PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 11,275,000,000	$ 6,540,400,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,198,400,000	1,247,600,000
Public Utilities Property Plant And Equipment Transmission	5,127,300,000	3,541,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,147,300,000	1,096,000,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	19,748,000,000	12,425,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	432,900,000	305,100,000
Public Utilities Property Plant And Equipment Plant In Service	20,180,900,000	12,731,000,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(4,990,400,000)	(3,035,500,000)
OtherAccumulatedDepreciation	(166,600,000)	(120,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,157,000,000)	(3,155,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	15,023,900,000	9,575,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	1,279,900,000	827,800,000
Property, Plant and Equipment, Net	16,303,805,000	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	4,628,100,000	4,419,600,000
Public Utilities Property Plant And Equipment Transmission	2,736,300,000	2,689,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,364,400,000	7,108,700,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,665,400,000)	(1,596,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,699,000,000	5,512,000,000
Public Utilities Property Plant And Equipment Construction Work In Progress	370,000,000	315,400,000
Property, Plant and Equipment, Net	6,069,033,000	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	1,498,000,000	1,451,600,000
Public Utilities Property Plant And Equipment Transmission	570,700,000	546,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,126,100,000	1,074,800,000
Public Utilities Property Plant And Equipment Plant In Service	3,194,800,000	3,072,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(935,000,000)	(893,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,259,800,000	2,179,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	71,600,000	77,500,000
Property, Plant and Equipment, Net	2,331,370,000	2,256,688,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	720,100,000	704,300,000
Public Utilities Property Plant And Equipment Transmission	361,800,000	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	1,103,100,000	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	851,000,000	782,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	406,300,000	295,400,000
Property, Plant and Equipment, Net	1,257,318,000	1,077,833,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,465,600,000	4,334,400,000
Public Utilities Property Plant And Equipment Transmission	1,438,200,000	1,386,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	5,903,800,000	5,721,300,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,524,000,000)	(1,436,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,379,800,000	4,285,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	249,000,000	162,000,000
Property, Plant and Equipment, Net	$ 4,628,823,000	$ 4,447,258,000