SEGMENT INFORMATION - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) companies segment
Segment Reporting Information [Line Items]
Number of companies that transmit hydro electricity imported from The Hydro-Quebec System in Canada | companies	2
Amount of natural gas transmission purchased from Enbridge by Yankee Gas and NSTAR Gas | $	$ 62.5
The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1
NSTAR Electric Company [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1
Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1
Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1