PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Schedule of Aggregate Composite Depreciation Rates (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	3.00%	2.90%	3.00%
CL&P
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	2.80%	2.80%	2.80%
NSTAR Electric
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	2.80%	2.80%	2.90%
PSNH
Property, Plant and Equipment [Line Items]
Composite depreciation rate for plant in service	2.80%	2.80%	3.10%