SHORT TERM AND LONG TERM DEBT (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 485,500,000	$ 435,500,000
Line of Credit Facility, Amount Outstanding	964,500,000	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 extended the expiration date of their joint $1.45 billion revolving credit facility for one 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 September 30, 2014 and December 31, 2013, NU had $964.5 million and $1.01 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, leaving $485.5 million and $435.5 million of available borrowing capacity as of September 30, 2014 and December 31, 2013, respectively. The weighted-average interest rate on these borrowings as of September 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 September 30, 2014, there were intercompany loans from NU of $105.4 million to CL&P, $153.3 million to PSNH and $13.2 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 extended the expiration date of its $450 million revolving credit facility for one additional year to September 6, 2019. This facility serves to backstop NSTAR Electric's existing $450 million commercial paper program. As of September 30, 2014 and December 31, 2013, NSTAR Electric had $159.5 million and $103.5 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $290.5 million and $346.5 million of available borrowing capacity as of September 30, 2014 and December 31, 2013, respectively. The weighted-average interest rate on these borrowings as of September 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.
Notes Payable	1,046,961,000	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. As the debt issuance refinanced short-term debt, the short-term debt was 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 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 borrowings. On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent that will mature on November 1, 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. As a result, the aggregate principal amount of PSNH’s outstanding Series S First Mortgage Bonds totals $325 million. The proceeds, net of issuance costs, were used to repay short-term borrowings. As the debt issuance refinanced short-term debt, the short-term debt was classified as Long-Term Debt on NU's balance sheet as of September 30, 2014.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	105,400,000	287,300,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	290,500,000	346,500,000
Line of Credit Facility, Amount Outstanding	159,500,000	103,500,000
WeightedAverageDiscountRatePercent	0.16%	0.13%
Notes Payable	159,500,000	103,500,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	153,300,000	86,500,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 13,200,000	$ 0