PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	$ 6,540,400,000	$ 6,197,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	1,247,600,000	1,126,600,000
Public Utilities Property Plant And Equipment Transmission	3,541,900,000	3,378,000,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,096,000,000	697,100,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	12,425,900,000	11,398,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	305,100,000	305,500,000
Public Utilities Property Plant And Equipment Plant In Service	12,731,000,000	11,704,400,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(3,035,500,000)	(2,862,300,000)
OtherAccumulatedDepreciation	(120,200,000)	(119,900,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(3,155,700,000)	(2,982,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	9,575,300,000	8,722,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	827,800,000	845,500,000
Property, Plant and Equipment, Net	10,403,065,000	9,567,726,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.60%	2.70%	2.90%
Advance Payments on Equipment Purchase Contracts	15,200,000	9,300,000
Distribution Average Depreciable Lives	38.8
Transmission Average Depreciable Lives	41.2
Generation Average Depreciable Lives	29.6
Other Average Depreciable Lives	17.7
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	4,419,600,000	4,180,700,000
Public Utilities Property Plant And Equipment Transmission	2,689,100,000	2,668,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,108,700,000	6,849,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,596,700,000)	(1,508,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,512,000,000	5,340,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	315,400,000	246,100,000
Property, Plant and Equipment, Net	5,827,384,000	5,586,504,000
Net Book Value Of Assets Sold	42,500,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.40%	2.70%	3.00%
Advance Payments on Equipment Purchase Contracts	1,300,000	1,300,000
Distribution Average Depreciable Lives	42.1
Transmission Average Depreciable Lives	40.6
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	1,451,600,000	1,375,400,000
Public Utilities Property Plant And Equipment Transmission	546,400,000	476,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,074,800,000	687,700,000
Public Utilities Property Plant And Equipment Plant In Service	3,072,800,000	2,539,200,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(893,600,000)	(837,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,179,200,000	1,701,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	77,500,000	351,400,000
Property, Plant and Equipment, Net	2,256,688,000	2,053,281,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.90%	2.80%	2.70%
Advance Payments on Equipment Purchase Contracts		4,900,000
Distribution Average Depreciable Lives	33.9
Transmission Average Depreciable Lives	41.9
Generation Average Depreciable Lives	29.6
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	704,300,000	673,700,000
Public Utilities Property Plant And Equipment Transmission	297,400,000	233,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,200,000	9,400,000
Public Utilities Property Plant And Equipment Plant In Service	1,022,900,000	916,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(240,500,000)	(228,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	782,400,000	688,100,000
Public Utilities Property Plant And Equipment Construction Work In Progress	295,400,000	129,000,000
Property, Plant and Equipment, Net	1,077,833,000	817,146,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.90%	2.80%	2.90%
Advance Payments on Equipment Purchase Contracts	13,900,000	3,100,000
Distribution Average Depreciable Lives	29.6
Transmission Average Depreciable Lives	47.0
Generation Average Depreciable Lives	25.0
Other Average Depreciable Lives	0
The Rocky River Realty Company [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	161,500,000	166,000,000
Northeast Utilities Service Company [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	$ 131,500,000	$ 126,600,000