PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, plant and equipment by asset (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 13,536,100	$ 13,054,800
Distribution - Natural Gas	2,865,800	2,727,200
Transmission - Electric	8,033,000	7,691,900
Generation	1,218,900	1,194,100
Electric and Natural Gas Utility	25,653,800	24,668,000
Other	635,300	558,600
Property, Plant and Equipment, Gross	26,289,100	25,226,600
Less: Accumulated Depreciation
Electric and Natural Gas Utility	(6,442,300)	(6,141,100)
Other	(285,800)	(255,600)
Total Accumulated Depreciation	(6,728,100)	(6,396,700)
Property, Plant and Equipment, Net	19,561,000	18,829,900
Construction Work in Progress	1,246,900	1,062,500
Net Plant	20,807,943	19,892,441
The Connecticut Light And Power Company [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	5,513,900	5,377,200
Transmission - Electric	3,752,700	3,618,000
Generation	0	0
Property, Plant and Equipment, Gross	9,266,600	8,995,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,102,300)	(2,041,900)
Property, Plant and Equipment, Net	7,164,300	6,953,300
Construction Work in Progress	288,900	203,500
Net Plant	7,453,200	7,156,809
NSTAR Electric Company [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	5,309,200	5,100,500
Transmission - Electric	2,211,700	2,131,300
Generation	0	0
Property, Plant and Equipment, Gross	7,520,900	7,231,800
Less: Accumulated Depreciation
Total Accumulated Depreciation	(1,996,800)	(1,886,800)
Property, Plant and Equipment, Net	5,524,100	5,345,000
Construction Work in Progress	351,800	310,500
Net Plant	5,875,874	5,655,458
Public Service Company Of New Hampshire [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	1,918,300	1,804,800
Transmission - Electric	1,005,500	928,200
Generation	1,182,900	1,158,100
Property, Plant and Equipment, Gross	4,106,700	3,891,100
Less: Accumulated Depreciation
Total Accumulated Depreciation	(1,243,800)	(1,171,000)
Property, Plant and Equipment, Net	2,862,900	2,720,100
Construction Work in Progress	110,200	135,300
Net Plant	2,973,057	2,855,363
Western Massachusetts Electric Company [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution - Electric	834,600	812,300
Transmission - Electric	1,014,900	964,900
Generation	36,000	36,000
Property, Plant and Equipment, Gross	1,885,500	1,813,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(330,600)	(307,000)
Property, Plant and Equipment, Net	1,554,900	1,506,200
Construction Work in Progress	88,400	69,100
Net Plant	$ 1,643,335	$ 1,575,306