PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 12,495,200,000	$ 11,950,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,595,400,000	2,425,900,000
Public Utilities Property Plant And Equipment Transmission	6,930,700,000	6,412,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,170,900,000	1,152,300,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	23,192,200,000	21,940,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	551,300,000	508,700,000
Public Utilities Property Plant And Equipment Plant In Service	23,743,500,000	22,449,600,000
Electric And Natural Gas Utility Accumulated Depreciation	(5,777,800,000)	(5,387,000,000)
Other Accumulated Depreciation	(231,800,000)	(196,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,009,600,000)	(5,583,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	17,733,900,000	16,866,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	913,100,000	709,800,000
Property, Plant and Equipment, Net	18,647,041,000	17,576,186,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	2.80%	2.50%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	34.9
Transmission Assets Average Useful Life	42.5
Generation Assets Average Useful Life	31.9
Other Assets Average Useful Life	14.2
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,158,800,000	4,930,700,000
Public Utilities Property Plant And Equipment Transmission	3,274,000,000	3,071,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,432,800,000	8,002,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,928,000,000)	(1,804,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,504,800,000	6,198,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	304,900,000	252,800,000
Property, Plant and Equipment, Net	6,809,664,000	6,451,259,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.70%	2.50%	2.50%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	37.5
Transmission Assets Average Useful Life	39.8
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,895,500,000	4,694,700,000
Public Utilities Property Plant And Equipment Transmission	1,928,500,000	1,772,300,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,824,000,000	6,467,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,761,400,000)	(1,631,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,062,600,000	4,835,700,000
Public Utilities Property Plant And Equipment Construction Work In Progress	272,800,000	208,200,000
Property, Plant and Equipment, Net	5,335,436,000	5,043,887,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	2.90%	2.80%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.3
Transmission Assets Average Useful Life	44.0
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,696,700,000	1,608,200,000
Public Utilities Property Plant And Equipment Transmission	789,700,000	695,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,136,500,000	1,131,200,000
Public Utilities Property Plant And Equipment Plant In Service	3,622,900,000	3,435,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,090,000,000)	(1,021,800,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,532,900,000	2,413,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	102,900,000	54,300,000
Property, Plant and Equipment, Net	2,635,844,000	2,467,556,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	3.00%	3.00%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.3
Transmission Assets Average Useful Life	43.7
Generation Assets Average Useful Life	32.1
Other Assets Average Useful Life	0
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	784,200,000	756,600,000
Public Utilities Property Plant And Equipment Transmission	891,000,000	826,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	34,400,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,709,600,000	1,604,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(297,400,000)	(271,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,412,200,000	1,332,600,000
Public Utilities Property Plant And Equipment Construction Work In Progress	49,100,000	48,500,000
Property, Plant and Equipment, Net	$ 1,461,321,000	$ 1,381,060,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.30%	2.90%	3.30%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	30.9
Transmission Assets Average Useful Life	49.9
Generation Assets Average Useful Life	25.0
Other Assets Average Useful Life	0