PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Average Remaining Useful Lives of Depreciable Assets (Details)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Distribution	34 years 7 months 6 days
Transmission	40 years 10 months 25 days
Water	32 years
Solar	25 years
Other	12 years 8 months 12 days
The Connecticut Light And Power Company
Property, Plant and Equipment [Line Items]
Distribution	35 years 9 months 18 days
Transmission	37 years 2 months 12 days
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution	31 years 8 months 12 days
Transmission	44 years 8 months 12 days
Solar	25 years
Public Service Company Of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution	31 years 3 months 19 days
Transmission	43 years 6 months