SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Sep. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Eversource Energy is a public utility holding company primarily engaged, through its wholly owned regulated utility subsidiaries, in the energy delivery business. Eversource Energy’s wholly owned regulated utility subsidiaries consist of CL&P, NSTAR Electric, PSNH, WMECO, Yankee Gas and NSTAR Gas. Eversource provides energy delivery service to approximately 3.6 million electric and natural gas customers through these six regulated utilities in Connecticut, Massachusetts and New Hampshire. The unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH include the accounts of each of their respective subsidiaries. Intercompany transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements of Eversource, NSTAR Electric and PSNH and the unaudited condensed financial statements of CL&P and WMECO are herein collectively referred to as the “financial statements.” The combined notes to the financial statements have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures included in annual financial statements prepared in accordance with GAAP have been omitted pursuant to such rules and regulations. The accompanying financial statements should be read in conjunction with the Combined Notes to Financial Statements included in Item 8, “Financial Statements and Supplementary Data,” of the Eversource 2015 Form 10-K, which was filed with the SEC. The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The financial statements contain, in the opinion of management, all adjustments (including normal, recurring adjustments) necessary to present fairly Eversource’s, CL&P’s, NSTAR Electric’s, PSNH’s and WMECO’s financial position as of September 30, 2016 and December 31, 2015, the results of operations and comprehensive income for the three and nine months ended September 30, 2016 and 2015, and the cash flows for the nine months ended September 30, 2016 and 2015. The results of operations and comprehensive income for the three and nine months ended September 30, 2016 and 2015 and the cash flows for the nine months ended September 30, 2016 and 2015 are not necessarily indicative of the results expected for a full year. Eversource consolidates CYAPC and YAEC because CL&P’s, NSTAR Electric’s, PSNH’s and WMECO’s combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Access Northeast is a natural gas pipeline and storage project (the Project) being developed jointly by Eversource, Spectra Energy Partners, LP (Spectra), and National Grid plc (National Grid) through Algonquin Gas Transmission, LLC (AGT). The Project will enhance the Algonquin and Maritimes & Northeast pipeline systems using existing routes and will include two new LNG storage tanks and liquefaction and vaporization facilities in Acushnet, Massachusetts that will be connected to the Algonquin natural gas pipeline. Eversource and Spectra each own a 40 percent interest in the Project, with the remaining 20 percent interest owned by National Grid. The total projected cost for both the pipeline and the LNG storage facilities will be funded in proportion to the respective ownership interest. Eversource’s cumulative equity investment in the Project as of September 30, 2016 of $28.6 million is presented in Other Long-Term Assets. Eversource’s utility subsidiaries’ distribution (including generation) and transmission businesses are subject to rate-regulation that is based on cost recovery and meets the criteria for application of accounting guidance for entities with rate-regulated operations, which considers the effect of regulation on the differences in the timing of the recognition of certain revenues and expenses from those of other businesses and industries. See Note 2, “Regulatory Accounting,” for further information. Certain reclassifications of prior period data were made in the accompanying financial statements to conform to the current period presentation and as a result of the adoption of new accounting guidance. See Note 1B, “Summary of Significant Accounting Policies – Accounting Standards,” for further information.
Accounting Standards	Accounting Standards Accounting Standards Issued but not Yet Effective: In May 2014, the Financial Accounting Standards Board (FASB) issued an Accounting Standards Update (ASU) 2014-9, Revenue from Contracts with Customers, which amends existing revenue recognition guidance and is required to be applied retrospectively (either to each reporting period presented or cumulatively at the date of initial application). In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers – Deferral of the Effective Date, which defers the effective date of ASU 2014-9 to the first quarter of 2018, with 2017 application permitted. The guidance continues to be interpreted on an industry specific level. The Company is evaluating the requirements and potential impacts of ASU 2014-9 and will implement the standard in the first quarter of 2018. The ASU is not currently expected to have a material impact on the financial statements of Eversource, CL&P, NSTAR Electric, PSNH or WMECO. In January 2016, the FASB issued ASU 2016-1, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Liabilities, which is required to be implemented in the first quarter of 2018. The Company is reviewing the requirements of the ASU. The ASU will remove the available-for-sale designation for equity securities, whereby changes in fair value are recorded in accumulated other comprehensive income within shareholders’ equity, and will require changes in fair value of all equity securities to be recorded in earnings beginning on January 1, 2018, with the unrealized gain or loss on available-for-sale equity securities as of that date reclassified to retained earnings as a cumulative effect of adoption. The fair value of available-for-sale equity securities subject to this guidance as of September 30, 2016 was approximately $50 million. The remaining available-for-sale equity securities included in marketable securities on the balance sheet are held in nuclear decommissioning trusts and are subject to regulatory accounting treatment and will not be impacted by this guidance. Implementation of the ASU for other financial instruments is not expected to have a