INCOME TAXES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	$ 168,938	$ 52,474
Income Tax Expense	120,487	55,964
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	350,455	156,719
Income Tax Expense	120,487	55,964
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	Tax Positions: In March 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) Appellate Division that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008, bringing closure to, and effective settlement of, issues concerning the deductibility of expenses and credits through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, recorded in the first quarter of 2013 that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively. Management estimates that resolution of this audit decreases NU's and CL&P's unrecognized tax benefits by approximately $49.8 million and $39.4 million, respectively, that was largely offset by NU's and CL&P's valuation allowance and other tax impacts of $44.9 million and $36.5 million, respectively.
Tax Years [Abstract]
Significant Change In Unrecognized Tax Benefits, Nature of Event	NU currently has open tax years. It is reasonably possible that at least one of these open tax years could be resolved within the next twelve months. Management estimates that the potential resolutions could result in a $1 million to $2 million decrease, net of any related valuation allowance, in unrecognized tax benefits for NU.
The Connecticut Light And Power Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	65,475	32,460
Income Tax Expense	39,188	29,672
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	124,196	83,657
Income Tax Expense	39,188	29,672
NSTAR Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	26,571	(19,446)
Income Tax Expense	31,265	2,035
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	79,387	5,975
Income Tax Expense	31,265	2,035
Public Service Company Of New Hampshire [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	10,143	8,908
Income Tax Expense	17,984	13,353
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	46,980	34,615
Income Tax Expense	17,984	13,353
Western Massachusetts Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	16,828	9,198
Income Tax Expense	11,698	9,171
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	30,301	23,357
Income Tax Expense	$ 11,698	$ 9,171