SHORT TERM DEBT (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Line of Credit Facility [Line Items]
Letters Of Credit Outstanding Under Line of Credit		$ 17.9
NU Money Pool [Abstract]
Line of Credit Facility, Description	CL&P Credit Agreement: On March 26, 2012, CL&P entered into a five-year unsecured revolving credit facility in the amount of $300 million, which expires on March 26, 2017. Under this facility, CL&P can borrow either on a short-term or a long-term basis subject to regulatory approval. 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.
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 November 30, 2011, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $450 million and $300 million, respectively, effective January 1, 2012 through December 31, 2013. On March 22, 2012, the FERC approved CL&P's application requesting to increase its total short-term borrowing capacity from a maximum of $450 million to a maximum of $600 million for the authorization period through December 31, 2013. 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. PSNH is authorized by regulation of the NHPUC to incur short-term borrowings up to 10 percent of net fixed plant plus an additional $60 million until further ordered by the NHPUC. As of December 31, 2012, PSNH's short-term debt authorization under the 10 percent of net fixed plant test plus $60 million totaled approximately $280 million. 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, 2012, CL&P had $482 million of unsecured debt capacity available under this authorization.
CommercialPaper	1,150.0
Description of commercial paper program	On July 25, 2012, NU, CL&P, NSTAR LLC, NSTAR Gas, PSNH, WMECO, and Yankee Gas jointly entered into a five-year $1.15 billion revolving credit facility. The new facility replaced (1) the NSTAR LLC revolving credit facility of $175 million that served to backstop a commercial paper program utilized by NSTAR LLC and was scheduled to expire on December 31, 2012, (2) the NSTAR Gas revolving credit facility of $75 million that expired on June 8, 2012, and (3) the CL&P, PSNH, WMECO, and Yankee Gas joint three-year $400 million and NU parent three-year $500 million unsecured revolving credit facilities that were scheduled to expire on September 24, 2013. The new facility expires on July 25, 2017. Management expects the new facility to be used primarily to backstop the $1.15 billion 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. Under the terms of the agreement, NU parent may provide intercompany loans to its subsidiaries, including CL&P, PSNH and WMECO. On July 25, 2012, NSTAR Electric entered into a five-year $450 million revolving credit facility. This new facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. The new facility expires on July 25, 2017. This new facility replaced a prior $450 million NSTAR Electric revolving credit facility that was scheduled to expire on December 31, 2012. As of December 31, 2012, NU had $1.15 billion in short-term borrowings outstanding under its commercial paper program. The weighted-average interest rate on these borrowings as of December 31, 2012 was 0.46 percent, which is generally based on money market rates. As of December 31, 2012, there were inter-company loans of $987.5 million from NU to its subsidiaries ($405.1 million for CL&P, $63.3 million for PSNH, and $31.9 million for WMECO). As of December 31, 2012, NSTAR Electric had $276 million in short-term borrowings outstanding under its commercial paper program, leaving $174 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of December 31, 2012 was 0.31 percent, which is generally based on money market rates.
Description of Working Capital	NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, pay 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 operate in an environment where recovery of its electric and natural gas distribution construction expenditures takes place over an extended period of time. 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 NU’s current liabilities exceeding current assets by approximately $1.4 billion, $268 million, $198 million and $60 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of December 31, 2012. As of December 31, 2012, approximately $730 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months, consisting of $550 million for NU parent, $55 million for WMECO, and $125 million for CL&P. Approximately $32 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, and WMECO will reduce their short-term borrowings with cash received from operating cash flows or with the issuance of new long-term debt, as deemed appropriate given capital requirements and maintenance of NU's credit rating and profile. Management expects the future operating cash flows of NU, CL&P, NSTAR Electric 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 15, 2013, CL&P issued $400 million of Series A First and Refunding Mortgage Bonds with a coupon rate of 2.5 percent and a maturity date of January 15, 2023. The proceeds, net of issuance costs, were used to pay short-term borrowings outstanding under the CL&P credit agreement and the NU commercial paper program. As a result, as of December 31, 2012, CL&P's credit agreement borrowings of $89 million and intercompany loans related to the commercial paper program of $305.8 million have been classified as Long-Term Debt on the accompanying consolidated balance sheet.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	89.0	31.0
Letters Of Credit Outstanding Under Line of Credit		4.0
NU Money Pool [Abstract]
Borrowings From Money Pool		58.5
Weighted Average Interest Rate on Money Pool Borrowings		0.08%
WeightedAverageDiscountRatePercent	3.33%	4.03%
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Letters Of Credit Outstanding Under Line of Credit		5.4
NU Money Pool [Abstract]
Borrowings From Money Pool		(55.9)
Weighted Average Interest Rate on Money Pool Borrowings		0.10%
Western Massachusetts Electric Company [Member]
NU Money Pool [Abstract]
Borrowings From Money Pool		(11.0)
Weighted Average Interest Rate on Money Pool Borrowings		0.10%
Nu Parent [Member]
NU Money Pool [Abstract]
WeightedAverageDiscountRatePercent	2.20%	0.46%
Yankee Gas Services Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding		30.0
NU Money Pool [Abstract]
WeightedAverageDiscountRatePercent		2.07%
NSTAR Electric Company [Member]
NU Money Pool [Abstract]
CommercialPaper	$ 276.0	$ 141.5
WeightedAverageDiscountRatePercent	0.31%	0.16%