PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, plant and equipment by asset (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 14,217,300	$ 13,716,900
Distribution - Natural Gas	3,158,100	3,010,400
Transmission	8,918,200	8,517,400
Generation	1,215,800	1,224,200
Electric and Natural Gas Utility	27,509,400	26,468,900
Other	679,900	591,600
Property, Plant and Equipment, Gross	28,189,300	27,060,500
Less: Accumulated Depreciation
Electric and Natural Gas Utility	(6,838,500)	(6,480,400)
Other	(274,400)	(242,000)
Total Accumulated Depreciation	(7,112,900)	(6,722,400)
Property, Plant and Equipment, Net	21,076,400	20,338,100
Construction Work in Progress	1,460,900	1,012,400
Net Plant	22,537,304	21,350,510
Construction work in progress related to NPT	201,000
The Connecticut Light And Power Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	5,797,600	5,562,900
Transmission	4,061,200	3,912,900
Generation	0	0
Property, Plant and Equipment, Gross	9,858,800	9,475,800
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,207,000)	(2,082,400)
Property, Plant and Equipment, Net	7,651,800	7,393,400
Construction Work in Progress	456,200	239,000
Net Plant	8,107,957	7,632,392
NSTAR Electric Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	5,543,100	5,402,300
Transmission	2,545,000	2,435,800
Generation	0	0
Property, Plant and Equipment, Gross	8,088,100	7,838,100
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,143,800)	(2,025,400)
Property, Plant and Equipment, Net	5,944,300	5,812,700
Construction Work in Progress	324,400	239,100
Net Plant	6,268,689	6,051,835
Public Service Company Of New Hampshire
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	2,048,800	1,949,800
Transmission	1,115,700	1,059,300
Generation	1,179,800	1,188,200
Property, Plant and Equipment, Gross	4,344,300	4,197,300
Less: Accumulated Depreciation
Total Accumulated Depreciation	(1,315,700)	(1,254,700)
Property, Plant and Equipment, Net	3,028,600	2,942,600
Construction Work in Progress	139,300	96,700
Net Plant	3,167,905	3,039,313
Western Massachusetts Electric Company
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	868,100	841,900
Transmission	1,147,900	1,061,100
Generation	36,000	36,000
Property, Plant and Equipment, Gross	2,052,000	1,939,000
Less: Accumulated Depreciation
Total Accumulated Depreciation	(356,500)	(338,800)
Property, Plant and Equipment, Net	1,695,500	1,600,200
Construction Work in Progress	74,100	78,100
Net Plant	$ 1,769,566	$ 1,678,262