Other Contingencies (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Loss Contingencies [Line Items]
Loss Contingency, Description	C.        Spent Nuclear Fuel Litigation - Yankee Companies The Yankee Companies have filed separate complaints against the DOE in the Court of Federal Claims seeking monetary damages resulting from the DOE's failure to provide for a permanent facility to store spent nuclear fuel pursuant to the terms of the 1983 spent fuel and high level waste disposal contracts between the Yankee Companies and the DOE. The court had previously awarded the Yankee Companies damages for Phase I and Phase II of litigation resulting from the DOE's failure to meet its contractual obligations. Phase I covered damages incurred in the years 1998 through 2002 and Phase II covered damages incurred in the years 2001 through 2008 for CYAPC and YAEC and from 2002 through 2008 for MYAPC. DOE Phase III Damages - In August 2013, the Yankee Companies each filed subsequent lawsuits against the DOE seeking recovery of actual damages incurred in the years 2009 through 2012. The DOE Phase III trial concluded on July 1, 2015, followed by a post-trial briefing that concluded on October 14, 2015. On March 25, 2016, the court issued its decision, awarding CYAPC, YAEC and MYAPC damages of $32.6 million, $19.6 million and $24.6 million, respectively. In total, the Yankee Companies were awarded $76.8 million of the $77.9 million in damages sought by the Yankee Companies in Phase III. The decision became final on July 18, 2016. Management cannot predict the timing or amount of any customer refunds. At this time, the proceeds are primarily expected to be used by the Yankee Companies to fund remaining fuel storage obligations, and management does not expect any significant amounts to be refunded to Eversource utilities (CL&P, NSTAR Electric, PSNH and WMECO). The utilities would then ultimately refund any amounts received to utility customers. D.        FERC ROE Complaints FERC ROE Complaints I, II and III: Three separate complaints have been filed at FERC by combinations of New England state attorneys general, state regulatory commissions, consumer advocates, consumer groups, municipal parties and other parties (collectively the "Complainants"). In these three separate complaints, the Complainants challenged the NETOs' base ROE of 11.14 percent that had been utilized since 2006 and sought an order to reduce it prospectively from the date of the final FERC order and for the 15-month complaint refund periods stipulated in the separate complaints. In 2014, the FERC ordered a 10.57 percent base ROE for the first complaint refund period and prospectively from October 16, 2014, and that a utility's total or maximum ROE shall not exceed the top of the new zone of reasonableness, which was set at 11.74 percent. In late 2014, the NETOs made a compliance filing, and the Company began issuing refunds to customers from the first complaint period. The Company has refunded all amounts associated with the first complaint period. As a result of developments in this matter, the Company recorded reserves across the complaint periods at its electric subsidiaries in the first half of 2015 and recognized a pre-tax charge to earnings (excluding interest) of $20 million, of which $12.5 million was recorded at CL&P, $2.4 million at NSTAR Electric, $1 million at PSNH, and $4.1 million at WMECO. The pre-tax charge was recorded as a regulatory liability and as a reduction to Operating Revenues. The NETOs and Complainants have filed appeals for the first complaint to the D.C. Circuit Court of Appeals. A court decision is expected in late 2016. For the second and third complaints, the state parties, municipal utilities and FERC trial staff each believe that the base ROE should be reduced to an amount lower than 10.57 percent. The NETOs believe that the Complainants' positions are without merit. On March 22, 2016, the FERC ALJ issued an initial decision on the second and third FERC ROE complaints. For the second complaint period, the FERC ALJ recommended a zone of reasonableness of 7.12 percent to 10.42 percent and a base ROE of 9.59 percent. For the third complaint period, the FERC ALJ recommended a zone of reasonableness of 7.04 percent to 12.19 percent and a base ROE of 10.90 percent. The FERC ALJ also affirmed that the maximum ROE for transmission incentive projects should be the top of the zone of reasonableness. The NETOs filed briefs on April 21, 2016, in which the NETOs identified corrections and requested changes that should be made to the FERC ALJ’s recommendations. A final FERC order is expected in late 2016 or early 2017. The Company believes that the range of potential loss for the second complaint period (the 15-month period beginning December 27, 2012) is from a base ROE of 10.57 percent to a base ROE of 9.59 percent. As the FERC ALJ initial decision on the third complaint recommended a base ROE of 10.90 percent, the Company concluded there is currently no range of potential loss for that complaint period. Given the differences between the recommended base ROEs in the FERC ALJ’s initial decision on the second and third complaints, as well as other factors, the Company is unable to predict the outcome of the final FERC order on these complaints. The Company does not believe any base ROE outcome within the 10.57 percent to 9.59 percent range is more likely than the base ROEs used to record the current revenues and reserves, and therefore the Company believes that the current reserves for the second complaint period are appropriate at this time. The impact of a 10 basis point change to the existing base ROE of 10.57 percent would affect Eversource's after-tax earnings by approximately $3 million for each of the 15-month second and third complaint periods.  