SHORT TERM DEBT (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Short Term Debt Regulatory Limits	The amount of short-term borrowings that may be incurred by CL&P, NSTAR Electric and WMECO is subject to periodic approval by the FERC. On July 31, 2013, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $600 million and $300 million, respectively, effective January 1, 2014 through December 31, 2015. On May 16, 2012, the FERC granted authorization to allow NSTAR Electric to issue total short-term debt securities in an aggregate principal amount not to exceed $655 million outstanding at any one time, effective October 23, 2012 through October 23, 2014. As a result of the NHPUC having jurisdiction over PSNH's short-term debt, PSNH is not currently required to obtain FERC approval for its short-term borrowings. CL&P's certificate of incorporation contains preferred stock provisions restricting the amount of unsecured debt that CL&P may incur, including limiting unsecured indebtedness with a maturity of less than 10 years to 10 percent of total capitalization. In November 2003, CL&P obtained from its preferred stockholders a waiver of such 10 percent limit for a ten-year period expiring in March 2014, provided that all unsecured indebtedness does not exceed 20 percent of total capitalization. As of December 31, 2013, CL&P had $776.9 million of unsecured debt capacity available under this authorization.
Description of commercial paper program	NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year revolving credit facility. The revolving credit facility is to be used primarily to backstop the commercial paper program at NU, which commenced July 25, 2012. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. On September 6, 2013, the $1.15 billion revolving credit facility dated July 25, 2012 was amended to increase the aggregate principal amount available thereunder by $300 million to $1.45 billion, to extend the expiration date from July 25, 2017 to September 6, 2018, and to increase CL&P's borrowing sublimit from $300 million to $600 million. PSNH and WMECO each have borrowing sublimits of $300 million. On September 6, 2013, NU parent’s $1.15 billion commercial paper program was increased by $300 million to $1.45 billion. NSTAR Electric has a five-year $450 million revolving credit facility. This facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. On September 6, 2013, NSTAR Electric amended its revolving credit facility dated July 25, 2012 to extend the expiration date from July 25, 2017 to September 6, 2018.
Description of Working Capital	Each of NU, CL&P, NSTAR Electric, PSNH and WMECO use its available capital resources to fund its respective construction expenditures, meet debt requirements, pay operating costs, including storm-related costs, pay dividends and fund other corporate obligations, such as pension contributions. The current growth in NU’s transmission construction expenditures utilizes a significant amount of cash for projects that have a long-term return on investment and recovery period. In addition, NU’s Regulated companies recover its electric and natural gas distribution construction expenditures as the related project costs are depreciated over the life of the assets. This impacts the timing of the revenue stream designed to fully recover the total investment plus a return on the equity portion of the cost and related financing costs. These factors have resulted in current liabilities exceeding current assets by approximately $1.2 billion, $398 million and $339 million at NU, CL&P and NSTAR Electric, respectively, as of December 31, 2013.As of December 31, 2013, $501.7 million of NU's obligations classified as current liabilities relates to long-term debt that will be paid in the next 12 months, consisting of $150 million for CL&P, $301.7 million for NSTAR Electric and $50 million for PSNH. In addition, $31.7 million relates to the amortization of the purchase accounting fair value adjustment that will be amortized in the next twelve months. NU, with its strong credit ratings, has several options available in the financial markets to repay or refinance these maturities with the issuance of new long-term debt. NU, CL&P, NSTAR Electric, PSNH and WMECO will reduce their short-term borrowings with cash received from operating cash flows or with the issuance of new long-term debt, determined considering capital requirements and maintenance of NU's credit rating and profile. Management expects the future operating cash flows of NU, CL&P, NSTAR Electric, PSNH and WMECO, along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities.
Description of ST debt reclassified to LT	On January 2, 2014, Yankee Gas issued $100 million of Series L First Mortgage Bonds. A portion of the proceeds was used to pay short-term borrowings outstanding under the NU commercial paper program. As a result and in accordance with applicable accounting guidance, $25 million of the NU commercial paper program borrowings have been classified as Long-Term Debt as of December 31, 2013.	As of December 31, 2012, CL&P had $89 million in borrowings outstanding under this credit agreement with a weighted average interest rate of 3.325 percent.
Notes Payable to Banks	$ 1,093,000,000	$ 1,120,196,000
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding		89,000,000
Short-term Debt, Weighted Average Interest Rate		3.325%
Notes Payable To Affiliated Companies	287,300,000	99,296,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	346,500,000
Line of Credit Facility, Remaining Borrowing Capacity	346,500,000	174,000,000
CommercialPaper	103,500,000	276,000,000
WeightedAverageDiscountRatePercent	0.13%	0.31%
Notes Payable to Banks	103,500,000	276,000,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	86,500,000	63,300,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	0	31,900,000
Nu Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	435,500,000
CommercialPaper	1,010,000,000	1,150,000,000
WeightedAverageDiscountRatePercent	0.24%	0.46%
Notes Payable to Banks	$ 1,014,500,000	$ 1,150,000,000