PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, Plant and Equipment By Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 15,071,100	$ 14,410,500
Distribution - Natural Gas	3,546,200	3,244,200
Transmission - Electric	10,153,900	9,270,900
Distribution - Water	1,639,800	1,558,400
Solar	164,100	36,200
Utility	30,575,100	28,520,200
Other Assets	778,600	693,700
Property, Plant and Equipment, Gross	31,353,700	29,213,900
Less: Accumulated Depreciation
Utility	(7,126,200)	(6,846,900)
Other	(336,700)	(286,900)
Total Accumulated Depreciation	(7,462,900)	(7,133,800)
Property, Plant and Equipment, Net	23,890,800	22,080,100
Construction Work in Progress	1,719,600	1,537,400
Total Property, Plant and Equipment, Net	25,610,428	23,617,463
The Connecticut Light And Power Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	6,176,400	5,888,300
Transmission - Electric	4,700,500	4,239,900
Solar	0	0
Property, Plant and Equipment, Gross	10,876,900	10,128,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,302,600)	(2,239,000)
Property, Plant and Equipment, Net	8,574,300	7,889,200
Construction Work in Progress	335,400	381,800
Total Property, Plant and Equipment, Net	8,909,701	8,271,030
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution - Electric	6,756,400	6,479,000
Transmission - Electric	4,065,900	3,821,200
Solar	164,100	36,200
Property, Plant and Equipment, Gross	10,986,400	10,336,400
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,702,000)	(2,550,200)
Property, Plant and Equipment, Net	8,284,400	7,786,200
Construction Work in Progress	510,300	460,300
Total Property, Plant and Equipment, Net	8,794,700	8,246,494
Public Service Company Of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution - Electric	2,178,600	2,083,400
Transmission - Electric	1,338,700	1,161,300
Solar	0	0
Property, Plant and Equipment, Gross	3,517,300	3,244,700
Less: Accumulated Depreciation
Total Accumulated Depreciation	(772,900)	(751,800)
Property, Plant and Equipment, Net	2,744,400	2,492,900
Construction Work in Progress	135,700	149,400
Total Property, Plant and Equipment, Net	$ 2,880,073	$ 2,642,274