SEGMENT INFORMATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) company segment	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of companies that transmit hydro electricity | company	2
Amount of natural gas transmission purchased | $	$ 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
Transmission Merger Cost Recovery
Segment Reporting Information [Line Items]
Amount of recovery through transmission rates | $		$ 27.5