If the Company adjusted its reserves based on the recommendations in the FERC ALJ initial decision (for both the base ROE and maximum ROE for transmission incentive projects) for the second and third complaints, then it would result in an after-tax increase of approximately $34 million and an after-tax decrease of approximately $8 million, respectively, to the existing reserves. FERC ROE Complaint IV: On April 29, 2016, a fourth complaint was filed with the FERC by certain municipal utilities claiming the current base ROE of 10.57 percent and the incentive cap of 11.74 percent are unjust and unreasonable. The NETOs answered on June 3, 2016 and requested that FERC dismiss the complaint. At this time FERC has not taken any action with respect to this complaint; therefore the Company is unable to predict the outcome of this complaint. E.        PSNH Generation Restructuring On June 10, 2015, Eversource and PSNH entered into the 2015 Public Service Company of New Hampshire Restructuring and Rate Stabilization Agreement (the Agreement) with the New Hampshire Office of Energy and Planning, certain members of the NHPUC staff, the Office of Consumer Advocate, two State Senators, and several other parties. The Agreement was filed with the NHPUC on the same day. Under the terms of the Agreement, PSNH agreed to divest its generation assets upon NHPUC approval. The Agreement is designed to provide a resolution of issues pertaining to PSNH's generation assets in pending regulatory proceedings before the NHPUC. The Agreement provided for the Clean Air Project prudence proceeding to be resolved and all remaining Clean Air Project costs to be included in rates effective January 1, 2016. As part of the Agreement, PSNH agreed to forego recovery of $25 million of the deferred equity return related to the Clean Air Project. In addition, PSNH will not seek a general distribution rate increase effective before July 1, 2017 and will contribute $5 million to create a clean energy fund, which will not be recoverable from its customers. In 2015, PSNH recorded the $5 million contribution as a long-term liability and an increase to Operations and Maintenance expense on the statements of income. On July 1, 2016, the NHPUC approved the Agreement in an order that, among other things, instructs PSNH to begin the process to divest its generation assets. The NHPUC is in the process of selecting an auction adviser to assist with the divestiture. Upon completion of the divestiture process, all remaining stranded costs will be recovered via bonds that will be secured by a non-bypassable charge or through recoveries in rates billed to PSNH's customers. If the NHPUC approves the sale of the plants, the Company expects the plants will be sold prior to the end of 2017. The sales price of the generating assets could be less than the carrying value, but the Company believes that full recovery of PSNH's generation assets is probable through a combination of cash flows during the remaining operating period, sales proceeds upon divestiture, and recovery of stranded costs in future rates. F.        Eversource and NSTAR Electric Boston Harbor Civil Action On July 15, 2016, the United States Army Corps of Engineers filed a civil action in the United States District Court for the District of Massachusetts under provisions of the Rivers and Harbors Act of 1899 and the Clean Water Act against NSTAR Electric, Harbor Electric Energy Company, a wholly-owned subsidiary of NSTAR Electric (“HEEC”), and the Massachusetts Water Resources Authority (together with NSTAR Electric and HEEC, the “Defendants”). The action alleges that the Defendants failed to comply with certain permitting requirements relating to the placement of the HEEC-owned electric distribution cable beneath Boston Harbor. The action seeks an order to force HEEC to comply with cable depth requirements in the U.S. Army Corps of Engineers' permit or alternatively to remove the electric distribution cable and cease unauthorized work in U.S. waterways. The action also seeks civil penalties and other costs. Management believes it has valid defenses to the claims and intends to defend it vigorously; concurrently, NSTAR Electric and HEEC are seeking to work collaboratively with all parties for a mutually beneficial resolution. At this time, management is unable to predict the outcome of this action or the impact on Eversource's and NSTAR Electric’s financial position, results of operations, or cash flows.
Public Utilities Disclosure Of Regulatory Matters
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period		$ 20.0
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period		12.5
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period		2.4
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period		1.0
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period		$ 4.1