SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The 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 consolidated financial statements of Eversource, NSTAR Electric and PSNH and the financial statements of CL&P are herein collectively referred to as the "financial statements." 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. Eversource consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's and PSNH's combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric and PSNH and the CYAPC and YAEC companies have been eliminated in consolidation of the Eversource financial statements. Eversource's utility subsidiaries' electric and natural gas distribution (including generation assets), transmission and water 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 year data were made in the accompanying financial statements to conform to the current year presentation. In accordance with accounting guidance on noncontrolling interests in consolidated financial statements, the Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric, which are not owned by Eversource or its consolidated subsidiaries and are not subject to mandatory redemption, have been presented as noncontrolling interests in the financial statements of Eversource. The Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric are considered to be temporary equity and have been classified between liabilities and permanent shareholders' equity on the balance sheets of Eversource, CL&P and NSTAR Electric due to a provision in the preferred stock agreements of both CL&P and NSTAR Electric that grant preferred stockholders the right to elect a majority of the CL&P and NSTAR Electric Boards of Directors, respectively, should certain conditions exist, such as if preferred dividends are in arrears for a specified amount of time. The Net Income reported in the statements of income and cash flows represents net income prior to apportionment to noncontrolling interests, which is represented by dividends on preferred stock of CL&P and NSTAR Electric.
Accounting Standards	Accounting Standards Accounting Standards Issued but Not Yet Effective: In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2014-09, Revenue from Contracts with Customers, which amends existing revenue recognition guidance and is required to be applied either fully retrospectively (to each reporting period presented) or under a modified retrospective method (cumulatively at the date of initial application). The FASB deferred implementation of ASU 2014-09 in ASU 2015-14, Revenue from Contracts with Customers (Topic: 606): Deferral of the Effective Date. The new accounting guidance is effective for interim and annual periods beginning in 2018 with early adoption permitted. The Company implemented the standard in the first quarter of 2018 using the modified retrospective method of adoption. Under this method of adoption, prior year reported results are not restated. Under the new standard, an entity must identify the performance obligations in a contract, determine the transaction price and allocate the price to specific performance obligations to recognize the revenue when the obligation is completed. The amendments in this ASU also require disclosure of sufficient information to allow users to understand the nature, amount, timing and uncertainty of revenue and cash flow arising from contracts. The Company has reviewed and performed accounting analyses of its revenue streams under contracts with customers. These accounting analyses included reviewing representative contracts and tariffs for each material revenue stream and evaluating them under the new guidance. The majority of the Company’s sales are derived from tariffs to provide electric and natural gas to customers. For such tariffs, the Company expects that the revenue from contracts with customers under ASU 2014-09 will be equivalent to revenue from electricity and natural gas supplied and billed in that period (including estimated unbilled revenues), which is consistent with current practice. Based on our assessments, the Company has identified one item that will be accounted for differently under the new revenue guidance as compared to current guidance. As a result of applying guidance on the unit of account under the new standard, purchases and sales of power from and to ISO-New England will be accounted for net by the hour, rather than net by the month, with no impact on net income. After taking into consideration this identified change, the Company has concluded that the new guidance will not have a material impact on the amounts or timing of revenue recognition. Implementation of the ASU will not have a material effect on the results of operations, financial position or cash flows of Eversource, CL&P, NSTAR Electric or PSNH. Significant additional disclosures of the nature, amount, timing and uncertainty of revenues and cash flows arising from contracts with customers will be presented beginning in the first quarter of 2018. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Liabilities, which is required to be implemented in the first quarter of 2018. 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 December 31, 2017 was approximately $51 million with an unrealized loss of $0.1 million. The unrealized loss recorded in AOCI will be recorded as an adjustment to the opening balance of retained earnings as of January 1, 2018. 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 or PSNH. In February 2016, the FASB issued ASU 2016-02, 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 lease criteria are required to be applied to leases and lease renewals entered into effective January 1, 2019, and leases entered into before that date are required to be recognized and measured using a modified retrospective approach. The Company is reviewing the requirements of ASU 2016-02, including balance sheet recognition of leases previously deemed to be operating leases, and expects to implement the ASU in the first quarter of 2019. In March 2017, the FASB issued ASU 2017-07, Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, required to be implemented in the first quarter of 2018. The ASU requires separate presentation of service cost from other components of net pension and PBOP costs, with the other components presented as non-operating income and not subject to capitalization. The ASU is required to be applied retrospectively for the separate presentation in the income statement of service costs and other components and prospectively in the balance sheet for the capitalization of only the service cost component. The implementation of the ASU will not have an impact on the net income of Eversource, CL&P, NSTAR Electric or PSNH.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash on hand and short-term cash investments that are highly liquid in nature and have original maturities of three months or less. At the end of each reporting period, any overdraft amounts are reclassified from Cash and Cash Equivalents to Accounts Payable on the balance sheets.
