PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$ 12,039,700,000	$ 11,950,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,447,900,000	2,425,900,000
Public Utilities Property Plant And Equipment Transmission	6,423,500,000	6,412,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,154,700,000	1,152,300,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	22,065,800,000	21,940,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	510,200,000	508,700,000
Public Utilities Property Plant And Equipment Plant In Service	22,576,000,000	22,449,600,000
Electric And Natural Gas Utility Accumulated Depreciation	(5,491,700,000)	(5,387,000,000)
Other Accumulated Depreciation	(204,700,000)	(196,200,000)
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(5,696,400,000)	(5,583,200,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	16,879,600,000	16,866,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	833,400,000	709,800,000
Property, Plant and Equipment, Net	17,713,027,000	17,576,186,000
Description Of Regulatory Costs Recently Approved	On March 12, 2014, the PURA issued a final decision on CL&P’s request to recover storm restoration costs associated with five major storms, which occurred in 2011 and 2012. The PURA approved recovery of $365 million of deferred storm restoration costs and ordered CL&P to capitalize approximately $18 million of the deferred storm restoration costs as utility plant, which will be recovered through depreciation expense in future rate proceedings. CL&P will recover the $365 million with carrying charges in its distribution rates over a six-year period beginning December 1, 2014. The remaining costs were either disallowed or we believe will be recovered from other sources. These costs did not have a material impact on CL&P’s financial position, results of operations or cash flows. As discussed in Note 2, "Regulatory Accounting," during the first quarter of 2014, as a result of a regulatory proceeding, CL&P reclassified approximately $18 million from Regulatory Assets to Property, Plant and Equipment, Net.
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,979,800,000	4,930,700,000
Public Utilities Property Plant And Equipment Transmission	3,074,800,000	3,071,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,054,600,000	8,002,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,838,500,000)	(1,804,100,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	6,216,100,000	6,198,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	290,100,000	252,800,000
Property, Plant and Equipment, Net	6,506,245,000	6,451,259,000
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,717,600,000	4,694,700,000
Public Utilities Property Plant And Equipment Transmission	1,769,000,000	1,772,300,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,486,600,000	6,467,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,664,600,000)	(1,631,300,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	4,822,000,000	4,835,700,000
Public Utilities Property Plant And Equipment Construction Work In Progress	247,200,000	208,200,000
Property, Plant and Equipment, Net	5,069,203,000	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	1,620,300,000	1,608,200,000
Public Utilities Property Plant And Equipment Transmission	701,700,000	695,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,133,600,000	1,131,200,000
Public Utilities Property Plant And Equipment Plant In Service	3,455,600,000	3,435,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(1,040,600,000)	(1,021,800,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	2,415,000,000	2,413,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	71,400,000	54,300,000
Property, Plant and Equipment, Net	2,486,440,000	2,467,556,000
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	762,000,000	756,600,000
Public Utilities Property Plant And Equipment Transmission	831,700,000	826,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,100,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,614,800,000	1,604,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	(278,400,000)	(271,500,000)
Property Plant And Equipment Net Excluding Construction Work In Progress	1,336,400,000	1,332,600,000
Public Utilities Property Plant And Equipment Construction Work In Progress	62,400,000	48,500,000
Property, Plant and Equipment, Net	$ 1,398,810,000	$ 1,381,060,000