SEGMENT INFORMATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments	1
Yankee Gas, NSTAR Gas and EGMA
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased | $	$ 77.7
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