PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 11,438,200,000	$ 6,540,400,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,274,200,000	1,247,600,000
Public Utilities Property Plant And Equipment Transmission	5,541,100,000	3,541,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,146,600,000	1,096,000,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	20,400,100,000	12,425,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	429,300,000	305,100,000
Public Utilities Property Plant And Equipment Plant In Service	20,829,400,000	12,731,000,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(5,065,100,000)	(3,035,500,000)
OtherAccumulatedDepreciation	(171,500,000)	(120,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,236,600,000)	(3,155,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	15,592,800,000	9,575,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	1,012,200,000	827,800,000
Property, Plant and Equipment, Net	16,605,010,000	10,403,065,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.50%	2.60%	2.70%
Distribution Average Depreciable Lives	42.1
Transmission Average Depreciable Lives	45.3
Generation Average Depreciable Lives	32.7
Other Average Depreciable Lives	16.7
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,691,300,000	4,419,600,000
Public Utilities Property Plant And Equipment Transmission	2,796,100,000	2,689,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,487,400,000	7,108,700,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,698,100,000)	(1,596,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,789,300,000	5,512,000,000
Public Utilities Property Plant And Equipment Construction Work In Progress	363,700,000	315,400,000
Property, Plant and Equipment, Net	6,152,959,000	5,827,384,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.50%	2.40%	2.70%
Distribution Average Depreciable Lives	41.8
Transmission Average Depreciable Lives	39.8
Generation Average Depreciable Lives	0
Other Average Depreciable Lives	0
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,520,100,000	1,451,600,000
Public Utilities Property Plant And Equipment Transmission	599,200,000	546,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,125,500,000	1,074,800,000
Public Utilities Property Plant And Equipment Plant In Service	3,244,800,000	3,072,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(954,000,000)	(893,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,290,800,000	2,179,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	61,700,000	77,500,000
Property, Plant and Equipment, Net	2,352,515,000	2,256,688,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	2.90%	2.80%
Distribution Average Depreciable Lives	33.8
Transmission Average Depreciable Lives	42.1
Generation Average Depreciable Lives	32.8
Other Average Depreciable Lives	0
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	724,200,000	704,300,000
Public Utilities Property Plant And Equipment Transmission	583,700,000	297,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,100,000	21,200,000
Public Utilities Property Plant And Equipment Plant In Service	1,329,000,000	1,022,900,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(252,100,000)	(240,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,076,900,000	782,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	213,600,000	295,400,000
Property, Plant and Equipment, Net	1,290,498,000	1,077,833,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.30%	2.90%	2.80%
Distribution Average Depreciable Lives	30.2
Transmission Average Depreciable Lives	47.5
Generation Average Depreciable Lives	25.0
Other Average Depreciable Lives	0
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,539,900,000	4,334,400,000
Public Utilities Property Plant And Equipment Transmission	1,529,700,000	1,386,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,069,600,000	5,721,300,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation		(1,436,000,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,540,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,529,500,000	4,285,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	205,800,000	162,000,000
Property, Plant and Equipment, Net	$ 4,735,297,000	$ 4,447,258,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.80%	3.00%	3.00%
Distribution Average Depreciable Lives	33.9
Transmission Average Depreciable Lives	46.3
Generation Average Depreciable Lives	0
Other Average Depreciable Lives	0