SHORT TERM AND LONG TERM DEBT (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 739,500,000	$ 435,500,000
Line of Credit Facility, Amount Outstanding	710,500,000	1,010,000,000
Short-term Debt, Weighted Average Interest Rate	0.25%	0.24%
Description of commercial paper program	Effective July 23, 2014, NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas amended their joint $1.45 billion revolving credit facility to extend the expiration date an additional year to September 6, 2019. The revolving credit facility is to be used primarily to backstop NU parent’s $1.45 billion commercial paper program. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. As of June 30, 2014 and December 31, 2013, NU had $710.5 million and $1.01 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, leaving $739.5 million and $435.5 million of available borrowing capacity as of June 30, 2014 and December 31, 2013, respectively. The weighted-average interest rate on these borrowings as of June 30, 2014 and December 31, 2013 was 0.25 percent and 0.24 percent, respectively, which is generally based on A2/P2 rated commercial paper. As of June 30, 2014, there were intercompany loans from NU of $6.4 million to CL&P, $95 million to PSNH and $15.9 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. Effective July 23, 2014, NSTAR Electric amended its $450 million revolving credit facility to extend the expiration date an additional year to September 6, 2019. This facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. As of June 30, 2014 and December 31, 2013, NSTAR Electric had $194.5 million and $103.5 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $255.5 million and $346.5 million of available borrowing capacity as of June 30, 2014 and December 31, 2013, respectively. The weighted-average interest rate on these borrowings as of June 30, 2014 and December 31, 2013 was 0.16 percent and 0.13 percent, respectively, which is generally based on A2/P1 rated commercial paper.
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 $730 million, $220 million and $200 million at NU, CL&P and NSTAR Electric, respectively, as of June 30, 2014. As of June 30, 2014, $366.7 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 $312 million for CL&P, $4.7 million at NSTAR Electric and $50 million for PSNH. In addition, $28.9 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
Notes Payable to Banks	905,000,000	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. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using 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. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term debt.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	6,400,000	287,300,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	255,500,000	346,500,000
Line of Credit Facility, Amount Outstanding		103,500,000
WeightedAverageDiscountRatePercent	0.16%	0.13%
Notes Payable to Banks	194,500,000	103,500,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	95,000,000	86,500,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 15,900,000	$ 0