PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 11,504,000,000	$ 11,438,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,290,800,000	2,274,200,000
Public Utilities Property Plant And Equipment Transmission	5,721,200,000	5,541,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,146,700,000	1,146,600,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	20,662,700,000	20,400,100,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	503,000,000	429,300,000
Public Utilities Property Plant And Equipment Plant In Service	21,165,700,000	20,829,400,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(5,157,700,000)	(5,065,100,000)
OtherAccumulatedDepreciation	(178,500,000)	(171,500,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,336,200,000)	(5,236,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	15,829,500,000	15,592,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	908,000,000	1,012,200,000
Property, Plant and Equipment, Net	16,737,549,000	16,605,010,000
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,724,300,000	4,691,300,000
Public Utilities Property Plant And Equipment Transmission	2,874,600,000	2,796,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,598,900,000	7,487,400,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,727,500,000)	(1,698,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,871,400,000	5,789,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	313,200,000	363,700,000
Property, Plant and Equipment, Net	6,184,628,000	6,152,959,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,552,000,000	4,539,900,000
Public Utilities Property Plant And Equipment Transmission	1,542,800,000	1,529,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,094,800,000	6,069,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,573,200,000)	(1,540,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,521,600,000	4,529,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	266,600,000	205,800,000
Property, Plant and Equipment, Net	4,788,158,000	4,735,297,000
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,535,700,000	1,520,100,000
Public Utilities Property Plant And Equipment Transmission	602,300,000	599,200,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,125,600,000	1,125,500,000
Public Utilities Property Plant And Equipment Plant In Service	3,263,600,000	3,244,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(968,400,000)	(954,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,295,200,000	2,290,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	65,600,000	61,700,000
Property, Plant and Equipment, Net	2,360,753,000	2,352,515,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	730,000,000	724,200,000
Public Utilities Property Plant And Equipment Transmission	656,700,000	583,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,100,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,407,800,000	1,329,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(257,500,000)	(252,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,150,300,000	1,076,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	155,400,000	213,600,000
Property, Plant and Equipment, Net	$ 1,305,743,000	$ 1,290,498,000