PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 12,701,600	$ 12,495,200
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,625,900	2,595,400
Public Utilities Property Plant And Equipment Transmission	7,196,300	6,930,700
Public Utilities Property Plant And Equipment Generation Or Processing	1,175,900	1,170,900
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	23,699,700	23,192,200
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	547,500	551,300
Public Utilities Property Plant And Equipment Plant In Service	24,247,200	23,743,500
Less Accumulated Depreciation [Abstract]
Electric And Natural Gas Utility Accumulated Depreciation	(5,950,400)	(5,777,800)
Other Accumulated Depreciation	(238,400)	(231,800)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,188,800)	(6,009,600)
Property Plant And Equipment Net Excluding Construction Work In Progress	18,058,400	17,733,900
Public Utilities Property Plant And Equipment Construction Work In Progress	1,020,800	913,100
Property, Plant and Equipment, Net	19,079,189	18,647,041
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,245,300	5,158,800
Public Utilities Property Plant And Equipment Transmission	3,455,500	3,274,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,700,800	8,432,800
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,975,900)	(1,928,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,724,900	6,504,800
Public Utilities Property Plant And Equipment Construction Work In Progress	209,700	304,900
Property, Plant and Equipment, Net	6,934,618	6,809,664
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,954,700	4,895,500
Public Utilities Property Plant And Equipment Transmission	1,965,200	1,928,500
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,919,900	6,824,000
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,824,900)	(1,761,400)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,095,000	5,062,600
Public Utilities Property Plant And Equipment Construction Work In Progress	356,700	272,800
Property, Plant and Equipment, Net	5,451,668	5,335,436
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,744,800	1,696,700
Public Utilities Property Plant And Equipment Transmission	821,200	789,700
Public Utilities Property Plant And Equipment Generation Or Processing	1,141,500	1,136,500
Public Utilities Property Plant And Equipment Plant In Service	3,707,500	3,622,900
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,127,600)	(1,090,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,579,900	2,532,900
Public Utilities Property Plant And Equipment Construction Work In Progress	135,000	102,900
Property, Plant and Equipment, Net	2,714,915	2,635,844
Public Service Company Of New Hampshire [Member] | Generation Plant [Member]
Less Accumulated Depreciation [Abstract]
Electric And Natural Gas Utility Accumulated Depreciation	(505,600)
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	796,800	784,200
Public Utilities Property Plant And Equipment Transmission	906,400	891,000
Public Utilities Property Plant And Equipment Generation Or Processing	34,400	34,400
Public Utilities Property Plant And Equipment Plant In Service	1,737,600	1,709,600
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(309,200)	(297,400)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,428,400	1,412,200
Public Utilities Property Plant And Equipment Construction Work In Progress	79,800	49,100
Property, Plant and Equipment, Net	$ 1,508,207	$ 1,461,321