PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 12,539,300,000	$ 12,495,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,584,800,000	2,595,400,000
Public Utilities Property Plant And Equipment Transmission	6,959,400,000	6,930,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,172,200,000	1,170,900,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	23,255,700,000	23,192,200,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	547,900,000	551,300,000
Public Utilities Property Plant And Equipment Plant In Service	23,803,600,000	23,743,500,000
Electric And Natural Gas Utility Accumulated Depreciation	(5,842,600,000)	(5,777,800,000)
Other Accumulated Depreciation	(232,300,000)	(231,800,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(6,074,900,000)	(6,009,600,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	17,728,700,000	17,733,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	1,082,000,000	913,100,000
Property, Plant and Equipment, Net	18,810,708,000	18,647,041,000
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,180,000,000	5,158,800,000
Public Utilities Property Plant And Equipment Transmission	3,274,700,000	3,274,000,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,454,700,000	8,432,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,950,700,000)	(1,928,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,504,000,000	6,504,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	370,900,000	304,900,000
Property, Plant and Equipment, Net	6,874,891,000	6,809,664,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,907,800,000	4,895,500,000
Public Utilities Property Plant And Equipment Transmission	1,950,600,000	1,928,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,858,400,000	6,824,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,783,600,000)	(1,761,400,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	5,074,800,000	5,062,600,000
Public Utilities Property Plant And Equipment Construction Work In Progress	289,500,000	272,800,000
Property, Plant and Equipment, Net	5,364,311,000	5,335,436,000
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,704,100,000	1,696,700,000
Public Utilities Property Plant And Equipment Transmission	794,600,000	789,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,137,800,000	1,136,500,000
Public Utilities Property Plant And Equipment Plant In Service	3,636,500,000	3,622,900,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,105,700,000)	(1,090,000,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,530,800,000	2,532,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	135,500,000	102,900,000
Property, Plant and Equipment, Net	2,666,312,000	2,635,844,000
Public Service Company Of New Hampshire [Member] | Generation Plant [Member]
Public Utility Property Plant And Equipment [Line Items]
Electric And Natural Gas Utility Accumulated Depreciation	(497,100,000)
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	787,400,000	784,200,000
Public Utilities Property Plant And Equipment Transmission	891,900,000	891,000,000
Public Utilities Property Plant And Equipment Generation Or Processing	34,400,000	34,400,000
Public Utilities Property Plant And Equipment Plant In Service	1,713,700,000	1,709,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(303,300,000)	(297,400,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,410,400,000	1,412,200,000
Public Utilities Property Plant And Equipment Construction Work In Progress	73,500,000	49,100,000
Property, Plant and Equipment, Net	$ 1,483,895,000	$ 1,461,321,000