PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, plant and equipment by asset (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 14,511,000	$ 14,410,500
Distribution - Natural Gas	3,273,200	3,244,200
Transmission - Electric	9,420,900	9,270,900
Distribution - Water	1,556,400	1,558,400
Generation and Solar	36,200	36,200
Utility	28,797,700	28,520,200
Other	698,400	693,700
Property, Plant and Equipment, Gross	29,496,100	29,213,900
Less: Accumulated Depreciation
Utility	(6,927,500)	(6,846,900)
Other	(300,000)	(286,900)
Total Accumulated Depreciation	(7,227,500)	(7,133,800)
Property, Plant and Equipment, Net	22,268,600	22,080,100
Construction Work in Progress	1,690,100	1,537,400
Net Plant	23,958,688	23,617,463
The Connecticut Light And Power Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	5,939,900	5,888,300
Transmission - Electric	4,322,600	4,239,900
Generation and Solar	0	0
Property, Plant and Equipment, Gross	10,262,500	10,128,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,274,400)	(2,239,000)
Property, Plant and Equipment, Net	7,988,100	7,889,200
Construction Work in Progress	418,500	381,800
Net Plant	8,406,595	8,271,030
NSTAR Electric Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	6,532,500	6,479,000
Transmission - Electric	3,842,700	3,821,200
Generation and Solar	36,200	36,200
Property, Plant and Equipment, Gross	10,411,400	10,336,400
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,599,300)	(2,550,200)
Property, Plant and Equipment, Net	7,812,100	7,786,200
Construction Work in Progress	511,700	460,300
Net Plant	8,323,826	8,246,494
Public Service Company Of New Hampshire
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	2,078,900	2,083,400
Transmission - Electric	1,207,100	1,161,300
Generation and Solar	0	0
Property, Plant and Equipment, Gross	3,286,000	3,244,700
Less: Accumulated Depreciation
Total Accumulated Depreciation	(734,900)	(751,800)
Property, Plant and Equipment, Net	2,551,100	2,492,900
Construction Work in Progress	142,900	149,400
Net Plant	$ 2,693,951	$ 2,642,274