PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Average Remaining Useful Lives of Depreciable Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Distribution - Electric	34 years 1 month 6 days
Distribution - Natural Gas	43 years 9 months 18 days
Distribution - Natural Gas	$ 3,546.2	$ 3,244.2
Transmission - Electric	41 years 3 months 18 days
Distribution - Water	33 years 3 months 18 days
Solar	24 years 10 months 24 days
Other	12 years 10 months 24 days
The Connecticut Light And Power Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	35 years 4 months 24 days
Transmission - Electric	38 years
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	33 years 8 months 24 days
Transmission - Electric	45 years 3 months 18 days
Solar	24 years 10 months 24 days
Public Service Company Of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution - Electric	32 years 3 months 18 days
Transmission - Electric	42 years 10 months 24 days