SHORT AND LONG-TERM TERM DEBT (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Line of Credit Facility, Description	Credit Agreements and Commercial Paper Programs: On October 26, 2015, Eversource parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas amended and restated their joint five-year $1.45 billion revolving credit facility and extended the termination date to September 4, 2020. The facility serves to backstop Eversource parent's $1.45 billion commercial paper program. The commercial paper program allows Eversource parent to issue commercial paper as a form of short-term debt. As of September 30, 2015 and December 31, 2014, Eversource parent had $757 million and approximately $1.1 billion, respectively, in short-term borrowings outstanding under the Eversource parent commercial paper program, leaving $693 million and $348.9 million of available borrowing capacity as of September 30, 2015 and December 31, 2014, respectively. The weighted-average interest rate on these borrowings as of September 30, 2015 and December 31, 2014 was 0.41 percent and 0.43 percent, respectively. As of September 30, 2015, there were intercompany loans from Eversource parent of $137.3 million to PSNH and $126.2 million to WMECO. As of December 31, 2014, there were intercompany loans from Eversource parent of $133.4 million to CL&P, $90.5 million to PSNH and $21.4 million to WMECO. On October 26, 2015, NSTAR Electric amended and restated its five-year $450 million revolving credit facility and extended the termination date to September 4, 2020. The facility serves to backstop NSTAR Electric's $450 million commercial paper program. As of September 30, 2015 and December 31, 2014, NSTAR Electric had $258.5 million and $302 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $191.5 million and $148 million of available borrowing capacity as of September 30, 2015 and December 31, 2014, respectively. The weighted-average interest rate on these borrowings as of September 30, 2015 and December 31, 2014 was 0.18 percent and 0.27 percent, respectively.
Notes Payable	$ 1,015,500	$ 956,825
Long-term Debt, Description	Long-Term Debt: On January 15, 2015, Eversource parent issued $150 million of 1.60 percent Series G Senior Notes, due to mature in 2018 and $300 million of 3.15 percent Series H Senior Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the Eversource parent commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014. On April 1, 2015, CL&P repaid at maturity the $100 million 5.00 percent 2005 Series A First and Refunding Mortgage Bonds using short-term borrowings. On April 1, 2015, CL&P also redeemed the $62 million 1996A Series 1.55 percent PCRBs that were subject to mandatory tender, using short term borrowings. On May 20, 2015, CL&P issued $300 million of 4.15 percent 2015 Series A First and Refunding Mortgage Bonds due to mature in 2045. The proceeds, net of issuance costs, were used to repay short-term borrowings. On August 3, 2015, WMECO repaid at maturity the $50 million 5.24 percent Series C Senior Notes using short-term borrowings. On September 10, 2015, Yankee Gas issued $75 million of 3.35 percent 2015 Series M First Mortgage Bonds due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 0	133,400
Long-term Debt, Description	On April 1, 2015, CL&P repaid at maturity the $100 million 5.00 percent 2005 Series A First and Refunding Mortgage Bonds using short-term borrowings. On April 1, 2015, CL&P also redeemed the $62 million 1996A Series 1.55 percent PCRBs that were subject to mandatory tender, using short term borrowings. On May 20, 2015, CL&P issued $300 million of 4.15 percent 2015 Series A First and Refunding Mortgage Bonds due to mature in 2045. The proceeds, net of issuance costs, were used to repay short-term borrowings.
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 450,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 191,500	$ 148,000
Short-term Debt, Weighted Average Interest Rate	0.18%	0.27%
CommercialPaper	$ 258,500	$ 302,000
Notes Payable	258,500	302,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	137,300	90,500
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 126,200	21,400
Long-term Debt, Description	On August 3, 2015, WMECO repaid at maturity the $50 million 5.24 percent Series C Senior Notes using short-term borrowings.
Eversource Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,450,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 693,000	$ 348,900
Short-term Debt, Weighted Average Interest Rate	0.41%	0.43%
CommercialPaper	$ 757,000	$ 1,100,000