SEGMENT INFORMATION - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment company	Jun. 30, 2017 USD ($)
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	$ 653,915	$ 549,704	$ 1,600,662	$ 1,303,353
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	234,335	207,211	$ 536,223	452,149
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	266,108	231,039	$ 598,687	504,891
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	$ 83,494	$ 60,442	$ 193,212	$ 122,189
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