PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, Plant and Equipment By Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Distribution	$ 14,410,500	$ 13,716,900
Distribution - Natural Gas	3,244,200	3,010,400
Transmission - Electric	9,270,900	8,517,400
Water	1,558,400	0
Generation and Solar(2)	36,200	1,224,200
Utility	28,520,200	26,468,900
Other Assets	693,700	591,600
Property, Plant and Equipment, Gross	29,213,900	27,060,500
Less: Accumulated Depreciation
Utility	(6,846,900)	(6,480,400)
Other	(286,900)	(242,000)
Total Accumulated Depreciation	(7,133,800)	(6,722,400)
Property, Plant and Equipment, Net	22,080,100	20,338,100
Construction Work in Progress	1,537,400	1,012,400
Total Property, Plant and Equipment, Net	23,617,463	21,350,510
The Connecticut Light And Power Company
Property, Plant and Equipment [Line Items]
Distribution	5,888,300	5,562,900
Distribution - Natural Gas	4,239,900	3,912,900
Generation and Solar(2)	0	0
Property, Plant and Equipment, Gross	10,128,200	9,475,800
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,239,000)	(2,082,400)
Property, Plant and Equipment, Net	7,889,200	7,393,400
Construction Work in Progress	381,800	239,000
Total Property, Plant and Equipment, Net	8,271,030	7,632,392
NSTAR Electric Company
Property, Plant and Equipment [Line Items]
Distribution	6,479,000	6,244,200
Distribution - Natural Gas	3,821,200	3,496,900
Generation and Solar(2)	36,200	36,000
Property, Plant and Equipment, Gross	10,336,400	9,777,100
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,550,200)	(2,364,200)
Property, Plant and Equipment, Net	7,786,200	7,412,900
Construction Work in Progress	460,300	317,200
Total Property, Plant and Equipment, Net	8,246,494	7,730,096
Public Service Company Of New Hampshire
Property, Plant and Equipment [Line Items]
Distribution	2,083,400	1,949,800
Distribution - Natural Gas	1,161,300	1,059,300
Generation and Solar(2)	0	1,188,200
Property, Plant and Equipment, Gross	3,244,700	4,197,300
Less: Accumulated Depreciation
Total Accumulated Depreciation	(751,800)	(1,254,700)
Property, Plant and Equipment, Net	2,492,900	2,942,600
Construction Work in Progress	149,400	96,700
Total Property, Plant and Equipment, Net	$ 2,642,274	$ 3,039,313