SHORT TERM AND LONG TERM DEBT (Details)	6 Months Ended
	Jun. 30, 2013
Line of Credit Facility [Line Items]
Description of commercial paper program	As of June 30, 2013 and December 31, 2012, NU had $541.5 million and $1.15 billion, respectively, in short-term borrowings outstanding under its commercial paper program, which provides $608.5 million of available borrowing capacity as of June 30, 2013. The weighted-average interest rate on these borrowings as of June 30, 2013 and December 31, 2012 was 0.3 percent and 0.46 percent, respectively, which is generally based on money market rates. As of June 30, 2013, there were inter-company loans from NU of $189.3 million to CL&P, $182.2 million to PSNH and $35.2 million to WMECO. As of December 31, 2012, there were inter-company loans from NU of $405.1 million to CL&P, $63.3 million to PSNH, and $31.9 million to WMECO. As of June 30, 2013 and December 31, 2012, NSTAR Electric had $253 million and $276 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $197 million and $174 million, respectively, of available borrowing capacity. The weighted-average interest rate on these borrowings as of June 30, 2013 and December 31, 2012 was 0.22 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 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 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 billion, $231 million, $338 million, $27 million and $50 million at NU, CL&P, NSTAR Electric, PSNH and WMECO, respectively, as of June 30, 2013. As of June 30, 2013, approximately $857 million of NU's current liabilities related to long-term debt will be paid in the next 12 months, primarily consisting of $300 million for NU parent, $125 million for CL&P, $302 million for NSTAR Electric and $55 million for WMECO. 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 and/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.
Long-term Debt, Description	Long-Term Debt Issuances: 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 balance sheet. 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. On May 13, 2013, NU parent issued $750 million of Senior Notes, consisting of $450 million of Series F Senior Notes at a coupon rate of 2.80 percent that will mature on May 1, 2023 and $300 million of Series E Senior Notes at a coupon rate of 1.45 percent that will mature on May 1, 2018. Part of the proceeds, net of issuance costs, was used to repay the NU parent $250 million Series C Senior Notes at a coupon rate of 5.65 percent that matured on June 1, 2013. In addition, part of the net proceeds will be used to repay the NU parent $300 million floating rate Series D Senior Notes due to mature on September 20, 2013. The remaining net proceeds were used to repay commercial paper borrowings and for other general corporate purposes. On May 17, 2013, NSTAR Electric issued $200 million of three-year floating rate debentures with an initial interest rate of 0.5141 percent due to mature on May 17, 2016. The proceeds, net of issuance costs, were used to repay commercial paper borrowings and for general corporate purposes.
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 balance sheet.
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Long-term Debt, Description	On May 17, 2013, NSTAR Electric issued $200 million of three-year floating rate debentures with an initial interest rate of 0.5141 percent due to mature on May 17, 2016. The proceeds, net of issuance costs, were used to repay commercial paper borrowings and for general corporate purposes
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Long-term Debt, Description	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.