SEGMENT INFORMATION - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) segment
Segment Reporting Information [Line Items]
Amount of natural gas transmission purchased | $	$ 62.5
CL&P
Segment Reporting Information [Line Items]
Number of reportable segments	1
NSTAR Electric
Segment Reporting Information [Line Items]
Number of reportable segments	1
PSNH
Segment Reporting Information [Line Items]
Number of reportable segments	1