from The Hydro-Quebec System in Canada | company			2
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas	$ 842,291	$ 651,776	$ 2,442,953	$ 1,955,129
The Connecticut Light and Power Company
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas	314,571	259,005	$ 850,794	711,154
Number of reportable segments | segment			1
NSTAR Electric Company
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas	383,208	294,115	$ 981,895	799,007
Number of reportable segments | segment			1
Public Service Company of New Hampshire
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas	$ 100,763	$ 57,099	$ 293,975	$ 179,289
Number of reportable segments | segment			1
Affiliated Entity
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas			$ 62,500