PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, Plant and Equipment By Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 15,880,000	$ 15,071,100
Distribution - Natural Gas	3,931,100	3,546,200
Transmission - Electric	10,958,400	10,153,900
Distribution - Water	1,726,500	1,639,800
Solar	200,200	164,100
Utility	32,696,200	30,575,100
Other Assets	1,025,600	778,600
Property, Plant and Equipment, Gross	33,721,800	31,353,700
Less: Accumulated Depreciation
Utility	(7,483,500)	(7,126,200)
Other	(387,400)	(336,700)
Total Accumulated Depreciation	(7,870,900)	(7,462,900)
Property, Plant and Equipment, Net	25,850,900	23,890,800
Construction Work in Progress	1,734,600	1,719,600
Total Property, Plant and Equipment, Net	27,585,470	25,610,428
CL&P
Property, Plant and Equipment [Line Items]
Distribution - Electric	6,485,500	6,176,400
Transmission - Electric	5,043,000	4,700,500
Solar	0	0
Property, Plant and Equipment, Gross	11,528,500	10,876,900
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,385,700)	(2,302,600)
Property, Plant and Equipment, Net	9,142,800	8,574,300
Construction Work in Progress	483,000	335,400
Total Property, Plant and Equipment, Net	9,625,765	8,909,701
NSTAR Electric
Property, Plant and Equipment [Line Items]
Distribution - Electric	7,163,700	6,756,400
Transmission - Electric	4,411,900	4,065,900
Solar	200,200	164,100
Property, Plant and Equipment, Gross	11,775,800	10,986,400
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,895,300)	(2,702,000)
Property, Plant and Equipment, Net	8,880,500	8,284,400
Construction Work in Progress	592,300	510,300
Total Property, Plant and Equipment, Net	9,472,770	8,794,700
PSNH
Property, Plant and Equipment [Line Items]
Distribution - Electric	2,271,100	2,178,600
Transmission - Electric	1,498,700	1,338,700
Solar	0	0
Property, Plant and Equipment, Gross	3,769,800	3,517,300
Less: Accumulated Depreciation
Total Accumulated Depreciation	(799,900)	(772,900)
Property, Plant and Equipment, Net	2,969,900	2,744,400
Construction Work in Progress	159,600	135,700
Total Property, Plant and Equipment, Net	$ 3,129,506	$ 2,880,073