SEGMENT INFORMATION - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) segment company
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 | $	$ 62.5
The Connecticut Light And Power Company
Segment Reporting Information [Line Items]
Number of reportable segments	1
NSTAR Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments	1
Public Service Company Of New Hampshire
Segment Reporting Information [Line Items]
Number of reportable segments	1
Western Massachusetts Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments	1