PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 11,168,100,000	$ 6,540,400,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,160,100,000	1,247,600,000
Public Utilities Property Plant And Equipment Transmission	5,061,100,000	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	19,536,600,000	12,425,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	420,200,000	305,100,000
Public Utilities Property Plant And Equipment Plant In Service	19,956,800,000	12,731,000,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	(4,910,200,000)	(3,035,500,000)
OtherAccumulatedDepreciation	(160,800,000)	(120,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,071,000,000)	(3,155,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	14,885,800,000	9,575,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	1,169,100,000	827,800,000
Property, Plant and Equipment, Net	16,054,913,000	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	4,590,200,000	4,419,600,000
Public Utilities Property Plant And Equipment Transmission	2,707,500,000	2,689,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,297,700,000	7,108,700,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,640,800,000)	(1,596,700,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,656,900,000	5,512,000,000
Public Utilities Property Plant And Equipment Construction Work In Progress	343,900,000	315,400,000
Property, Plant and Equipment, Net	6,000,783,000	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	1,481,600,000	1,451,600,000
Public Utilities Property Plant And Equipment Transmission	558,200,000	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	3,165,900,000	3,072,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(916,700,000)	(893,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,249,200,000	2,179,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	66,800,000	77,500,000
Property, Plant and Equipment, Net	2,315,954,000	2,256,688,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	713,800,000	704,300,000
Public Utilities Property Plant And Equipment Transmission	352,600,000	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	1,087,600,000	1,022,900,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(247,300,000)	(240,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	840,300,000	782,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	376,100,000	295,400,000
Property, Plant and Equipment, Net	1,216,370,000	1,077,833,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,418,700,000	4,334,400,000
Public Utilities Property Plant And Equipment Transmission	1,424,800,000	1,386,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	5,843,500,000	5,721,300,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,498,800,000)	(1,436,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,344,700,000	4,285,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	210,800,000	162,000,000
Property, Plant and Equipment, Net	$ 4,555,454,000	$ 4,447,258,000