material impact on the financial statements of Eversource, CL&P, NSTAR Electric, PSNH or WMECO. In February 2016, the FASB issued ASU 2016-2, Leases, which changes existing lease accounting guidance and is required to be applied in the first quarter of 2019, with earlier application permitted. The ASU is required to be implemented for leases beginning on the date of initial application. For prior periods presented, leases are required to be recognized and measured using a modified retrospective approach. The Company is reviewing the requirements of ASU 2016-2, including balance sheet recognition of leases previously deemed operating leases, and expects to implement the ASU in the first quarter of 2019. Recently Adopted Accounting Standards: In March 2016, the FASB issued ASU 2016-9, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting. The ASU is intended to simplify some aspects of the accounting for share-based payment transactions. The Company implemented this guidance in the first quarter of 2016, as permitted by early adoption. Beginning in the first quarter of 2016, the excess tax benefits associated with the distribution of stock compensation awards, previously recognized in Capital Surplus, Paid In within Common Shareholders’ Equity on the balance sheet, are recognized in income tax expense in the income statement. The implementation reduced income tax expense by $2.9 million for the nine months ended September 30, 2016. Also, beginning in 2016, in the statement of cash flows, the excess tax benefits are presented as an operating activity rather than a financing activity, and in both periods presented, cash paid to satisfy the statutory income tax withholding obligation previously reflected within operating activities in 2015 is now treated as a financing activity. The cash payments to satisfy this obligation for the nine months ended September 30, 2016 and 2015 were $9.1 million and $9.7 million, respectively, and are included in Other Financing Activities on the statements of cash flows.
Provision for Uncollectible Accounts	Provision for Uncollectible Accounts Eversource, including CL&P, NSTAR Electric, PSNH and WMECO, presents its receivables at estimated net realizable value by maintaining a provision for uncollectible accounts. This provision is determined based upon a variety of judgments and factors, including the application of an estimated uncollectible percentage to each receivable aging category. The estimate is based upon historical collection and write-off experience and management’s assessment of collectability from customers. Management continuously assesses the collectability of receivables and adjusts collectability estimates based on actual experience. Receivable balances are written off against the provision for uncollectible accounts when the customer accounts are terminated and these balances are deemed to be uncollectible. The PURA allows CL&P and Yankee Gas to accelerate the recovery of accounts receivable balances attributable to qualified customers under financial or medical duress (uncollectible hardship accounts receivable) outstanding for greater than 180 days and 90 days, respectively. The DPU allows WMECO and NSTAR Gas to also recover in rates amounts associated with certain uncollectible hardship accounts receivable. Certain of NSTAR Electric’s uncollectible hardship accounts receivable are expected to be recovered in future rates, similar to WMECO and NSTAR Gas. These uncollectible customer account balances are included in Regulatory Assets or Other Long-Term Assets on the balance sheets.
Fair Value Measurements	Fair Value Measurements Fair value measurement guidance is applied to derivative contracts that are not elected or designated as “normal purchases or normal sales” (normal) and to the marketable securities held in trusts. Fair value measurement guidance is also applied to valuations of the investments used to calculate the funded status of pension and PBOP plans, the nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs, and the estimated fair value of preferred stock and long-term debt. Fair Value Hierarchy: In measuring fair value, Eversource uses observable market data when available in order to minimize the use of unobservable inputs. Inputs used in fair value measurements are categorized into three fair value hierarchy levels for disclosure purposes. The entire fair value measurement is categorized based on the lowest level of input that is significant to the fair value measurement. Eversource evaluates the classification of assets and liabilities measured at fair value on a quarterly basis, and Eversource’s policy is to recognize transfers between levels of the fair value hierarchy as of the end of the reporting period. The three levels of the fair value hierarchy are described below: Level 1 - Inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities as of the reporting date. Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis. Level 2 - Inputs are quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs are observable. Level 3 - Quoted market prices are not available. Fair value is derived from valuation techniques in which one or more significant inputs or assumptions are unobservable. Where possible, valuation techniques incorporate observable market inputs that can be validated to external sources such as industry exchanges, including prices of energy and energy-related products.
Other Income, Net	Other Income, Net Items included within Other Income, Net on the statements of income primarily consist of investment income/(loss), interest income, AFUDC related to equity funds, and equity in earnings of equity method investees. Investment income/(loss) primarily relates to debt and equity securities held in trust. For further information, see Note 5, “Marketable Securities,” to the financial statements.
Other Taxes	As agents for state and local governments, Eversource’s companies that serve customers in Connecticut and Massachusetts collect certain sales taxes that are recorded on a net basis with no impact on the statements of income. Other Taxes Gross receipts taxes levied by the state of Connecticut are collected by CL&P and Yankee Gas from their respective customers.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.