PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 14,794,900	$ 14,410,500
Distribution - Natural Gas	3,371,200	3,244,200
Transmission - Electric	9,733,600	9,270,900
Distribution - Water	1,594,300	1,558,400
Solar	109,300	36,200
Utility	29,603,300	28,520,200
Other	759,000	693,700
Property, Plant and Equipment, Gross	30,362,300	29,213,900
Less: Accumulated Depreciation
Utility	(7,065,400)	(6,846,900)
Other	(325,400)	(286,900)
Total Accumulated Depreciation	(7,390,800)	(7,133,800)
Property, Plant and Equipment, Net	22,971,500	22,080,100
Construction Work in Progress	1,996,200	1,537,400
Net Plant	24,967,702	23,617,463
The Connecticut Light and Power Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,027,100	5,888,300
Transmission - Electric	4,513,700	4,239,900
Solar	0	0
Property, Plant and Equipment, Gross	10,540,800	10,128,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,297,400)	(2,239,000)
Property, Plant and Equipment, Net	8,243,400	7,889,200
Construction Work in Progress	510,300	381,800
Net Plant	8,753,744	8,271,030
NSTAR Electric Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,651,100	6,479,000
Transmission - Electric	3,915,500	3,821,200
Solar	109,300	36,200
Property, Plant and Equipment, Gross	10,675,900	10,336,400
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,673,000)	(2,550,200)
Property, Plant and Equipment, Net	8,002,900	7,786,200
Construction Work in Progress	573,200	460,300
Net Plant	8,576,096	8,246,494
Public Service Company of New Hampshire
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	2,157,000	2,083,400
Transmission - Electric	1,255,600	1,161,300
Solar	0	0
Property, Plant and Equipment, Gross	3,412,600	3,244,700
Less: Accumulated Depreciation
Total Accumulated Depreciation	(762,100)	(751,800)
Property, Plant and Equipment, Net	2,650,500	2,492,900
Construction Work in Progress	176,000	149,400
Net Plant	$ 2,826,541	$ 2,642,274