PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 6,645,000,000	$ 6,540,400,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	1,259,900,000	1,247,600,000
Public Utilities Property Plant And Equipment Transmission	3,590,700,000	3,541,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,132,000,000	1,096,000,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	12,627,600,000	12,425,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	308,400,000	305,100,000
Public Utilities Property Plant And Equipment Plant In Service	12,936,000,000	12,731,000,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(3,077,500,000)	(3,035,500,000)
OtherAccumulatedDepreciation	(122,500,000)	(120,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(3,200,000,000)	(3,155,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	9,736,000,000	9,575,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	877,200,000	827,800,000
Property, Plant and Equipment, Net	10,613,199,000	10,403,065,000
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,507,600,000	4,419,600,000
Public Utilities Property Plant And Equipment Transmission	2,693,700,000	2,689,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,201,300,000	7,108,700,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,618,700,000)	(1,596,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,582,600,000	5,512,000,000
Public Utilities Property Plant And Equipment Construction Work In Progress	325,800,000	315,400,000
Property, Plant and Equipment, Net	5,908,385,000	5,827,384,000
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,464,500,000	1,451,600,000
Public Utilities Property Plant And Equipment Transmission	548,100,000	546,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,110,800,000	1,074,800,000
Public Utilities Property Plant And Equipment Plant In Service	3,123,400,000	3,072,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(905,400,000)	(893,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,218,000,000	2,179,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	70,000,000	77,500,000
Property, Plant and Equipment, Net	2,287,951,000	2,256,688,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	708,600,000	704,300,000
Public Utilities Property Plant And Equipment Transmission	337,700,000	297,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,200,000	21,200,000
Public Utilities Property Plant And Equipment Plant In Service	1,067,500,000	1,022,900,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(243,300,000)	(240,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	824,200,000	782,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	328,800,000	295,400,000
Property, Plant and Equipment, Net	1,152,976,000	1,077,833,000
Rocky River Realty Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	162,200,000	161,500,000
Northeast Utilities Service Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	$ 134,100,000	$ 131,500,000