PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 13,227,400	$ 13,054,800
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,759,100	2,727,200
Public Utilities Property Plant And Equipment Transmission	7,725,300	7,691,900
Public Utilities Property Plant And Equipment Generation Or Processing	1,210,300	1,194,100
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	24,922,100	24,668,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	575,500	558,600
Public Utilities Property Plant And Equipment Plant In Service	25,497,600	25,226,600
Less Accumulated Depreciation [Abstract]
Electric And Natural Gas Utility Accumulated Depreciation	(6,249,000)	(6,141,100)
Other Accumulated Depreciation	(264,800)	(255,600)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,513,800)	(6,396,700)
Property Plant And Equipment Net Excluding Construction Work In Progress	18,983,800	18,829,900
Public Utilities Property Plant And Equipment Construction Work In Progress	1,112,900	1,062,500
Property, Plant and Equipment, Net	20,096,693	19,892,441
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,429,500	5,377,200
Public Utilities Property Plant And Equipment Transmission	3,631,300	3,618,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	9,060,800	8,995,200
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(2,073,700)	(2,041,900)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,987,100	6,953,300
Public Utilities Property Plant And Equipment Construction Work In Progress	244,100	203,500
Property, Plant and Equipment, Net	7,231,214	7,156,809
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,176,100	5,100,500
Public Utilities Property Plant And Equipment Transmission	2,137,700	2,131,300
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,313,800	7,231,800
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,924,400)	(1,886,800)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,389,400	5,345,000
Public Utilities Property Plant And Equipment Construction Work In Progress	310,700	310,500
Property, Plant and Equipment, Net	5,700,068	5,655,458
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,839,500	1,804,800
Public Utilities Property Plant And Equipment Transmission	936,100	928,200
Public Utilities Property Plant And Equipment Generation Or Processing	1,174,300	1,158,100
Public Utilities Property Plant And Equipment Plant In Service	3,949,900	3,891,100
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,192,000)	(1,171,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,757,900	2,720,100
Public Utilities Property Plant And Equipment Construction Work In Progress	123,500	135,300
Property, Plant and Equipment, Net	2,881,435	2,855,363
Public Service Company Of New Hampshire [Member] | Generation Plant [Member]
Less Accumulated Depreciation [Abstract]
Electric And Natural Gas Utility Accumulated Depreciation	(531,700)
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	822,200	812,300
Public Utilities Property Plant And Equipment Transmission	971,000	964,900
Public Utilities Property Plant And Equipment Generation Or Processing	36,000	36,000
Public Utilities Property Plant And Equipment Plant In Service	1,829,200	1,813,200
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(317,000)	(307,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,512,200	1,506,200
Public Utilities Property Plant And Equipment Construction Work In Progress	78,300	69,100
Property, Plant and Equipment, Net	$ 1,590,524	$ 1,575,306