PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Summary of Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 16,941,800	$ 16,703,200
Distribution - Natural Gas	6,158,400	6,111,200
Transmission - Electric	12,099,400	11,954,000
Distribution - Water	1,749,000	1,743,100
Solar	200,400	201,500
Utility	37,149,000	36,713,000
Other	1,365,200	1,269,000
Property, Plant and Equipment, Gross	38,514,200	37,982,000
Less:  Accumulated Depreciation
Utility	(8,628,000)	(8,476,300)
Other	(499,000)	(477,600)
Total Accumulated Depreciation	(9,127,000)	(8,953,900)
Property, Plant and Equipment, Net	29,387,200	29,028,100
Construction Work in Progress	1,878,600	1,854,400
Total Property, Plant and Equipment, Net	31,265,819	30,882,523
CL&P
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,919,200	6,820,700
Transmission - Electric	5,561,000	5,512,000
Solar	0	0
Property, Plant and Equipment, Gross	12,480,200	12,332,700
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(2,523,800)	(2,475,400)
Property, Plant and Equipment, Net	9,956,400	9,857,300
Construction Work in Progress	388,400	377,300
Total Property, Plant and Equipment, Net	10,344,782	10,234,556
NSTAR Electric
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	7,640,400	7,544,400
Transmission - Electric	4,782,900	4,701,300
Solar	200,400	201,500
Property, Plant and Equipment, Gross	12,623,700	12,447,200
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(3,130,900)	(3,074,100)
Property, Plant and Equipment, Net	9,492,800	9,373,100
Construction Work in Progress	759,500	750,000
Total Property, Plant and Equipment, Net	10,252,264	10,123,062
PSNH
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	2,422,500	2,378,400
Transmission - Electric	1,757,200	1,742,400
Solar	0	0
Property, Plant and Equipment, Gross	4,179,700	4,120,800
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(862,700)	(848,900)
Property, Plant and Equipment, Net	3,317,000	3,271,900
Construction Work in Progress	95,800	102,400
Total Property, Plant and Equipment, Net	$ 3,412,781	$ 3,374,270