Provision for Uncollectible Accounts	Provision for Uncollectible Accounts Eversource, including CL&P, NSTAR Electric and PSNH, 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 NSTAR Electric and NSTAR Gas to recover in rates, amounts associated with certain uncollectible hardship accounts receivable. These uncollectible hardship customer account balances are included in Regulatory Assets or Other Long-Term Assets on the balance sheets.
Fuel, Materials, Supplies and Inventory	Fuel, Materials, Supplies and Inventory Fuel, Materials, Supplies and Inventory include natural gas, coal, biomass and oil inventories, materials and supplies purchased primarily for construction or operation and maintenance purposes, RECs and emission allowances. Inventory is valued at the lower of cost or net realizable value. RECs are purchased from suppliers of renewable sources of generation and are used to meet state mandated Renewable Portfolio Standards requirements. PSNH is subject to federal and state laws and regulations that regulate emissions of air pollutants, including SO₂, CO₂, and NO_x related to its regulated generation units, and used SO₂, CO₂, and NO_x emissions allowances. SO₂, CO₂, and NO_x emissions allowances were charged to expense based on their average cost as they were utilized against emissions volumes at PSNH's generating units. On October 11, 2017, PSNH entered into two Purchase and Sale Agreements ("Agreements") to sell its thermal and hydroelectric generation assets. The NHPUC approved the Agreements in late November 2017 and on January 10, 2018, PSNH completed the sale of its thermal generation assets. As of December 31, 2017, PSNH has classified its generation assets, which included coal, biomass and oil inventories and emission allowances, as held for sale. As of December 31, 2016, these inventories were recorded within Fuel, Materials, Supplies and Inventory on the balance sheet
Deposits	Deposits As of December 31, 2017, Eversource, CL&P, NSTAR Electric and PSNH had $24.5 million, $3.1 million, $12.8 million and $0.5 million, respectively, of cash collateral posted not subject to master netting agreements, with ISO-NE related to energy transactions, which was included in Prepayments and Other Current 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. Determination of Fair Value: The valuation techniques and inputs used in Eversource's fair value measurements are described in Note 4, "Derivative Instruments," Note 5, "Marketable Securities," Note 6, "Asset Retirement Obligations," Note 9A, "Employee Benefits – Pension Benefits and Postretirement Benefits Other Than Pensions," and Note 14, "Fair Value of Financial Instruments" to the financial statements.
Derivative Accounting	Derivative Accounting Many of the electric and natural gas companies' contracts for the purchase and sale of energy or energy-related products are derivatives. The accounting treatment for energy contracts entered into varies and depends on the intended use of the particular contract and on whether or not the contract is a derivative. For the regulated companies, regulatory assets or regulatory liabilities are recorded to offset the fair values of derivative contracts related to energy and energy-related products, as contract settlements are recovered from, or refunded to, customers in future rates. The application of derivative accounting is complex and requires management judgment in the following respects: identification of derivatives and embedded derivatives, election and designation of a contract as normal, and determination of the fair value of derivative contracts. All of these judgments can have a significant impact on the financial statements. The judgment applied in the election of a contract as normal (and resulting accrual accounting) includes the conclusion that it is probable at the inception of the contract and throughout its term that it will result in physical delivery of the underlying product and that the quantities will be used or sold by the business in the normal course of business. If facts and circumstances change and management can no longer support this conclusion, then a contract cannot be considered normal and accrual accounting is terminated, and fair value accounting is applied prospectively. The fair value of derivative contracts is based upon the contract terms and conditions and the underlying market price or fair value per unit. When quantities are not specified in the contract, the Company determines whether the contract has a determinable quantity by using amounts referenced in default provisions and other relevant sections of the contract. The fair value of derivative assets and liabilities with the same counterparty are offset and recorded as a net derivative asset or liability on the balance sheets. All changes in the fair value of derivative contracts are recorded as regulatory assets or liabilities and do not impact net income.
Investments	Investments Investments are included in Other Long-Term Assets on the balance sheets and earnings impacts from equity investments are included in Other Income, Net on the statements of income.
