SHORT-TERM DEBT AND LONG-TERM DEBT (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Line of Credit Facility, Description	Commercial Paper Programs and Credit Agreements: Eversource parent has a $1.45 billion commercial paper program allowing Eversource parent to issue commercial paper as a form of short-term debt. As of June 30, 2016 and December 31, 2015, Eversource parent had $438 million and approximately $1.1 billion, respectively, in short-term borrowings outstanding under the Eversource parent commercial paper program, leaving $1.0 billion and $351.5 million of available borrowing capacity as of June 30, 2016 and December 31, 2015, respectively. The weighted-average interest rate on these borrowings as of June 30, 2016 and December 31, 2015 was 0.66 percent and 0.72 percent, respectively. As of June 30, 2016, there were intercompany loans from Eversource parent of $93 million to CL&P, $120.9 million to PSNH and $53.7 million to WMECO. As of December 31, 2015, there were intercompany loans from Eversource parent of $277.4 million to CL&P, $231.3 million to PSNH and $143.4 million to WMECO. Eversource parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year $1.45 billion revolving credit facility, which terminates on September 4, 2020. The revolving credit facility serves to backstop Eversource parent's $1.45 billion commercial paper program. NSTAR Electric has a $450 million commercial paper program allowing NSTAR Electric to issue commercial paper as a form of short-term debt. As of June 30, 2016 and December 31, 2015, NSTAR Electric had $329 million and $62.5 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $121 million and $387.5 million of available borrowing capacity as of June 30, 2016 and December 31, 2015, respectively. The weighted-average interest rate on these borrowings as of June 30, 2016 and December 31, 2015 was 0.44 percent and 0.40 percent, respectively. NSTAR Electric is a party to a five-year $450 million revolving credit facility, which terminates on September 4, 2020. The revolving credit facility serves to backstop NSTAR Electric's $450 million commercial paper program.
Notes Payable	$ 767,000	$ 1,160,953
Long-term Debt, Description	Long-Term Debt Issuances: Long-Term Debt Issuances: In March 2016, Eversource parent issued $250 million of 2.50 percent Series I Senior Notes due to mature in 2021 and $250 million of 3.35 percent Series J Senior Notes due to mature in 2026. The proceeds, net of issuance costs, were used to repay short-term borrowings under the Eversource parent commercial paper program. In May 2016, NSTAR Electric issued $250 million of 2.70 percent debentures, due to mature in 2026. The proceeds, net of issuance costs, were used to repay short-term borrowings under the NSTAR Electric commercial paper program and fund capital expenditures and working capital. In June 2016, WMECO issued $50 million of 2.75 percent Series H Senior Notes, due to mature in 2026. The proceeds, net of issuance costs, were used to repay short-term borrowings. Long-Term Debt Repayments: In May 2016, NSTAR Electric repaid at maturity $200 million variable rate debentures, using short-term borrowings.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	$ 93,000	277,400
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	$ 121,000	$ 388,000
Short-term Debt, Weighted Average Interest Rate	0.44%	0.40%
CommercialPaper	$ 329,000	$ 63,000
Notes Payable	329,000	62,500
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	120,900	231,300
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	53,700	143,400
Eversource Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity
Line of Credit Facility, Remaining Borrowing Capacity	1,000,000	351,500
Letters Of Credit Outstanding Under Line of Credit	$ 438,000	$ 1,100
Short-term Debt, Weighted Average Interest Rate	0.66%	0.72%