PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, Plant and Equipment By Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 16,703,200	$ 15,880,000
Distribution - Natural Gas	6,111,200	3,931,100
Transmission - Electric	11,954,000	10,958,400
Distribution - Water	1,743,100	1,726,500
Solar	201,500	200,200
Utility	36,713,000	32,696,200
Other	1,269,000	1,025,600
Property, Plant and Equipment, Gross	37,982,000	33,721,800
Less:  Accumulated Depreciation
Utility	(8,476,300)	(7,483,500)
Other	(477,600)	(387,400)
Total Accumulated Depreciation	(8,953,900)	(7,870,900)
Property, Plant and Equipment, Net	29,028,100	25,850,900
Construction Work in Progress	1,854,400	1,734,600
Total Property, Plant and Equipment, Net	30,882,523	27,585,470
The Connecticut Light and Power Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	6,820,700	6,485,500
Transmission - Electric	5,512,000	5,043,000
Solar	0	0
Property, Plant and Equipment, Gross	12,332,700	11,528,500
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(2,475,400)	(2,385,700)
Property, Plant and Equipment, Net	9,857,300	9,142,800
Construction Work in Progress	377,300	483,000
Total Property, Plant and Equipment, Net	10,234,556	9,625,765
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	7,544,400	7,163,700
Transmission - Electric	4,701,300	4,411,900
Solar	201,500	200,200
Property, Plant and Equipment, Gross	12,447,200	11,775,800
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(3,074,100)	(2,895,300)
Property, Plant and Equipment, Net	9,373,100	8,880,500
Construction Work in Progress	750,000	592,300
Total Property, Plant and Equipment, Net	10,123,062	9,472,770
Public Service Company of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution - Electric	2,378,400	2,271,100
Transmission - Electric	1,742,400	1,498,700
Solar	0	0
Property, Plant and Equipment, Gross	4,120,800	3,769,800
Less:  Accumulated Depreciation
Total Accumulated Depreciation	(848,900)	(799,900)
Property, Plant and Equipment, Net	3,271,900	2,969,900
Construction Work in Progress	102,400	159,600
Total Property, Plant and Equipment, Net	$ 3,374,270	$ 3,129,506