SHORT TERM AND LONG TERM DEBT (Details)	3 Months Ended
	Mar. 31, 2013
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 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.
Description of commercial paper program	As of March 31, 2013 and December 31, 2012, NU had $979 million and $1.15 billion, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $171 million of available borrowing capacity as of March 31, 2013. The weighted-average interest rate on these borrowings as of March 31, 2013 and December 31, 2012 was 0.35 percent and 0.46 percent, respectively, which is generally based on money market rates. As of March 31, 2013, there were inter-company loans of $832 million from NU to its subsidiaries ($210.4 million at CL&P, $53.4 million at PSNH and $43.4 million at WMECO). As of December 31, 2012, there were inter-company loans of $987.5 million from NU to its subsidiaries ($405.1 million at CL&P, $63.3 million at PSNH, and $31.9 million at WMECO). As of March 31, 2013 and December 31, 2012, NSTAR Electric had $308 million and $276 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $142 million and $174 million, respectively, of available borrowing capacity. The weighted-average interest rate on these borrowings as of March 31, 2013 and December 31, 2012 was 0.28 percent and 0.31 percent, respectively, which is generally based on money market rates. Amounts outstanding under the commercial paper program are included in Notes Payable for NU and NSTAR Electric and classified in current liabilities on the consolidated balance sheets as management anticipates that all borrowings under these credit facilities will be outstanding for no more than 364 days at one time. Intercompany loans from NU to CL&P, PSNH and WMECO are included in Notes Payable to Affiliated Companies and classified in current liabilities on the consolidated balance sheets.
Description of Working Capital	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 $1.7 billion, $231 million, $226 million, $34 million and $48 million at NU, CL&P, NSTAR Electric, PSNH and WMECO, respectively, as of March 31, 2013. As of March 31, 2013, $947.3 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, $125 million for CL&P, $109 million for PSNH, $75 million for Yankee Gas and $55 million for WMECO. 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, 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, 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, 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 LT debt reclassified to ST	On May 1, 2013, PSNH redeemed at par approximately $109 million of the 2001 Series C PCRBs that were due to mature in 2021 with short-term debt. As a result, the PCRBs were recorded as Long-Term Debt - Current Portion.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Long-term Debt, Description	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. Therefore, 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 consolidated balance sheet.