PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 11,950,200,000	$ 11,438,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,425,900,000	2,274,200,000
Public Utilities Property Plant And Equipment Transmission	6,412,500,000	5,541,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,152,300,000	1,146,600,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	21,940,900,000	20,400,100,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	508,700,000	429,300,000
Public Utilities Property Plant And Equipment Plant In Service	22,449,600,000	20,829,400,000
Electric And Natural Gas Utility Accumulated Depreciation	(5,387,000,000)	(5,065,100,000)
Other Accumulated Depreciation	(196,200,000)	(171,500,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,583,200,000)	(5,236,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	16,866,400,000	15,592,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	709,800,000	1,012,200,000
Property, Plant and Equipment, Net	17,576,186,000	16,605,010,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.80%	2.50%	2.60%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	36.1
Transmission Assets Average Useful Life	43.0
Generation Assets Average Useful Life	32.2
Other Assets Average Useful Life	14.6
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,930,700,000	4,691,300,000
Public Utilities Property Plant And Equipment Transmission	3,071,900,000	2,796,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,002,600,000	7,487,400,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,804,100,000)	(1,698,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,198,500,000	5,789,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	252,800,000	363,700,000
Property, Plant and Equipment, Net	6,451,259,000	6,152,959,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.50%	2.50%	2.40%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	42.0
Transmission Assets Average Useful Life	39.6
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,694,700,000	4,539,900,000
Public Utilities Property Plant And Equipment Transmission	1,772,300,000	1,529,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,467,000,000	6,069,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,631,300,000)	(1,540,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,835,700,000	4,529,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	208,200,000	205,800,000
Property, Plant and Equipment, Net	5,043,887,000	4,735,297,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.90%	2.80%	3.00%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.9
Transmission Assets Average Useful Life	47.2
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,608,200,000	1,520,100,000
Public Utilities Property Plant And Equipment Transmission	695,700,000	599,200,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,131,200,000	1,125,500,000
Public Utilities Property Plant And Equipment Plant In Service	3,435,100,000	3,244,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,021,800,000)	(954,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,413,300,000	2,290,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	54,300,000	61,700,000
Property, Plant and Equipment, Net	2,467,556,000	2,352,515,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	3.00%	2.90%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.7
Transmission Assets Average Useful Life	42.3
Generation Assets Average Useful Life	32.4
Other Assets Average Useful Life	0
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	756,600,000	724,200,000
Public Utilities Property Plant And Equipment Transmission	826,400,000	583,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,100,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,604,100,000	1,329,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(271,500,000)	(252,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,332,600,000	1,076,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	48,500,000	213,600,000
Property, Plant and Equipment, Net	$ 1,381,060,000	$ 1,290,498,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.90%	3.30%	2.90%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	29.8
Transmission Assets Average Useful Life	49.5
Generation Assets Average Useful Life	25.0
Other Assets Average Useful Life	0