PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 13,054,800	$ 12,495,200
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,727,200	2,595,400
Public Utilities Property Plant And Equipment Transmission	7,691,900	6,930,700
Public Utilities Property Plant And Equipment Generation Or Processing	1,194,100	1,170,900
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	24,668,000	23,192,200
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	558,600	551,300
Public Utilities Property Plant And Equipment Plant In Service	25,226,600	23,743,500
Less Accumulated Depreciation [Abstract]
Electric And Natural Gas Utility Accumulated Depreciation	(6,141,100)	(5,777,800)
Other Accumulated Depreciation	(255,600)	(231,800)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,396,700)	(6,009,600)
Property Plant And Equipment Net Excluding Construction Work In Progress	18,829,900	17,733,900
Public Utilities Property Plant And Equipment Construction Work In Progress	1,062,500	913,100
Property, Plant and Equipment, Net	$ 19,892,441	$ 18,647,041
Public Utilities Property Plant and Equipment Depreciation Rates	2.90%	3.00%	2.80%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	34.8
Transmission Assets Average Useful Life	41.6
Generation Assets Average Useful Life	30.7
Other Assets Average Useful Life	14.1
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 5,377,200	$ 5,158,800
Public Utilities Property Plant And Equipment Transmission	3,618,000	3,274,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,995,200	8,432,800
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(2,041,900)	(1,928,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,953,300	6,504,800
Public Utilities Property Plant And Equipment Construction Work In Progress	203,500	304,900
Property, Plant and Equipment, Net	$ 7,156,809	$ 6,809,664
Public Utilities Property Plant and Equipment Depreciation Rates	2.70%	2.70%	2.50%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	37.3
Transmission Assets Average Useful Life	38.7
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	$ 5,100,500	$ 4,895,500
Public Utilities Property Plant And Equipment Transmission	2,131,300	1,928,500
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,231,800	6,824,000
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,886,800)	(1,761,400)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,345,000	5,062,600
Public Utilities Property Plant And Equipment Construction Work In Progress	310,500	272,800
Property, Plant and Equipment, Net	$ 5,655,458	$ 5,335,436
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	3.00%	2.90%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	31.9
Transmission Assets Average Useful Life	43.8
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	$ 1,804,800	$ 1,696,700
Public Utilities Property Plant And Equipment Transmission	928,200	789,700
Public Utilities Property Plant And Equipment Generation Or Processing	1,158,100	1,136,500
Public Utilities Property Plant And Equipment Plant In Service	3,891,100	3,622,900
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,171,000)	(1,090,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,720,100	2,532,900
Public Utilities Property Plant And Equipment Construction Work In Progress	135,300	102,900
Property, Plant and Equipment, Net	$ 2,855,363	$ 2,635,844
Public Utilities Property Plant and Equipment Depreciation Rates	3.20%	3.00%	3.00%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	31.3
Transmission Assets Average Useful Life	41.6
Generation Assets Average Useful Life	30.9
Other Assets Average Useful Life	0
Public Service Company Of New Hampshire [Member] | Generation Plant [Member]
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	$ (522,400)
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	812,300	$ 784,200
Public Utilities Property Plant And Equipment Transmission	964,900	891,000
Public Utilities Property Plant And Equipment Generation Or Processing	36,000	34,400
Public Utilities Property Plant And Equipment Plant In Service	1,813,200	1,709,600
Less Accumulated Depreciation [Abstract]
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(307,000)	(297,400)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,506,200	1,412,200
Public Utilities Property Plant And Equipment Construction Work In Progress	69,100	49,100
Property, Plant and Equipment, Net	$ 1,575,306	$ 1,461,321
Public Utilities Property Plant and Equipment Depreciation Rates	2.70%	3.30%	2.90%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	30.5
Transmission Assets Average Useful Life	50.0
Generation Assets Average Useful Life	25.0
Other Assets Average Useful Life	0