PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Average Remaining Useful Lives of Depreciable Assets (Details)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Distribution - Electric	34 years 3 months 18 days
Distribution - Natural Gas	41 years 6 months
Transmission - Electric	40 years 8 months 12 days
Distribution - Water	34 years 1 month 6 days
Solar	24 years 3 months 18 days
Other	11 years
Computer Software, Hardware and Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of property, plant and equipment	5 years
Computer Software, Hardware and Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of property, plant and equipment	15 years
Buildings
Property, Plant and Equipment [Line Items]
Estimated useful life of property, plant and equipment	40 years
The Connecticut Light and Power Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	35 years 4 months 24 days
Transmission - Electric	36 years 10 months 24 days
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	33 years 8 months 12 days
Transmission - Electric	45 years 6 months
Solar	24 years 3 months 18 days
Public Service Company of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution - Electric	33 years 3 months 18 days
Transmission - Electric	42 years 7 months 6 days