PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, Plant and Equipment By Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Distribution - Electric	$ 13,716,900	$ 13,054,800
Distribution - Natural Gas	3,010,400	2,727,200
Transmission - Electric	8,517,400	7,691,900
Generation	1,224,200	1,194,100
Electric and Natural Gas Utility	26,468,900	24,668,000
Other	591,600	558,600
Property, Plant and Equipment, Gross	27,060,500	25,226,600
Less: Accumulated Depreciation
Electric and Natural Gas Utility	(6,480,400)	(6,141,100)
Other	(242,000)	(255,600)
Total Accumulated Depreciation	(6,722,400)	(6,396,700)
Property, Plant and Equipment, Net	20,338,100	18,829,900
Construction Work in Progress	1,012,400	1,062,500
Total Property, Plant and Equipment, Net	21,350,510	19,892,441
The Connecticut Light And Power Company [Member]
Property, Plant and Equipment [Line Items]
Distribution - Electric	5,562,900	5,377,200
Transmission - Electric	3,912,900	3,618,000
Generation	0	0
Property, Plant and Equipment, Gross	9,475,800	8,995,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,082,400)	(2,041,900)
Property, Plant and Equipment, Net	7,393,400	6,953,300
Construction Work in Progress	239,000	203,500
Total Property, Plant and Equipment, Net	7,632,392	7,156,809
NSTAR Electric Company [Member]
Property, Plant and Equipment [Line Items]
Distribution - Electric	5,402,300	5,100,500
Transmission - Electric	2,435,800	2,131,300
Generation	0	0
Property, Plant and Equipment, Gross	7,838,100	7,231,800
Less: Accumulated Depreciation
Total Accumulated Depreciation	(2,025,400)	(1,886,800)
Property, Plant and Equipment, Net	5,812,700	5,345,000
Construction Work in Progress	239,100	310,500
Total Property, Plant and Equipment, Net	6,051,835	5,655,458
Public Service Company Of New Hampshire [Member]
Property, Plant and Equipment [Line Items]
Distribution - Electric	1,949,800	1,804,800
Transmission - Electric	1,059,300	928,200
Generation	1,188,200	1,158,100
Property, Plant and Equipment, Gross	4,197,300	3,891,100
Less: Accumulated Depreciation
Total Accumulated Depreciation	(1,254,700)	(1,171,000)
Property, Plant and Equipment, Net	2,942,600	2,720,100
Construction Work in Progress	96,700	135,300
Total Property, Plant and Equipment, Net	3,039,313	2,855,363
Western Massachusetts Electric Company [Member]
Property, Plant and Equipment [Line Items]
Distribution - Electric	841,900	812,300
Transmission - Electric	1,061,100	964,900
Generation	36,000	36,000
Property, Plant and Equipment, Gross	1,939,000	1,813,200
Less: Accumulated Depreciation
Total Accumulated Depreciation	(338,800)	(307,000)
Property, Plant and Equipment, Net	1,600,200	1,506,200
Construction Work in Progress	78,100	69,100
Total Property, Plant and Equipment, Net	$ 1,678,262	$ 1,575,306