SHORT TERM AND LONG TERM DEBT (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Line of Credit Facility, Description	Credit Agreements and Commercial Paper Programs: ES parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year $1.45 billion revolving credit facility that expires September 6, 2019. The revolving credit facility is used primarily to backstop ES parent's $1.45 billion commercial paper program. The commercial paper program allows ES parent to issue commercial paper as a form of short-term debt. As of March 31, 2015 and December 31, 2014, ES parent had $788 million and approximately $1.1 billion, respectively, in short-term borrowings outstanding under the ES parent commercial paper program, leaving $662 million and $348.9 million of available borrowing capacity as of March 31, 2015 and December 31, 2014, respectively. The weighted-average interest rate on these borrowings as of March 31, 2015 and December 31, 2014 was 0.53 percent and 0.43 percent, respectively, which is generally based on A2/P2 rated commercial paper. As of March 31, 2015, there were intercompany loans from ES parent of $190.1 million to CL&P, $82 million to PSNH and $70.5 million to WMECO. As of December 31, 2014, there were intercompany loans from ES parent of $133.4 million to CL&P, $90.5 million to PSNH and $21.4 million to WMECO. NSTAR Electric has a five-year $450 million revolving credit facility that expires September 6, 2019. This facility serves to backstop NSTAR Electric's existing $450 million commercial paper program. As of March 31, 2015 and December 31, 2014, NSTAR Electric had $215.5 million and $302 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $234.5 million and $148 million of available borrowing capacity as of March 31, 2015 and December 31, 2014, respectively. The weighted-average interest rate on these borrowings as of March 31, 2015 and December 31, 2014 was 0.35 percent and 0.27 percent, respectively, which is generally based on A2/P1 rated commercial paper.
Notes Payable	$ 1,003,500,000	$ 956,825,000
Long Term Debt Description [Abstract]
Long-term Debt, Description	Long-Term Debt: On January 15, 2015, ES 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 ES 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. Long-Term Debt Issuance Authorization: On April 3, 2015, the DPU authorized NSTAR Gas to issue up to $100 million in long-term debt for the period through December 31, 2015.
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	190,100,000	133,400,000
Long Term Debt Description [Abstract]
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.
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	450,000,000
Line of Credit Facility, Remaining Borrowing Capacity	234,500,000	148,000,000
Short-term Debt, Weighted Average Interest Rate	0.35%	0.27%
Commercial Paper Borrowings	215,500,000	302,000,000
Notes Payable	215,500,000	302,000,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	82,000,000	90,500,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	70,500,000	21,400,000
Nu Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,450,000,000
Line of Credit Facility, Remaining Borrowing Capacity	662,000,000	348,900,000
Short-term Debt, Weighted Average Interest Rate	0.53%	0.43%
Commercial Paper Borrowings	$ 788,000,000	$ 1,100,000,000