PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Aggregate Composite Depreciation Rates (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	3.00%	3.00%	2.90%
The Connecticut Light And Power Company
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	2.80%	2.70%	2.70%
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	2.90%	2.90%	2.90%
Public Service Company Of New Hampshire
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	3.10%	3.10%	3.20%