Other Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies [Line Items]
Loss Contingency, Description	E.         FERC ROE Complaints E.         FERC ROE Complaints 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 (the "Complainants"). In the first complaint, filed in 2011, the Complainants alleged that the NETOs' base ROE that had been utilized since 2006 was unjust and unreasonable, asserted that the rate was excessive due to changes in the capital markets, and sought an order to reduce it prospectively from the date of the final FERC order and for the 15-month period beginning October 1, 2011 to December 31, 2012. In the second and third complaints, filed in 2012 and 2014, the Complainants challenged the NETOs' base ROE and sought refunds for the respective 15-month periods beginning December 27, 2012 and July 31, 2014. As a result of the actions taken by the FERC and other developments in the first complaint matter, the Company recorded additional reserves at its electric subsidiaries in 2015 and 2014. In 2015, Eversource 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. In 2014, the net aggregate pre-tax charge to earnings (excluding interest) totaled $37 million, of which $20.7 million was recorded at CL&P, $7.9 million at NSTAR Electric, $2.8 million at PSNH and $5.6 million at WMECO. In 2013, the net aggregate pre-tax charge to earnings (excluding interest) totaled $23.7 million, of which $12.8 million was recorded at CL&P, $5.7 million at NSTAR Electric, $2.3 million at PSNH and $2.9 million at WMECO. The second and third complaint proceedings are ongoing and a final FERC order is expected in late 2016 or early 2017. Although management is uncertain on the final outcome of the second and third complaints regarding the ROE, management believes the current reserves established are appropriate to reflect probable and reasonably estimable refunds. F.        NSTAR Electric and NSTAR Gas Comprehensive Settlement Agreement On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2009 through 2011 (11 dockets in total). In the first quarter of 2015, as a result of the DPU order, NSTAR Electric and NSTAR Gas commenced refunding a combined $44.7 million to customers, which was recorded as a regulatory liability. Refunds to customers will continue through December 2016. As a result of the Settlement, NSTAR Electric increased its operating revenues and decreased its amortization expense in 2015, resulting in the recognition of a $21.7 million pre-tax benefit in 2015. G.        NSTAR Electric Basic Service Bad Debt Adder On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had removed energy-related bad debt costs from base distribution rates effective January 1, 2006. As a result of the DPU order, in the first quarter of 2015, NSTAR Electric increased its regulatory assets and reduced its operations and maintenance expense by an under recovered amount of $24.2 million for energy-related bad debt costs through 2014, resulting in a pre-tax benefit in 2015. NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers and on November 20, 2015the DPU approved NSTAR Electric’s proposed rate increase, to recover these costs over a 12-month period, effective January 1, 2016. H.        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 has 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 has 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. 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 other recoveries in rates billed to PSNH customers. On January 26, 2016, Advisory Staff of the NHPUC and the parties to the Agreement filed a stipulation with the NHPUC agreeing that near-term divestiture of PSNH’s generation was in the public interest and that the Agreement should be approved. Implementation of the Agreement is subject to NHPUC approval, which is expected in early 2016. If the NHPUC approves the settlements and the sale of the plants, then management expects to sell the plants in the first half of 2017. The sales price of the generating assets could be less than the carrying value, but we believe 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. I.        Litigation and Legal Proceedings Eversource, including CL&P, NSTAR Electric, PSNH and WMECO, are involved in legal, tax and regulatory proceedings regarding matters arising in the ordinary course of business, which involve management's assessment to determine the probability of whether a loss will occur and, if probable, its best estimate of probable loss. The Company records and discloses losses when these losses are probable and reasonably estimable, and discloses matters when losses are probable but not estimable or when losses are reasonably possible. Legal costs related to the defense of loss contingencies are expensed as incurred.
Public Utilities Disclosure Of Regulatory Matters	2015 Regulatory Developments: FERC ROE Complaints: As a result of the actions taken by the FERC and other developments in the pending ROE complaint proceedings described in Note 11E, "Commitments and Contingencies – FERC ROE Complaints," Eversource recorded reserves for the first and second ROE complaints, which were recorded as a regulatory liability and as a reduction to operating revenues. The cumulative pre-tax reserves (excluding interest) as of December 31, 2015, which include the impact of refunds given to customers, totaled $39.1 million for Eversource (including $21.4 million for CL&P, $8.5 million for NSTAR Electric, $3.1 million for PSNH, and $6.1 million for WMECO). NSTAR Electric and NSTAR Gas Comprehensive Settlement Agreement: On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2009 through 2011 (11 dockets in total). In 2015, as a result of the DPU order, NSTAR Electric and NSTAR Gas commenced refunding a combined $44.7 million to customers, which was recorded as a regulatory liability. Refunds to customers will continue through December 2016. As a result of the Settlement, NSTAR Electric increased its operating revenues and decreased its amortization expense in 2015, resulting in the recognition of a $21.7 million pre-tax benefit in 2015. NSTAR Electric Basic Service Bad Debt Adder: On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had removed energy-related bad debt costs from base distribution rates effective January 1, 2006. As a result of the DPU order, in the first quarter of 2015, NSTAR Electric increased its regulatory assets and reduced its operations and maintenance expense by an under recovered amount of $24.2 million for energy-related bad debt costs through 2014, resulting in a pre-tax benefit in 2015. NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers and on November 20, 2015 the DPU approved NSTAR Electric’s proposed rate increase to recover these costs over a 12-month period, effective January 1, 2016. CL&P Distribution Rates: On July 2, 2015, PURA issued a final order that approved a settlement agreement filed on May 19, 2015, which allows for an increase to rate base of approximately $163 million associated with ADIT, including a regulatory asset to recover the incremental revenue requirement for the period December 1, 2014 through November 30, 2015 over a subsequent 24-month period. The rate base increase provided an increase to total allowed annual revenue requirements of $18.4 million beginning December 1, 2014. As part of the settlement agreement, the $18.4 million for the period December 1, 2014 through November 30, 2015 was recorded as a regulatory asset with a corresponding increase in Operating Revenues, and is being collected from customers in rates over a 24-month period beginning December 1, 2015. NSTAR Gas Distribution Rates: On October 30, 2015, the DPU issued its order in the NSTAR Gas distribution rate case, which approved an annualized base rate increase of $15.8 million, plus other increases of approximately $11.5 million, mostly relating to recovery of pension and PBOP expenses and the Hopkinton GSA, effective January 1, 2016. In the order, the DPU also approved an authorized regulatory ROE of 9.8 percent, the establishment of a revenue decoupling mechanism, the recovery of certain bad debt expenses, and a 52.1 percent equity component of its capital structure. On November 19, 2015, NSTAR Gas filed a motion for reconsideration of the order with the DPU seeking the correction of mathematical errors and other plant and cost of service items. As a result of this order, Eversource recorded regulatory deferrals for costs that have been approved for recovery or are expected to be approved for recovery in future rate proceedings, which resulted in the recognition of a $17.2 million pre-tax benefit in 2015. Included in this amount is a $10.5 million pre-tax benefit recorded at NSTAR Electric for certain uncollectible hardship accounts receivable that are expected to be recovered in future rates given the allowed recoveries of uncollectible hardship accounts receivable by WMECO and NSTAR Gas.
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period	$ 20.0	$ 37.0	$ 23.7
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	20.7	12.8
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | NSTAR Electric Company [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period	2.4	7.9	5.7
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	2.8	2.3
FERC ROE Reserve - Cumulative - Pre Tax Charge [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies [Line Items]
Loss Contingency Loss in Period	$ 4.1	$ 5.6	$ 2.9