SHORT TERM AND LONG TERM DEBT (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 631,500,000	$ 435,500,000
Line of Credit Facility, Amount Outstanding	818,500,000	1,010,000,000
Short-term Debt, Weighted Average Interest Rate	0.23%	0.24%
Description of commercial paper program	NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year $1.45 billion revolving credit facility due to expire on September 6, 2018. The revolving credit facility is to be used primarily to backstop the $1.45 billion commercial paper program at NU. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. As of March 31, 2014 and December 31, 2013, NU had approximately $818.5 million and $1.01 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, leaving $631.5 million and $435.5 million of available borrowing capacity as of March 31, 2014 and December 31, 2013, respectively. The weighted-average interest rate on these borrowings as of March 31, 2014 and December 31, 2013 was 0.23 percent and 0.24 percent, respectively, which is generally based on money market rates. As of March 31, 2014, there were intercompany loans from NU of $351.6 million to CL&P, $39.9 million to PSNH and $37.4 million to WMECO. As of December 31, 2013, there were intercompany loans from NU of $287.3 million to CL&P and $86.5 million to PSNH. NSTAR Electric has a five-year $450 million revolving credit facility due to expire on September 6, 2018. This facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. As of March 31, 2014, NSTAR Electric had no borrowings outstanding under its commercial paper program. As of December 31, 2013, NSTAR Electric had $103.5 million in short-term borrowings outstanding under its commercial paper program, leaving $346.5 million of available borrowing capacity. The weighted-average interest rate on these borrowings as of December 31, 2013 was 0.13 percent, which is generally based on money market rates.
Description of Working Capital	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 their 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 $424 million, $355 million and $177 million at NU, CL&P and NSTAR Electric, respectively, as of March 31, 2014. As of March 31, 2014, $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, $28.8 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 April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in April 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. In accordance with applicable accounting guidance, Notes Payable of $247.4 million were classified as Long-Term Debt on NU’s balance sheet as of March 31, 2014.
Notes Payable to Banks	571,147,000	1,093,000,000
Long-term Debt, Description	: On January 2, 2014, Yankee Gas issued $100 million of 4.82 percent Series L First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $75 million 4.80 percent Series G First Mortgage Bonds that matured on January 1, 2014 and to pay $25 million in short-term borrowings. In accordance with applicable accounting guidance, these amounts were classified as Long-Term Debt on NU’s balance sheet as of December 31, 2013. On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014. On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in April 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. In accordance with applicable accounting guidance, Notes Payable of $247.4 million were classified as Long-Term Debt on NU’s balance sheet as of March 31, 2014.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	351,600,000	287,300,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity		346,500,000
Line of Credit Facility, Amount Outstanding		103,500,000
Short-term Debt, Weighted Average Interest Rate		0.13%
Notes Payable to Banks	0	103,500,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	39,900,000	86,500,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 37,400,000	$ 0