PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Average Remaining Useful Lives of Depreciable Assets (Details)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Distribution	35 years 1 month 6 days
Transmission	41 years 6 months
Generation	29 years
Other	11 years
The Connecticut Light And Power Company [Member]
Property, Plant and Equipment [Line Items]
Distribution	37 years 8 months 12 days
Transmission	38 years
NSTAR Electric Company [Member]
Property, Plant and Equipment [Line Items]
Distribution	32 years
Transmission	43 years 9 months 18 days
Public Service Company Of New Hampshire [Member]
Property, Plant and Equipment [Line Items]
Distribution	31 years 3 months 18 days
Transmission	43 years 6 months
Generation	29 years 1 month 6 days
Western Massachusetts Electric Company [Member]
Property, Plant and Equipment [Line Items]
Distribution	30 years 8 months 12 days
Transmission	49 years 8 months 12 days
Generation	25 years