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PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)
Sep. 30, 2011
Dec. 31, 2010
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Distribution$ 6,427,000,000$ 6,197,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas1,194,000,0001,126,600,000
Public Utilities Property Plant And Equipment Transmission3,410,200,0003,378,000,000
Public Utilities Property Plant And Equipment Generation Or Processing1,043,900,000697,100,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total12,075,100,00011,398,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment306,000,000305,500,000
Public Utilities Property Plant And Equipment Plant In Service12,381,100,00011,704,400,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation(2,986,000,000)(2,862,300,000)
OtherAccumulatedDepreciation(122,300,000)(119,900,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation(3,108,300,000)(2,982,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress9,272,800,0008,722,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress823,300,000845,500,000
Property, Plant and Equipment, Net10,096,063,0009,567,726,000
The Connecticut Light and Power Company [Member]
  
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Distribution4,354,200,0004,180,700,000
Public Utilities Property Plant And Equipment Transmission2,650,400,0002,668,400,000
Public Utilities Property Plant And Equipment Generation Or Processing00
Public Utilities Property Plant And Equipment Plant In Service7,004,600,0006,849,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation(1,570,200,000)(1,508,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress5,434,400,0005,340,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress294,900,000246,100,000
Property, Plant and Equipment, Net5,729,334,0005,586,504,000
Net Book Value Of Assets Sold42,500,000 
Public Service Company of New Hampshire [Member]
  
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Distribution1,417,100,0001,375,400,000
Public Utilities Property Plant And Equipment Transmission499,700,000476,100,000
Public Utilities Property Plant And Equipment Generation Or Processing1,034,500,000687,700,000
Public Utilities Property Plant And Equipment Plant In Service2,951,300,0002,539,200,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation(876,400,000)(837,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress2,074,900,0001,701,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress106,300,000351,400,000
Property, Plant and Equipment, Net2,181,234,0002,053,281,000
Western Massachusetts Electric Company [Member]
  
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Distribution690,000,000673,700,000
Public Utilities Property Plant And Equipment Transmission260,100,000233,500,000
Public Utilities Property Plant And Equipment Generation Or Processing9,400,0009,400,000
Public Utilities Property Plant And Equipment Plant In Service959,500,000916,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation(238,000,000)(228,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress721,500,000688,100,000
Public Utilities Property Plant And Equipment Construction Work In Progress264,800,000129,000,000
Property, Plant and Equipment, Net986,310,000817,146,000
The Rocky River Realty Company [Member]
  
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Other Property Plant And Equipment163,200,000166,000,000
Northeast Utilities Service Company [Member]
  
Public Utility Property Plant And Equipment Line Items  
Public Utilities Property Plant And Equipment Other Property Plant And Equipment$ 130,900,000$ 126,600,000