INCOME TAXES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			$ 334,225	$ 186,181
Income Tax Expense	109,351	117,360	325,442	199,379
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	320,731	326,868	939,855	555,861
Income Tax Expense	109,351	117,360	325,442	199,379
Other Tax Disclosures [Abstract]
Other Information Pertaining To Income Taxes			2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changes the income tax rate applicable to utility companies effective January 1, 2014, from 6.5 percent to 8 percent. The tax law change required NU to remeasure its deferred taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.4 million at NSTAR Electric and $9.8 million at WMECO). 2013 Federal: On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations. The final regulations are meant to simplify, clarify and make more administrable the previously issued temporary and proposed regulations. In the third quarter of 2013, CL&P recorded an after-tax valuation allowance of $10.5 million against its deferred tax assets as a result of these regulations. NU continues to evaluate the implications of these new regulations, including several new elections. Therefore, a change to the valuation allowance at CL&P could result once NU completes the review of the impact of the final regulations.
The Connecticut Light And Power Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			89,084	97,224
Income Tax Expense	36,136	34,121	113,149	63,917
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	102,459	109,007	332,337	199,721
Income Tax Expense	36,136	34,121	113,149	63,917
NSTAR Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			26,358	(20,250)
Income Tax Expense	68,558	69,373	137,499	102,220
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	175,567	176,135	350,661	258,404
Income Tax Expense	68,558	69,373	137,499	102,220
Public Service Company Of New Hampshire [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			57,066	39,108
Income Tax Expense	18,196	21,106	52,797	48,037
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	46,549	48,338	137,340	117,763
Income Tax Expense	18,196	21,106	52,797	48,037
Western Massachusetts Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			79,401	30,565
Income Tax Expense	8,588	7,977	30,424	24,385
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	23,613	22,061	80,465	63,793
Income Tax Expense	$ 8,588	$ 7,977	$ 30,424	$ 24,385