Revenues	Revenues Retail Revenues: Retail revenues are based on rates approved by respective state regulatory commissions. In general, rates can only be changed through formal proceedings with the state regulatory commissions. These rates are designed to recover the costs to provide service to customers, and include a return on investment. Regulatory commission-approved tracking mechanisms are also used to recover certain costs on a fully-reconciling basis. These tracking mechanisms require rates to be changed periodically to ensure recovery of actual costs incurred. Certain Eversource electric, natural gas and water companies, including CL&P and NSTAR Electric (for a portion of its customers), have a regulatory commission approved revenue decoupling mechanism ("decoupled companies"). Distribution revenues are decoupled from customer sales volumes, where applicable, which breaks the relationship between sales volumes and revenues recognized. The decoupled companies reconcile their annual base distribution rate recovery to pre-established levels of baseline distribution delivery service revenues. Any difference between the allowed level of distribution revenue and the actual amount realized is adjusted through rates in a subsequent period. A significant portion of the electric and natural gas companies' retail revenues relate to the recovery of costs incurred for the sale of electricity and natural gas purchased on behalf of customers. These energy supply costs are recovered from customers in rates through cost tracking mechanisms. Energy purchases are recorded in Purchased Power, Fuel and Transmission, and the sales of energy associated with these purchases are recorded in Operating Revenues on the statements of income. Unbilled Revenues: Because customers are billed throughout the month based on pre-determined cycles rather than on a calendar month basis, an estimate of electricity, natural gas or water delivered to customers for which the customers have not yet been billed is calculated as of the balance sheet date. Unbilled revenues are included in Operating Revenues on the statements of income and in Current Assets on the balance sheets. Actual amounts billed to customers when meter readings become available may vary from the estimated amount. Unbilled revenues are recognized by allocating estimated unbilled sales volumes to the respective customer classes, and then applying an estimated rate by customer class to those sales volumes. Unbilled revenues can vary significantly from period to period as a result of seasonality, weather, customer usage patterns, customer rates in effect for customer classes, and the timing of customer billing. The estimate of unbilled revenues can significantly impact the amount of revenues recorded at the companies that do not have a revenue decoupling mechanism. Companies that do have a decoupling mechanism record a regulatory deferral to reflect the actual allowed amount of revenue associated with their respective decoupled distribution rate design. Transmission Revenues - Wholesale Rates: The Eversource electric transmission-owning companies have a combination of FERC-approved regional and local formula rates that work in tandem to recover all their transmission costs. These rates are part of the ISO-NE Tariff. Regional rates recover the costs of higher voltage transmission facilities that benefit the region, and are collected from all New England transmission customers, including the Eversource distribution businesses. Eversource and NSTAR Electric each have two sets of local rates that recover the companies' total transmission revenue requirements, less revenues received from regional rates and other sources, and are collected from Eversource's distribution businesses and other transmission customers. The distribution businesses of Eversource, in turn, recover the FERC- approved charges from retail customers through annual or semiannual tracking mechanisms. The transmission formula rates provide for the annual reconciliation and recovery or refund of estimated costs to actual costs. The financial impacts of differences between actual and estimated costs are deferred for future recovery from, or refund to, transmission customers. See Note 11E, "Commitments and Contingencies – FERC ROE Complaints," for complaints filed at the FERC relating to Eversource's ROE. Transmission Revenues - Retail Rates: A significant portion of the Eversource electric transmission segment revenue comes from ISO-NE charges to the distribution businesses of CL&P, NSTAR Electric, and PSNH, each of which recovers these costs through rates charged to their retail customers. CL&P, NSTAR Electric and PSNH each have a retail transmission cost tracking mechanism as part of their rates, which allows the electric distribution companies to charge their retail customers for transmission costs on a timely basis.
Allowance for Funds Used During Construction	Allowance for Funds Used During Construction AFUDC represents the cost of borrowed and equity funds used to finance construction and is included in the cost of the electric, natural gas and water companies' utility plant on the balance sheet. The portion of AFUDC attributable to borrowed funds is recorded as a reduction of Interest Expense, and the AFUDC related to equity funds is recorded as Other Income, Net on the statements of income. AFUDC costs are recovered from customers over the service life of the related plant in the form of increased revenue collected as a result of higher depreciation expense. The average AFUDC rate is based on a FERC-prescribed formula using the cost of a company's short-term financings and capitalization (preferred stock, long-term debt and common equity), as appropriate. The average rate is applied to average eligible CWIP amounts to calculate AFUDC.
Other Income, Net	Other Income, Net Items included within Other Income, Net on the statements of income primarily consist of investment income/(loss) related to debt and equity securities held in trust, market value changes related to deferred compensation plans, interest income, AFUDC related to equity funds, and income/(loss) related to equity method investees.
Other Taxes	Other Taxes Eversource's companies that serve customers in Connecticut collect gross receipts taxes levied by the state of Connecticut from their customers.
Related Parties	Related Parties Eversource Service, Eversource's service company, provides centralized accounting, administrative, engineering, financial, information technology, legal, operational, planning, purchasing, and other services to Eversource's companies. The Rocky River Realty Company, Renewable Properties, Inc. and Properties, Inc., three other Eversource subsidiaries, construct, acquire or lease some of the property and facilities used by Eversource's companies. As of both December 31, 2017 and 2016, CL&P, NSTAR Electric and PSNH had long-term receivables from Eversource Service in the amounts of $25.0 million, $3.8 million and $5.5 million, respectively, which were included in Other Long-Term Assets on the balance sheets. These amounts related to the funding of investments held in trust by Eversource Service in connection with certain postretirement benefits for CL&P, NSTAR Electric and PSNH employees and have been eliminated in consolidation on the Eversource financial statements. Included in the CL&P, NSTAR Electric and PSNH balance sheets as of December 31, 2017 and 2016 were Accounts Receivable from Affiliated Companies and Accounts Payable to Affiliated Companies relating to transactions between CL&P, NSTAR Electric and PSNH and other subsidiaries that are wholly-owned by Eversource. These amounts have been eliminated in consolidation on the Eversource financial statements.