SHORT TERM DEBT (Details)	9 Months Ended
Sep. 30, 2012
Line of Credit Facility [Line Items]
Working capital discussion	NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, to pay costs, including storm-related costs, and dividends and to 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 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, $558 million, $111 million and $217 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of September 30, 2012.As of September 30, 2012, approximately $880 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months. NU, with credit ratings among the highest in the industry, 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 and its subsidiaries, along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Short-term Debt, Terms	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 September 30, 2012, there were no borrowings under this facility. Under this facility, CL&P may borrow at prime rates or LIBOR-based rates, plus an applicable margin based on the higher of S&P’s or Moody’s credit ratings. In addition, CL&P must comply with certain financial and non-financial covenants, including a consolidated debt to total capitalization ratio. CL&P was in compliance with these covenants as of September 30, 2012. If CL&P was not in compliance with these covenants, an event of default would occur requiring all outstanding borrowings to be repaid and additional borrowings would not be permitted under this credit facility. Commercial Paper Programs: 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. 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 September 30, 2012, NU had $1,098.3 million in short-term borrowings outstanding under its commercial paper program, leaving $51.8 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2012 was 0.459 percent, which is generally based on money market rates. As of September 30, 2012, there were inter-company loans of $1,040.9 million from NU to the subsidiaries ($372.8 million for CL&P, $44.2 million for PSNH, and $172.5 million for WMECO). As of September 30, 2012, NSTAR Electric had $261 million in short-term borrowings outstanding under its commercial paper program, leaving $189 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2012 was 0.349 percent, which is generally based on money market rates. NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, to pay costs, including storm-related costs, and dividends and to 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 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, $558 million, $111 million and $217 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of September 30, 2012.As of September 30, 2012, approximately $880 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months. NU, with credit ratings among the highest in the industry, 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 and its subsidiaries, along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities.
Short Term Debt Regulatory Limits	Limits: The amount of short-term borrowings that may be incurred by CL&P are subject to periodic approval by the FERC. On November 30, 2011, the FERC granted authorization to allow CL&P to incur total short-term borrowings up to a maximum of $450 million effective January 1, 2012 through December 31, 2013. On March 22, 2012, 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. The amount of short-term borrowings that may be incurred by NSTAR Electric is subject to periodic approval by the FERC. 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.

SHORT TERM DEBT (Details)

9 Months Ended

Sep. 30, 2012

Line of Credit Facility [Line Items]

NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, to pay costs, including storm-related costs, and dividends and to 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 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, $558 million, $111 million and $217 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of September 30, 2012.As of September 30, 2012, approximately $880 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months. NU, with credit ratings among the highest in the industry, 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 and its subsidiaries, along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities

The Connecticut Light And Power Company [Member]

Line of Credit Facility [Line Items]

Short-term Debt, Terms

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 September 30, 2012, there were no borrowings under this facility. Under this facility, CL&P may borrow at prime rates or LIBOR-based rates, plus an applicable margin based on the higher of S&P’s or Moody’s credit ratings. In addition, CL&P must comply with certain financial and non-financial covenants, including a consolidated debt to total capitalization ratio. CL&P was in compliance with these covenants as of September 30, 2012. If CL&P was not in compliance with these covenants, an event of default would occur requiring all outstanding borrowings to be repaid and additional borrowings would not be permitted under this credit facility. Commercial Paper Programs: 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. 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 September 30, 2012, NU had $1,098.3 million in short-term borrowings outstanding under its commercial paper program, leaving $51.8 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2012 was 0.459 percent, which is generally based on money market rates. As of September 30, 2012, there were inter-company loans of $1,040.9 million from NU to the subsidiaries ($372.8 million for CL&P, $44.2 million for PSNH, and $172.5 million for WMECO). As of September 30, 2012, NSTAR Electric had $261 million in short-term borrowings outstanding under its commercial paper program, leaving $189 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2012 was 0.349 percent, which is generally based on money market rates. NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, to pay costs, including storm-related costs, and dividends and to 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 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, $558 million, $111 million and $217 million at NU, CL&P, NSTAR Electric and WMECO, respectively, as of September 30, 2012.As of September 30, 2012, approximately $880 million of NU's current liabilities relates to long-term debt that will be paid in the next 12 months. NU, with credit ratings among the highest in the industry, 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 and its subsidiaries, along with the access to financial markets, will be sufficient to meet any future operating requirements and capital investment forecasted opportunities.

Short Term Debt Regulatory Limits

Limits: The amount of short-term borrowings that may be incurred by CL&P are subject to periodic approval by the FERC. On November 30, 2011, the FERC granted authorization to allow CL&P to incur total short-term borrowings up to a maximum of $450 million effective January 1, 2012 through December 31, 2013. On March 22, 2012, 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. The amount of short-term borrowings that may be incurred by NSTAR Electric is subject to periodic approval by the FERC. 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.

- Definition

Description of the short-term debt arrangement, and disclosures pertaining to the underlying arrangements, including repayment terms, interest rates, restrictions on assets and activities, debt covenants, and other matters important to users of the financial statements.

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