PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 12,306,600,000	$ 11,950,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,494,500,000	2,425,900,000
Public Utilities Property Plant And Equipment Transmission	6,564,200,000	6,412,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,168,600,000	1,152,300,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	22,533,900,000	21,940,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	541,700,000	508,700,000
Public Utilities Property Plant And Equipment Plant In Service	23,075,600,000	22,449,600,000
Electric And Natural Gas Utility Accumulated Depreciation	(5,687,000,000)	(5,387,000,000)
Other Accumulated Depreciation	(216,300,000)	(196,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,903,300,000)	(5,583,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	17,172,300,000	16,866,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	1,082,300,000	709,800,000
Property, Plant and Equipment, Net	18,254,610,000	17,576,186,000
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,104,800,000	4,930,700,000
Public Utilities Property Plant And Equipment Transmission	3,125,200,000	3,071,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,230,000,000	8,002,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,908,200,000)	(1,804,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,321,800,000	6,198,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	379,000,000	252,800,000
Property, Plant and Equipment, Net	6,700,798,000	6,451,259,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,812,400,000	4,694,700,000
Public Utilities Property Plant And Equipment Transmission	1,824,600,000	1,772,300,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,637,000,000	6,467,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,726,700,000)	(1,631,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,910,300,000	4,835,700,000
Public Utilities Property Plant And Equipment Construction Work In Progress	301,400,000	208,200,000
Property, Plant and Equipment, Net	5,211,681,000	5,043,887,000
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,654,500,000	1,608,200,000
Public Utilities Property Plant And Equipment Transmission	715,800,000	695,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,134,700,000	1,131,200,000
Public Utilities Property Plant And Equipment Plant In Service	3,505,000,000	3,435,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,069,400,000)	(1,021,800,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,435,600,000	2,413,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	125,000,000	54,300,000
Property, Plant and Equipment, Net	2,560,635,000	2,467,556,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	774,800,000	756,600,000
Public Utilities Property Plant And Equipment Transmission	851,800,000	826,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	33,900,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,660,500,000	1,604,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(291,800,000)	(271,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,368,700,000	1,332,600,000
Public Utilities Property Plant And Equipment Construction Work In Progress	63,700,000	48,500,000
Property, Plant and Equipment, Net	$ 1,432,368,000	$ 1,381,060,000