PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	$ 6,350,200,000	$ 6,197,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	1,168,500,000	1,126,600,000
Public Utilities Property Plant And Equipment Transmission	3,407,100,000	3,378,000,000
Public Utilities Property Plant And Equipment Generation Or Processing	720,600,000	697,100,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	11,646,400,000	11,398,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	304,400,000	305,500,000
Public Utilities Property Plant And Equipment Plant In Service	11,950,800,000	11,704,400,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(2,938,000,000)	(2,862,300,000)
OtherAccumulatedDepreciation	(119,300,000)	(119,900,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(3,057,300,000)	(2,982,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	8,893,500,000	8,722,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	970,300,000	845,500,000
Property, Plant and Equipment, Net	9,863,789,000	9,567,726,000
The Connecticut Light and Power Company [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	4,293,600,000	4,180,700,000
Public Utilities Property Plant And Equipment Transmission	2,649,200,000	2,668,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,942,800,000	6,849,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,543,600,000)	(1,508,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,399,200,000	5,340,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	256,000,000	246,100,000
Property, Plant and Equipment, Net	5,655,205,000	5,586,504,000
Net Book Value Of Assets Sold	42,500,000
Public Service Company of New Hampshire [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	1,405,600,000	1,375,400,000
Public Utilities Property Plant And Equipment Transmission	497,000,000	476,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	711,100,000	687,700,000
Public Utilities Property Plant And Equipment Plant In Service	2,613,700,000	2,539,200,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(862,800,000)	(837,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,750,900,000	1,701,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	385,000,000	351,400,000
Property, Plant and Equipment, Net	2,135,883,000	2,053,281,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment Line Items
Public Utilities Property Plant And Equipment Distribution	684,800,000	673,700,000
Public Utilities Property Plant And Equipment Transmission	260,900,000	233,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	9,500,000	9,400,000
Public Utilities Property Plant And Equipment Plant In Service	955,200,000	916,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(234,600,000)	(228,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	720,600,000	688,100,000
Public Utilities Property Plant And Equipment Construction Work In Progress	188,100,000	129,000,000
Property, Plant and Equipment, Net	908,654,000	817,146,000
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	162,800,000	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	$ 129,700,000	$ 126,600,000